16.6 Creating new types during runtime

System.Reflection.Emit contains classes such as AssemblyBuilder , ModuleBuilder , TypeBuilder , and MethodBuilder which are used to create new types during runtime. Rarely used in conventional programming, the ability to create new Type s during runtime may be useful when you want to dynamically create new proxy classes for remote operations.

Since this is an advanced topic, I will give only a simple code example to give a clue as to which classes to use. Refer to the API documentation for more information about each class in System.Reflection.Emit .

 1: using System;  2: using System.Reflection;  3: using System.Reflection.Emit;  4:  5: public class TestClass{  6:  7:   public static Type CreateType (){  8:  9:     AppDomain       ad; 10:     AssemblyName    an; 11:     AssemblyBuilder ab; 12:     ModuleBuilder   mb; 13:     TypeBuilder     tb; 14:     MethodBuilder   methb; 15:     ILGenerator     ilg; 16: 17:     // AppDomain.CurrentDomain returns the current process 18:     // that is running 19:     ad = AppDomain.CurrentDomain; 20: 21:     // create a new assemby called 'MyAssembly' 22:     an      = new AssemblyName(); 23:     an.Name = "MyAssembly"; 24:     ab      = ad.DefineDynamicAssembly 25:             (an, AssemblyBuilderAccess.Run); 26: 27:     // create a new module called 'MyModule' 28:     mb = ab.DefineDynamicModule("MyModule"); 29: 30:     // create new Type called 'MyType' 31:     tb = mb.DefineType("MyType", TypeAttributes.Public); 32: 33:     // create new method called DoThis which takes in no 34:     // parameters and is public. 35:     methb = tb.DefineMethod 36:       ("DoThis", MethodAttributes.Public, null, null); 37: 38:     // Generate IL codes for DoThis method. 39:     // All DoThis does is to print out a string and return. 40:     ilg = methb.GetILGenerator(); 41:     ilg.EmitWriteLine("i love computers!"); 42:     ilg.Emit(OpCodes.Ret); 43: 44:     // complete creating the Type. Call tb.CreateType only 45:     // after all the members within have been defined. 46:     return tb.CreateType(); 47:   } 48: 49:   public static void Main(){ 50:     // CreateType returns a dynamically generated Type 51:     Type t = CreateType(); 52: 53:     // create new instance of the dynamically generated type 54:     object o = Activator.CreateInstance(t); 55: 56:     // retrieve and invoke the DoThis method of the object 57:     MethodInfo m = t.GetMethod("DoThis"); 58:     m.Invoke(o, null); 59:   } 60: } 

Output:

 c:\expt>test i love computers! 

The action happens in the CreateType method. When CreateType runs, it dynamically creates an assembly ( MyAssembly ) (lines 22 “ 25) which contains a module ( MyModule ) (line 28), which in turn contains a new Type ( MyType ) (line 31). MyType contains a single method ( DoThis ) which returns void and takes in no parameters (lines 35 “ 42). All DoThis does is print out a string and return (coded in lines 41 “ 42).

Reflection is also used extensively together with custom attributes to retrieve attribute values during runtime from an entity. Custom attributes are covered in Chapter 28.