18.3 Reflection Emit

So far we've seen reflection used for three purposes: viewing metadata, type discovery, and dynamic invocation. You might use these techniques when building tools (such as a development environment) or when processing scripts. The most powerful use of reflection, however, is with reflection emit.

Reflection emit supports the dynamic creation of new types at runtime. You can define an assembly to run dynamically or to save itself to disk, and you can define modules and new types with methods that you can then invoke.

The use of dynamic invocation and reflection emit should be considered an advanced topic. Most developers will never have need to use reflection emit. This demonstration is based on an example provided at the Microsoft Author's Summit, Fall 2000.

To understand the power of reflection emit, you must first consider a slightly more complicated example of dynamic invocation.

Problems can have general solutions that are relatively slow and specific solutions that are fast. To keep things manageably simple, consider a DoSum( ) method, which provides the sum of a string of integers from 1...n, where n will be supplied by the user.

Thus, DoSum(3) is equal to 1+2+3, or 6. DoSum(10) is 55. Writing this in C# is very simple:

public int DoSum1(int n) {     int result = 0;     for(int i = 1;i <= n; i++)     {         result += i;     }     return result; }

The method simply loops, adding the requisite number. If you pass in 3, the method adds 1+2+3 and returns an answer of 6.

With large numbers, and when run many times, this might be a bit slow. Given the value 20, this method would be considerably faster if you removed the loop:

public int DoSum2( ) {     return 1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20; }

DoSum2 runs more quickly than DoSum1 does. How much more quickly? To find out, you'll need to put a timer on both methods. To do so, use a DateTime object to mark the start time and a TimeSpan object to compute the elapsed time.

For this experiment, you need to create two DoSum( ) methods: the first will use the loop and the second will not. Call each 1,000,000 times. (Computers are very fast, so to see a difference you have to work hard!) Then compare the times. Example 18-8 illustrates the entire test program.

Example 18-8. Comparing loop to brute force

namespace Programming_CSharp {    using System;    using System.Diagnostics;    using System.Threading;    public class MyMath    {       // sum numbers with a loop       public int DoSum(int n)       {          int result = 0;          for(int i = 1; i <= n; i++)          {             result += i;          }          return result;       }       // brute force by hand       public int DoSum2( )       {          return 1+2+3+4+5+6+7+8+9+10+11             +12+13+14+15+16+17+18+19+20;       }    }    public class TestDriver    {       public static void Main( )       {          const int val = 20;  // val to sum          // 1,000,000 iterations          const int iterations = 1000000;          // hold the answer          int result = 0;          MyMath m = new MyMath( );                      // mark the start time             DateTime startTime = DateTime.Now;          // run the experiment          for (int i = 0;i < iterations;i++)          {             result = m.DoSum(val);          }          // mark the elapsed time          TimeSpan elapsed =              DateTime.Now - startTime;          // display the results          Console.WriteLine(             "Loop: Sum of ({0}) = {1}",                val, result);          Console.WriteLine(             "The elapsed time in milliseconds is: " +              elapsed.TotalMilliseconds.ToString( ));          // mark a new start time          startTime = DateTime.Now;          // run the experiment          for (int i = 0;i < iterations;i++)          {             result = m.DoSum2( );          }          // mark the new elapsed time          elapsed = DateTime.Now - startTime;          // display the results          Console.WriteLine(             "Brute Force: Sum of ({0}) = {1}",                val, result);          Console.WriteLine(             "The elapsed time in milliseconds is: " +              elapsed.TotalMilliseconds);       }    } } Output: Loop: Sum of (20) = 210 The elapsed time in milliseconds is: 187.5 Brute Force: Sum of (20) = 210 The elapsed time in milliseconds is: 31.25

As you can see, both methods returned the same answer (one million times!), but the brute-force method was six times faster.

Is there a way to avoid the loop and still provide a general solution? In traditional programming, the answer would be no, but with reflection you do have one other option. You can, at runtime, take the value the user wants (20, in this case) and write out to disk a class that implements the brute-force solution. You can then use dynamic invocation to invoke that method.

There are at least three ways to achieve this result, each increasingly elegant. The third, reflection emit, is the best, but a close look at two other techniques is instructive. If you are pressed for time, you might wish to jump ahead to Section 18.3.3, later in this chapter.

18.3.1 Dynamic Invocation with InvokeMember( )

The first approach will be to dynamically create a class named BruteForceSums at runtime. The BruteForceSums class will contain a method, ComputeSum( ), that implements the brute-force approach. You'll write that class to disk, compile it, and then use dynamic invocation to invoke its brute-force method by means of the InvokeMember( ) method of the Type class. The key point is that BruteForceSums.cs won't exist until you run the program. You'll create it when you need it and supply its arguments then.

To accomplish this, you'll create a new class named ReflectionTest. The job of the ReflectionTest class is to create the BruteForceSums class, write it to disk, and compile it. ReflectionTest has only two methods: DoSum and GenerateCode.

ReflectionTest.DoSum( ) is a public method that returns the sum, given a value. That is, if you pass in 10, it returns the sum of 1+2+3+4+5+6+7+8+9+10. It does this by creating the BruteForceSums class and delegating the job to its ComputeSum method.

ReflectionTest has two private fields:

Type theType = null; object theClass = null;

The first is an object of type Type, that you use to load your class from disk; the second is an object of type object, that you use to dynamically invoke the ComputeSums( ) method of the BruteForceSums class you'll create.

The driver program instantiates an instance of ReflectionTest and calls its DoSum( ) method, passing in the value. For this version of the program, the value is increased to 200.

The DoSum( ) method checks whether theType is null; if it is, the class has not been created yet. DoSum( ) calls the helper method GenerateCode to generate the code for the BruteForceSums class and the class's ComputeSums method. GenerateCode then writes this newly created code to a .cs file on disk and runs the compiler to turn it into an assembly on disk. Once this is completed, DoSum( ) can call the method using reflection.

Once the class and method are created, load the assembly from disk and assign the class type information to theType DoSum( ) can use that to invoke the method dynamically to get the correct answer.

You begin by creating a constant for the value to which you'll sum:

const int val = 200;

Each time you compute a sum, it will be the sum of the values 1 to 200.

Before you create the dynamic class, you need to go back and recreate MyMath:

MyMath m = new MyMath( );                      

Give MyMath a method DoSumLooping, much as you did in the previous example:

public int DoSumLooping (int initialVal) {     int result = 0;     for(int i = 1;i <=initialVal;i++)     {         result += i;     }     return result; }

This serves as a benchmark against which you can compare the performance of the brute-force method.

Now you're ready to create the dynamic class and compare its performance with the looping version. First, instantiate an object of type ReflectionTest and invoke the DoSum( ) method on that object:

ReflectionTest t = new ReflectionTest( ); result = t.DoSum(val);

ReflectionTest.DoSum( ) checks to see if its Type field, theType, is null. If it is, you haven't yet created and compiled the BruteForceSums class and must do so now:

if (theType == null) {     GenerateCode(theValue); }

The GenerateCode method takes the value (in this case, 200) as a parameter to know how many values to add.

GenerateCode begins by creating a file on disk. The details of file I/O will be covered in Chapter 21. For now, I'll walk you through this quickly. First, call the static method File.Open, and pass in the filename and a flag indicating that you want to create the file. File.Open returns a Stream object:

string fileName = "BruteForceSums"; Stream s = File.Open(fileName + ".cs", FileMode.Create);

Once you have the Stream, you can create a StreamWriter so that you can write into that file:

StreamWriter wrtr = new StreamWriter(s);

You can now use the WriteLine( ) methods of StreamWriter to write lines of text into the file. Begin the new file with a comment:

wrtr.WriteLine("// Dynamically created BruteForceSums class");

This writes the text:

// Dynamically created BruteForceSums class 

to the file you've just created (BruteForceSums.cs). Next, write out the class declaration:

string className = "BruteForceSums"; wrtr.WriteLine("class {0}", className); wrtr.WriteLine("{");

Within the braces of the class, create the ComputeSum( ) method:

wrtr.WriteLine("\tpublic double ComputeSum( )"); wrtr.WriteLine("\t{"); wrtr.WriteLine("\t// Brute force sum method"); wrtr.WriteLine("\t// For value = {0}", theVal);

Now it is time to write out the addition statements. When you are done, you want the file to have this line:

return 0+1+2+3+4+5+6+7+8+9...

continuing up to value (in this case, 200):

wrtr.Write("\treturn 0"); for (int i = 1;i<=theVal;i++) {    wrtr.Write("+ {0}",i); }

Notice how this works. What will be written to the file is:

\treturn 0+ 1+ 2+ 3+...

The initial \t causes the code to be indented in the source file.

When the loop completes, end the return statement with a semicolon and then close the method and the class:

wrtr.WriteLine(";"); wrtr.WriteLine("\t}"); wrtr.WriteLine("}");

Close the StreamWriter and the Stream, thus closing the file:

wrtr.Close( ); s.Close( );

When this runs, the BruteForceSums.cs file will be written to disk. It will look like this:

// Dynamically created BruteForceSums class class BruteForceSums {     public double ComputeSum( )     {     // Brute force sum method     // For value = 200     return 0+ 1+ 2+ 3+ 4+ 5+ 6+ 7+ 8+ 9+ 10+  11+ 12+ 13+ 14+ 15+ 16+ 17+ 18+ 19+ 20+ 21+  22+ 23+ 24+ 25+ 26+ 27+ 28+ 29+ 30+ 31+ 32+  33+ 34+ 35+ 36+ 37+ 38+ 39+ 40+ 41+ 42+ 43+  44+ 45+ 46+ 47+ 48+ 49+ 50+ 51+ 52+ 53+ 54+  55+ 56+ 57+ 58+ 59+ 60+ 61+ 62+ 63+ 64+ 65+  66+ 67+ 68+ 69+ 70+ 71+ 72+ 73+ 74+ 75+ 76+  77+ 78+ 79+ 80+ 81+ 82+ 83+ 84+ 85+ 86+ 87+  88+ 89+ 90+ 91+ 92+ 93+ 94+ 95+ 96+ 97+ 98+  99+ 100+ 101+ 102+ 103+ 104+ 105+ 106+ 107+  108+ 109+ 110+ 111+ 112+ 113+ 114+ 115+ 116+  117+ 118+ 119+ 120+ 121+ 122+ 123+ 124+ 125+  126+ 127+ 128+ 129+ 130+ 131+ 132+ 133+ 134+  135+ 136+ 137+ 138+ 139+ 140+ 141+ 142+ 143+  144+ 145+ 146+ 147+ 148+ 149+ 150+ 151+ 152+  153+ 154+ 155+ 156+ 157+ 158+ 159+ 160+ 161+  162+ 163+ 164+ 165+ 166+ 167+ 168+ 169+ 170+  171+ 172+ 173+ 174+ 175+ 176+ 177+ 178+ 179+  180+ 181+ 182+ 183+ 184+ 185+ 186+ 187+ 188+  189+ 190+ 191+ 192+ 193+ 194+ 195+ 196+ 197+  198+ 199+ 200;     } }

This accomplishes the goal of dynamically creating a class with a method that finds the sum through brute force.

The only remaining task is to build the file and then use the method. To build the file, you must start a new process (processes are explained in some detail in Chapter 20). The best way to launch this process is with a ProcessStartInfo structure that will hold the command line. Instantiate a ProcessStartInfo and set its filename to cmd.exe:

ProcessStartInfo psi = new ProcessStartInfo( );  psi.FileName = "cmd.exe";

You need to pass in the string you want to invoke at the command line. The ProcessStartInfo.Arguments property specifies the command-line arguments to use when starting the program. The command-line argument to the cmd.exe program will be /c to tell cmd.exe to exit after it executes the command, and then the command for cmd.exe. The command for cmd.exe is the command-line compiler:

string compileString = "/c {0}csc /optimize+ "; compileString += " /target:library "; compileString += "{1}.cs > compile.out";

The string compileString will invoke the C# compiler (csc), telling it to optimize the code (after all, you're doing this to gain performance) and to build a dynamic link library (DLL) file (/target:library). Redirect the output of the compile to a file named compile.out so that you can examine it if there are errors.

Combine compileString with the filename, using the static method Format of the string class, and assign the combined string to psi.Arguments. The first placeholder, {0}, holds the location of the compiler (%SystemRoot%\Microsoft.NET\Framework\<version>), and the second placeholder, {1}, holds the source code filename:

string frameworkDir =   RuntimeEnvironment.GetRuntimeDirectory( ); psi.Arguments = String.Format(compileString, frameworkDir, fileName);

The effect of all this is to set the Arguments property of the ProcessStartInfo object psi to:

/c csc /optimize+ /target:library  BruteForceSums.cs > compile.out

Before invoking cmd.exe, set the WindowStyle property of psi to Minimized so that when the command executes, the window does not flicker onto and then off of the user's display:

psi.WindowStyle = ProcessWindowStyle.Minimized;

You are now ready to start the cmd.exe process wait until it finishes before proceeding with the rest of the GenerateCode method:

Process proc = Process.Start(psi); proc.WaitForExit( );

Once the process is done, you can get the assembly; from the assembly, you can get the class you've created. Finally, you can ask that class for its type and assign that to your theType member variable:

Assembly a = Assembly.LoadFrom(fileName + ".dll"); theClass = a.CreateInstance(className); theType = a.GetType(className);

You can now delete the .cs file you generated:

File.Delete(fileName + ".cs");

You've now filled theType, and you're ready to return to DoSum( ) to invoke the ComputeSum( ) method dynamically. The Type object has a method InvokeMember( ), which can be used to invoke a member of the class described by the Type object. The InvokeMember( ) method is overloaded; the version you'll use takes five arguments:

public object InvokeMember(    string name,    BindingFlags invokeAttr,    Binder binder,    object target,    object[] args );

name

The name of the method you wish to invoke.

invokeAttr

A bit mask of BindingFlags that specifies how the search of the object is conducted. In this case, you'll use the InvokeMethod flag OR'd with the Default flag. These are the standard flags for invoking a method dynamically.

binder

Used to assist in type conversions. By passing in null, you'll specify that you want the default binder.

target

The object on which you'll invoke the method. In this case, you'll pass in theClass, which is the class you just created from the assembly you just built.

args

An array of arguments to pass to the method you're invoking.

The complete invocation of InvokeMember looks like this:

object[] arguments = new object[0]; object retVal =     theType.InvokeMember("ComputeSum",     BindingFlags.Default |      BindingFlags.InvokeMethod,     null,     theClass,     arguments); return (double) retVal;

The result of invoking this method is assigned to the local variable retVal, which is then returned, as a double, to the driver program. The complete listing is shown in Example 18-9.

Example 18-9. Dynamic invocation with Type and InvokeMember( )

namespace Programming_CSharp {    using System;    using System.Diagnostics;    using System.IO;    using System.Reflection;    using System.Runtime.InteropServices; // provides RuntimeEnvironment    // used to benchmark the looping approach    public class MyMath    {       // sum numbers with a loop       public int DoSumLooping(int initialVal)       {          int result = 0;          for(int i = 1;i <=initialVal;i++)          {             result += i;          }          return result;       }    }    // responsible for creating the BruteForceSums    // class and compiling it and invoking the    // DoSums method dynamically    public class ReflectionTest    {       Type theType = null;       object theClass = null;       // the public method called by the driver       public double DoSum(int theValue)       {          // if you don't have a reference          // to the dynamically created class          // create it          if (theType == null)          {             GenerateCode(theValue);          }          // with the reference to the dynamically           // created class you can invoke the method           object[] arguments = new object[0];          object retVal =             theType.InvokeMember("ComputeSum",             BindingFlags.Default |              BindingFlags.InvokeMethod,             null,             theClass,             arguments);          return (double) retVal;       }       // generate the code and compile it       private void GenerateCode(int theVal)       {          // open the file for writing          string fileName = "BruteForceSums";          Stream s =              File.Open(fileName + ".cs", FileMode.Create);          StreamWriter wrtr = new StreamWriter(s);          wrtr.WriteLine(             "// Dynamically created BruteForceSums class");          // create the class          string className = "BruteForceSums";          wrtr.WriteLine("class {0}", className);          wrtr.WriteLine("{");          // create the method          wrtr.WriteLine("\tpublic double ComputeSum( )");          wrtr.WriteLine("\t{");          wrtr.WriteLine("\t// Brute force sum method");          wrtr.WriteLine("\t// For value = {0}", theVal);          // write the brute force additions          wrtr.Write("\treturn 0");          for (int i = 1;i<=theVal;i++)          {             wrtr.Write("+ {0}",i);          }          wrtr.WriteLine(";");    // finish method          wrtr.WriteLine("\t}");    // end method          wrtr.WriteLine("}");    // end class          // close the writer and the stream          wrtr.Close( );          s.Close( );          // Build the file          ProcessStartInfo psi =              new ProcessStartInfo( );          psi.FileName = "cmd.exe";          string compileString = "/c {0}csc /optimize+ ";          compileString += "/target:library ";          compileString += "{1}.cs > compile.out";          string frameworkDir =            RuntimeEnvironment.GetRuntimeDirectory( );          psi.Arguments =              String.Format(compileString, frameworkDir, fileName);          psi.WindowStyle = ProcessWindowStyle.Minimized;          Process proc = Process.Start(psi);          proc.WaitForExit(2000);            // Open the file, and get a           // pointer to the method info          Assembly a =              Assembly.LoadFrom(fileName + ".dll");          theClass = a.CreateInstance(className);          theType = a.GetType(className);          // File.Delete(fileName + ".cs");  // clean up       }    }    public class TestDriver    {       public static void Main( )       {          const int val = 200;  // 1..200          const int iterations = 100000;          double result = 0;          // run the benchmark          MyMath m = new MyMath( );           DateTime startTime = DateTime.Now;                      for (int i = 0;i < iterations;i++)          {             result = m.DoSumLooping(val);          }          TimeSpan elapsed =              DateTime.Now - startTime;          Console.WriteLine(             "Sum of ({0}) = {1}",val, result);          Console.WriteLine(             "Looping. Elapsed milliseconds: " +              elapsed.TotalMilliseconds +              " for {0} iterations", iterations);          // run our reflection alternative          ReflectionTest t = new ReflectionTest( );          startTime = DateTime.Now;           for (int i = 0;i < iterations;i++)          {             result = t.DoSum(val);          }          elapsed = DateTime.Now - startTime;          Console.WriteLine(             "Sum of ({0}) = {1}",val, result);          Console.WriteLine(             "Brute Force. Elapsed milliseconds: " +              elapsed.TotalMilliseconds  +              " for {0} iterations", iterations);       }    } } Output: Sum of (200) = 20100 Looping. Elapsed milliseconds:  78.125 for 100000 iterations Sum of (200) = 20100 Brute Force. Elapsed milliseconds:  3843.75 for 100000 iterations

Notice that the dynamically invoked method is far slower than the loop. This is not a surprise; writing the file to disk, compiling it, reading it from disk, and invoking the method all bring significant overhead. You accomplished your goal, but it was a Pyrrhic victory .

18.3.2 Dynamic Invocation with Interfaces

It turns out that dynamic invocation is particularly slow. You want to maintain the general approach of writing the class at runtime and compiling it on the fly. But rather than using dynamic invocation, you'd just like to call the method. One way to speed things up is to use an interface to call the ComputeSums( ) method directly.

To accomplish this, you need to change ReflectionTest.DoSum( ) from:

public double DoSum(int theValue) {     if (theType == null)     {         GenerateCode(theValue);     }     object[] arguments = new object[0];     object retVal =         theType.InvokeMember("ComputeSum",         BindingFlags.Default | BindingFlags.InvokeMethod,         null,         theFunction,         arguments);     return (double) retVal; }

to the following:

public double DoSum(int theValue) {     if (theComputer == null)     {         GenerateCode(theValue);     }     return (theComputer.ComputeSum( )); }

In this example, theComputer is an interface to an object of type BruteForceSums. It must be an interface and not an object because when you compile this program, theComputer won't yet exist; you'll create it dynamically.

Remove the declarations for theType and theClass and replace them with:

IComputer theComputer = null;

This declares theComputer to be an IComputer interface. At the top of your program, declare the interface:

public interface IComputer {     double ComputeSum( ); }

When you create the BruteForceSum class, you must make it implement IComputer:

wrtr.WriteLine( "class {0} : Programming_CSharp.IComputer ",  className); 

Save your program in a project file named Reflection, and modify compileString in GenerateCode as follows:

string compileString = "/c csc /optimize+ "; compileString += "/r:\"Reflection.exe\" "; compileString += "/target:library "; compileString += "{0}.cs > compile.out";

The compile string will need to reference the ReflectionTest program itself (Reflection.exe) so that the dynamically called compiler will know where to find the declaration of IComputer.

After you build the assembly, you will no longer assign the instance to theClass and then get the type for theType, as these variables are gone. Instead, you will assign the instance to the interface IComputer:

theComputer = (IComputer) a.CreateInstance(className);

Use the interface to invoke the method directly in DoSum( ):

return (theComputer.ComputeSum( ));

Example 18-10 is the complete source code.

Example 18-10. Dynamic invocation with interfaces

namespace Programming_CSharp {    using System;    using System.Diagnostics;    using System.IO;    using System.Reflection;    using System.Runtime.InteropServices; // provides RuntimeEnvironment    // used to benchmark the looping approach    public class MyMath    {       // sum numbers with a loop       public int DoSumLooping(int initialVal)       {          int result = 0;          for(int i = 1;i <=initialVal;i++)          {             result += i;          }          return result;       }    } public interface IComputer    {       double ComputeSum( );    }    // responsible for creating the BruteForceSums    // class and compiling it and invoking the    // DoSums method dynamically    public class ReflectionTest    {       IComputer theComputer = null;       // the public method called by the driver       public double DoSum(int theValue)       {          if (theComputer == null)                   {             GenerateCode(theValue);          }         return (theComputer.ComputeSum( ));       }       // generate the code and compile it       private void GenerateCode(int theVal)       {          // open the file for writing          string fileName = "BruteForceSums";          Stream s =              File.Open(fileName + ".cs", FileMode.Create);             StreamWriter wrtr = new StreamWriter(s);             wrtr.WriteLine(             "// Dynamically created BruteForceSums class");          // create the class          string className = "BruteForceSums";          wrtr.WriteLine(             "class {0} : Programming_CSharp.IComputer ",              className);          wrtr.WriteLine("{");          // create the method          wrtr.WriteLine("\tpublic double ComputeSum( )");          wrtr.WriteLine("\t{");          wrtr.WriteLine("\t// Brute force sum method");          wrtr.WriteLine("\t// For value = {0}", theVal);          // write the brute force additions          wrtr.Write("\treturn 0");          for (int i = 1;i<=theVal;i++)          {             wrtr.Write("+ {0}",i);          }          wrtr.WriteLine(";");    // finish method          wrtr.WriteLine("\t}");    // end method          wrtr.WriteLine("}");    // end class          // close the writer and the stream          wrtr.Close( );          s.Close( );          // Build the file          ProcessStartInfo psi =              new ProcessStartInfo( );          psi.FileName = "cmd.exe";          string compileString = "/c {0}csc /optimize+ ";          compileString += "/r:\"Reflection.exe\" ";          compileString += "/target:library ";          compileString += "{1}.cs > compile.out";          string frameworkDir =            RuntimeEnvironment.GetRuntimeDirectory( );          psi.Arguments =              String.Format(compileString, frameworkDir, fileName);          psi.WindowStyle = ProcessWindowStyle.Minimized;          Process proc = Process.Start(psi);          proc.WaitForExit( );    // wait at most 2 seconds          // Open the file, and get a           // pointer to the method info          Assembly a =              Assembly.LoadFrom(fileName + ".dll");          theComputer = (IComputer)  a.CreateInstance(className);          File.Delete(fileName + ".cs");  // clean up       }    }    public class TestDriver    {       public static void Main( )       {          const int val = 200;  // 1..200          const int iterations = 1000000;          double result = 0;          // run the benchmark          MyMath m = new MyMath( );           DateTime startTime = DateTime.Now;                      for (int i = 0;i < iterations;i++)          {             result = m.DoSumLooping(val);          }          TimeSpan elapsed =              DateTime.Now - startTime;          Console.WriteLine(             "Sum of ({0}) = {1}",val, result);          Console.WriteLine(             "Looping. Elapsed milliseconds: " +              elapsed.TotalMilliseconds +              " for {0} iterations", iterations);          // run our reflection alternative          ReflectionTest t = new ReflectionTest( );          startTime = DateTime.Now;           for (int i = 0;i < iterations;i++)          {             result = t.DoSum(val);          }          elapsed = DateTime.Now - startTime;          Console.WriteLine(             "Sum of ({0}) = {1}",val, result);          Console.WriteLine(             "Brute Force. Elapsed milliseconds: " +              elapsed.TotalMilliseconds  +              " for {0} iterations", iterations);       }    } } Output: Sum of (200) = 20100 Looping. Elapsed milliseconds:  951.368 for 1000000 iterations Sum of (200) = 20100 Brute Force. Elapsed milliseconds:  530.7632 for 1000000 iterations

This output is much more satisfying: our dynamically created brute-force method now runs twice as fast as the loop does. But you can do a lot better than that by using reflection emit.

18.3.3 Dynamic Invocation with Reflection Emit

So far you've created an assembly on the fly by writing its source code to disk and then compiling that source code. You then dynamically invoked the method you wanted to use from that assembly, which was compiled on disk. That brings a lot of overhead, and what have you accomplished? When you're done with writing the file to disk, you have source code you can compile; when you're done compiling, you have IL (Intermediate Language) op codes on disk that you can ask the .NET Framework to run.

Reflection emit allows you to skip a few steps and just "emit" the op codes directly. This is writing assembly code directly from your C# program and then invoking the result. It just doesn't get any cooler than that.

You start much as you did in the previous examples. Create a constant for the number to add to (200) and the number of iterations (1,000,000). You then recreate the myMath class as a benchmark.

Once again you have a ReflectionTest class, and once again you call DoSum( ), passing in the value:

ReflectionTest t = new ReflectionTest( ); result = t.DoSum(val);

DoSum( ) itself is virtually unchanged:

public double DoSum(int theValue) {    if (theComputer == null)    {       GenerateCode(theValue);    }    // call the method through the interface    return (theComputer.ComputeSum( )); }

As you can see, you will use an interface again, but this time you are not going to write a file to disk.

GenerateCode is quite different now. You no longer write the file to disk and compile it; instead you call the helper method EmitAssembly and get back an assembly. You then create an instance from that assembly and cast that instance to your interface.

public void GenerateCode(int theValue) {    Assembly theAssembly = EmitAssembly(theValue);    theComputer = (IComputer)        theAssembly.CreateInstance("BruteForceSums"); }

As you might have guessed, the magic is stashed away in the EmitAssembly method:

private Assembly EmitAssembly(int theValue)

The value you pass in is the sum you want to compute. To see the power of reflection emit, you'll increase that value from 200 to 2,000.

The first thing to do in EmitAssembly is to create an object of type AssemblyName and give that AssemblyName object the name "DoSumAssembly":

AssemblyName assemblyName = new AssemblyName( ); assemblyName.Name = "DoSumAssembly";

An AssemblyName is an object that fully describes an assembly's unique identity. As discussed in Chapter 13, an assembly's identity consists of a simple name (DoSumAssembly), a version number, a cryptographic key pair, and a supported culture.

With this object in hand, you can create a new AssemblyBuilder object. To do so, call DefineDynamicAssembly on the current domain, which is done by calling the static GetDomain( ) method of the Thread object. Domains are discussed in detail in Chapter 19.

The parameters to the GetDomain( ) method are the AssemblyName object you just created and an AssemblyBuilderAccess enumeration value (one of Run, RunandSave, or Save). You'll use Run in this case to indicate that the assembly can be run but not saved:

AssemblyBuilder newAssembly =      Thread.GetDomain( ).DefineDynamicAssembly(assemblyName,           AssemblyBuilderAccess.Run);

With this newly created AssemblyBuilder object, you are ready to create a ModuleBuilder object. The job of the ModuleBuilder, not surprisingly, is to build a module dynamically. Modules are discussed in Chapter 17. Call the DefineDynamicModule method, passing in the name of the method you want to create:

ModuleBuilder newModule =  newAssembly.DefineDynamicModule("Sum");

Now, given that module, you can define a public class and get back a TypeBuilder object. TypeBuilder is the root class used to control the dynamic creation of classes. With a TypeBuilder object, you can define classes and add methods and fields:

TypeBuilder myType = newModule.DefineType("BruteForceSums", TypeAttributes.Public);

You are now ready to mark the new class as implementing the IComputer interface:

myType.AddInterfaceImplementation(typeof(IComputer));

You're almost ready to create the ComputeSum method, but first you must set up the array of parameters. Because you have no parameters at all, create an array of zero length:

Type[] paramTypes = new Type[0]; 

Then create a Type object to hold the return type for your method:

Type returnType = typeof(int);

You're ready to create the method. The DefineMethod( ) method of TypeBuilder will both create the method and return an object of type MethodBuilder, which you will use to generate the IL code:

MethodBuilder simpleMethod = myType.DefineMethod("ComputeSum",                MethodAttributes.Public |                 MethodAttributes.Virtual,                returnType,                paramTypes);

Pass in the name of the method, the flags you want (public and virtual), the return type (int), and the paramTypes (the zero length array).

Then use the MethodBuilder object you created to get an ILGenerator object:

ILGenerator generator = simpleMethod.GetILGenerator( );

With your precious ILGenerator object in hand, you are ready to emit the op codes. These are the very op codes that the C# compiler would have created. (In fact, the best way to get the op codes is to write a small C# program, compile it, and then examine the op codes in ILDasm!)

First emit the value 0 to the stack. Then loop through the number values you want to add (1 through 200), adding each to the stack in turn, adding the previous sum to the new number, and leaving the result on the stack:

generator.Emit(OpCodes.Ldc_I4, 0); for (int i = 1; i <= theValue;i++) {     generator.Emit(OpCodes.Ldc_I4, i);     generator.Emit(OpCodes.Add); }

The value that remains on the stack is the sum you want, so you'll return it:

generator.Emit(OpCodes.Ret);

You're ready now to create a MethodInfo object that will describe the method:

MethodInfo computeSumInfo =     typeof(IComputer).GetMethod("ComputeSum");

Now you must specify the implementation that will implement the method. Call DefineMethodOverride on the TypeBuilder object you created earlier, passing in the MethodBuilder you created along with the MethodInfo object you just created:

myType.DefineMethodOverride(simpleMethod, computeSumInfo);

You're just about done; create the class and return the assembly:

myType.CreateType( ); return newAssembly;

Okay, I didn't say it was easy, but it is really cool, and the resulting code runs very fast. The normal loop runs 1,000,000 iterations in 11.5 seconds, but the emitted code runs in .4 seconds! A full 3,000% faster. Example 18-11 is the full source code.

Example 18-11. Dynamic invocation with reflection emit

namespace Programming_CSharp {    using System;    using System.Diagnostics;    using System.IO;    using System.Reflection;    using System.Reflection.Emit;    using System.Threading;    // used to benchmark the looping approach    public class MyMath    {       // sum numbers with a loop       public int DoSumLooping(int initialVal)       {          int result = 0;          for(int i = 1;i <=initialVal;i++)          {             result += i;          }          return result;       }    }    // declare the interface    public interface IComputer    {       int ComputeSum( );    }    public class ReflectionTest    {       // private member data       IComputer theComputer = null;       // the private method which emits the assembly       // using op codes       private Assembly EmitAssembly(int theValue)       {          // Create an assembly name          AssemblyName assemblyName =              new AssemblyName( );          assemblyName.Name = "DoSumAssembly";          // Create a new assembly with one module          AssemblyBuilder newAssembly =             Thread.GetDomain( ).DefineDynamicAssembly(             assemblyName, AssemblyBuilderAccess.Run);          ModuleBuilder newModule =             newAssembly.DefineDynamicModule("Sum");          //  Define a public class named "BruteForceSums "           //  in the assembly.          TypeBuilder myType =             newModule.DefineType(             "BruteForceSums", TypeAttributes.Public);          // Mark the class as implementing IComputer.          myType.AddInterfaceImplementation(             typeof(IComputer));          // Define a method on the type to call. Pass an          // array that defines the types of the parameters,          // the type of the return type, the name of the           // method, and the method attributes.          Type[] paramTypes = new Type[0];          Type returnType = typeof(int);          MethodBuilder simpleMethod =             myType.DefineMethod(             "ComputeSum",             MethodAttributes.Public |              MethodAttributes.Virtual,             returnType,             paramTypes);          // Get an ILGenerator. This is used          // to emit the IL that you want.          ILGenerator generator =              simpleMethod.GetILGenerator( );          // Emit the IL that you'd get if you           // compiled the code example           // and then ran ILDasm on the output.          // Push zero onto the stack. For each 'i'           // less than 'theValue',           // push 'i' onto the stack as a constant          // add the two values at the top of the stack.          // The sum is left on the stack.          generator.Emit(OpCodes.Ldc_I4, 0);          for (int i = 1; i <= theValue;i++)          {             generator.Emit(OpCodes.Ldc_I4, i);             generator.Emit(OpCodes.Add);          }          // return the value          generator.Emit(OpCodes.Ret);          //Encapsulate information about the method and          //provide access to the method's metadata          MethodInfo computeSumInfo =             typeof(IComputer).GetMethod("ComputeSum");          // specify the method implementation.          // Pass in the MethodBuilder that was returned           // by calling DefineMethod and the methodInfo           // just created          myType.DefineMethodOverride(simpleMethod, computeSumInfo);          // Create the type.          myType.CreateType( );          return newAssembly;       }       // check if the interface is null       // if so, call Setup.       public double DoSum(int theValue)       {          if (theComputer == null)          {             GenerateCode(theValue);          }          // call the method through the interface          return (theComputer.ComputeSum( ));       }       // emit the assembly, create an instance        // and get the interface       public void GenerateCode(int theValue)       {          Assembly theAssembly = EmitAssembly(theValue);          theComputer = (IComputer)              theAssembly.CreateInstance("BruteForceSums");       }    }    public class TestDriver    {       public static void Main( )       {          const int val = 2000;  // Note 2,000          // 1 million iterations!          const int iterations = 1000000;          double result = 0;          // run the benchmark          MyMath m = new MyMath( );           DateTime startTime = DateTime.Now;                      for (int i = 0;i < iterations;i++)          {             result = m.DoSumLooping(val);          }          TimeSpan elapsed =              DateTime.Now - startTime;          Console.WriteLine(             "Sum of ({0}) = {1}",val, result);          Console.WriteLine(             "Looping. Elapsed milliseconds: " +              elapsed.TotalMilliseconds +              " for {0} iterations", iterations);          // run our reflection alternative          ReflectionTest t = new ReflectionTest( );          startTime = DateTime.Now;           for (int i = 0;i < iterations;i++)          {             result = t.DoSum(val);          }          elapsed = DateTime.Now - startTime;          Console.WriteLine(             "Sum of ({0}) = {1}",val, result);          Console.WriteLine(             "Brute Force. Elapsed milliseconds: " +              elapsed.TotalMilliseconds  +              " for {0} iterations", iterations);       }    } } Output: Sum of (2000) = 2001000 Looping. Elapsed milliseconds:  11468.75 for 1000000 iterations Sum of (2000) = 2001000 Brute Force. Elapsed milliseconds:  406.25 for 1000000 iterations

Reflection emit is a powerful technique for emitting op codes. Although today's compilers are fast and today's machines have lots of memory and processing speed, it is comforting to know that when you must, you can get right down to the virtual metal.