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:
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.