10.6 The is operator (Java s instanceof operator)

10.6 The is operator (Java's instanceof operator)

C#'s is operator is used to determine if the runtime type of an object is equivalent to another, much like how Java's instanceof is used.

It goes like this:

 <expression> is <type> 

The is operator returns a boolean result ( true or false ).

The is operator returns true if:

• <expression> is not null AND

• <expression> is an object which is either a subclass of <type> or is a class that implemented <type> (in this case, <type> is an interface) “ in other words, if <expression> can be successfully cast into <type> .

Let's study an example. The output is shown as comments. For convenience, the inheritance relations between the classes in this example are shown in Figure 10.2.

 1: using System;  2:  3: interface ITest1 {}  4: interface ITest2 {}  5: class GrandParent:ITest1{}  6: class Parent:GrandParent {}  7:  8: class Child: Parent,ITest2 {  9:   public static void Main(){ 10: 11:     Child c = new Child(); 12:     Console.WriteLine(c is Child);        // True 13:     Console.WriteLine(c is ITest1);       // True 14:     Console.WriteLine(c is ITest2);       // True 15:     Console.WriteLine(c is Parent);       // True 16:     Console.WriteLine(c is GrandParent);  // True 17: 18:     Parent p = new Parent(); 19:     Console.WriteLine(p is Child);        // False 20:     Console.WriteLine(p is ITest1);       // True 21:     Console.WriteLine(p is ITest2);       // False 22:     Console.WriteLine(p is Parent);       // True 23:     Console.WriteLine(p is GrandParent);  // True 24: 25:     ITest2 it2 = new Child(); 26:     Console.WriteLine(it2 is Child);       // True 27:     Console.WriteLine(it2 is ITest1);      // True 28:     Console.WriteLine(it2 is ITest2);      // True 29:     Console.WriteLine(it2 is Parent);      // True 30:     Console.WriteLine(it2 is GrandParent); // True 31:   } 32: } 

Figure 10.2. Class diagram showing inheritance relations between the classes in this example.

Compilation warning:

[View full width]

 
 [View full width] 
 test.cs(12,23): warning CS0183: The given expression is always of the provided ('Child') type test.cs(13,23): warning CS0183: The given expression is always of the provided ('ITest1') type test.cs(14,23): warning CS0183: The given expression is always of the provided ('ITest2') type test.cs(15,23): warning CS0183: The given expression is always of the provided ('Parent') type test.cs(16,23): warning CS0183: The given expression is always of the provided  ('GrandParent') type test.cs(20,23): warning CS0183: The given expression is always of the provided ('ITest1') type test.cs(22,23): warning CS0183: The given expression is always of the provided ('Parent') type test.cs(23,23): warning CS0183: The given expression is always of the provided  ('GrandParent') type test.cs(28,23): warning CS0183: The given expression is always of the provided ('ITest2') type 

The compilation warnings warn that some of the statements containing the is operator can already be determined during compile time.

Output:

 c:\expt>test True True True True True False True False True True True True True True True 

If you want to perform a cast of <expression> to <type> if (<expression> is <type>) , C# has a convenient as operator which you can use.