Recipe 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 type if the cast succeeds. If the cast fails, a null is returned, and an InvalidCastException is not thrown. Because of this, you should always check the result for null .

This operator does not work with user-defined conversions (both explicit and implicit). A user -defined conversion method extends one type to allow it to be converted to another type. This is done by adding a method, such as the following, to a class or structure:

 public struct MyPoint {     public static explicit operator MyPoint(System.Drawing.Point pt)      {         // Convert a Point structure to a MyPoint structure type         return (new MyPoint( ));     } } 

This method allows a System.Drawing.Point structure to be cast to an object of type MyPoint . Due to the use of the explicit keyword, the cast must be explicitly defined:

 System.Drawing.Point systemPt = new System.Drawing.Point(0, 0); MyPoint pt = (MyPoint)systemPt; 

If you attempt to use the as operator in a user-defined conversion, the following compiler error is shown:

 Cannot convert type 'Chapter_Code.Vector32' to 'string' via a built-in conversion 

This type of conversion does not work with unboxing conversions, either. An unboxing conversion converts a previously boxed value type to its original value type, such as with the following code:

 int x = 5; object obj = x;         // Box x int originalX = obj as int;   // Attempt to unbox obj into an integer 

If you attempt to use the as operator in an unboxing conversion, the following compiler error is shown:

 The as operator must be used with a reference type ('int' is a value type) 

because as indicates that the cast cannot be performed by returning null , but there is no such thing as a null value for an int .

See Also

See Recipe 3.14 and Recipe 3.16; see the "( ) Operator," "as Operator," and "is Operator" topics in the MSDN documentation.