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as (C# Reference)

 

You can use the as operator to perform certain types of conversions between compatible reference types or nullable types. The following code shows an example.


class csrefKeywordsOperators
{
    class Base
    {
        public override string  ToString()
        {
           return "Base";
        }
    }
    class Derived : Base 
    { }

    class Program
    {
        static void Main()
        {

            Derived d = new Derived();

            Base b = d as Base;
            if (b != null)
            {
                Console.WriteLine(b.ToString());
            }

        }
    }
}

The as operator is like a cast operation. However, if the conversion isn't possible, as returns null instead of raising an exception. Consider the following example:

expression as type

The code is equivalent to the following expression except that the expression variable is evaluated only one time.

expression is type ? (type)expression : (type)null

Note that the as operator performs only reference conversions, nullable conversions, and boxing conversions. The as operator can't perform other conversions, such as user-defined conversions, which should instead be performed by using cast expressions.

Example

class ClassA { }
class ClassB { }

class MainClass
{
    static void Main()
    {
        object[] objArray = new object[6];
        objArray[0] = new ClassA();
        objArray[1] = new ClassB();
        objArray[2] = "hello";
        objArray[3] = 123;
        objArray[4] = 123.4;
        objArray[5] = null;

        for (int i = 0; i < objArray.Length; ++i)
        {
            string s = objArray[i] as string;
            Console.Write("{0}:", i);
            if (s != null)
            {
                Console.WriteLine("'" + s + "'");
            }
            else
            {
                Console.WriteLine("not a string");
            }
        }
    }
}
/*
Output:
0:not a string
1:not a string
2:'hello'
3:not a string
4:not a string
5:not a string
*/

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

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