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is (C# Reference)

Checks if an object is compatible with a given type. For example, the following code can determine if an object is an instance of the MyObject type, or a type that derives from MyObject:

if (obj is MyObject)
{
}

An is expression evaluates to true if the provided expression is non-null, and the provided object can be cast to the provided type without causing an exception to be thrown. Otherwise, the expression evaluates to false.

The is keyword causes a compile-time warning if the expression is known to always be true or to always be false, but typically evaluates type compatibility at run time.

The is operator cannot be overloaded.

Note that the is operator only considers reference conversions, boxing conversions, and unboxing conversions. Other conversions, such as user-defined conversions, are not considered.

Anonymous methods are not allowed on the left side of the is operator. This exception includes lambda expressions.

class Class1 {}
class Class2 {}
class Class3 : Class2 { }

class IsTest
{
    static void Test(object o)
    {
        Class1 a;
        Class2 b;

        if (o is Class1)
        {
            Console.WriteLine("o is Class1");
            a = (Class1)o;
            // Do something with "a."
        }
        else if (o is Class2)
        {
            Console.WriteLine("o is Class2");
            b = (Class2)o;
            // Do something with "b."
        }

        else
        {
            Console.WriteLine("o is neither Class1 nor Class2.");
        }
    }
    static void Main()
    {
        Class1 c1 = new Class1();
        Class2 c2 = new Class2();
        Class3 c3 = new Class3();
        Test(c1);
        Test(c2);
        Test(c3);
        Test("a string");
    }
}
/*
Output:
o is Class1
o is Class2
o is Class2
o is neither Class1 nor Class2.
*/

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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