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C# Cookbook by Jay Hilyard, Stephen Teilhet

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3.15. Casting with the as Operator

Problem

Ordinarily, when you attempt a casting operation, the .NET Common Language Runtime generates an InvalidCastException if the cast fails. Often, though, you cannot guarantee in advance that a cast will succeed, but you also do not want the overhead of handling an InvalidCastException.

Solution

Use the as operator. The as operator attempts the casting operation, but if the cast fails, the expression returns a null instead of throwing an exception. If the cast succeeds, the expression returns the converted value. The following code shows how the as operator is used:

public static void ConvertObj(Base baseObj)
{
    Specific specificObj = baseObj as Specific;
    if (specificObj == null)
    {
        // Cast failed
    }
    else
    {
        // Cast was successful
    }
}

where the Specific type derives from the Base type:

public class Base {}
public class Specific : Base {}

In this code fragment, the as operator is used to attempt to cast the specificObj to the type Base. The next lines contain an if-else statement that tests the variable baseObj to determine whether it is equal to null. If it is equal to null, you should prevent any use of this variable, since it might cause a NullReferenceException to be thrown.

Discussion

The as operator has the following syntax:

               expression 
               as 
               type

The expression and type are defined as follows:

expression

A reference type.

type

The type to which to cast the object defined by expression.

This operation returns expression cast to the type defined by ...

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