Casting: (NewType) vs. Object as NewType

 

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Casting vs using the 'as' keyword in the CLR

What is actually the difference between these two casts? 

SomeClass sc = (SomeClass)SomeObject;
SomeClass sc2 = SomeObject as SomeClass;

Normally, they should both be explicit casts to the specified type?

The former will throw an exception if the source type can't be cast to the target type. The latter will result in sc2 being a null reference, but no exception.

[Edit]

My original answer is certainly the most pronounced difference, but as Eric Lippert points out, it's not the only one. Other differences include:

  • You can't use the 'as' operator to cast to a type that doesn't accept 'null' as a value
  • You can't use 'as' to convert things, like numbers to a different representation (float to int, for example).

    • And finally, using 'as' vs. the cast operator, you're also saying "I'm not sure if this will succeed."

    Also note that you can only use the as keyword with a reference type or a nullable type

    ie: 

    double d = 5.34;
int i = d as int;

    will not compile 

    double d = 5.34;
int i = (int)d;

    will compile.

    Typecasting using "as" is of course much faster when the cast fails, as it avoids the expense of throwing an exception.

    But it is not faster when the cast succeeds. The graph at http://www.codeproject.com/KB/cs/csharpcasts.aspx is misleading because it doesn't explain what it's measuring.

    The bottom line is:

  • If you expect the cast to succeed (ie a failure would be exceptional), use a cast.

  • If you don't know if it will succeed, use the "as" operator and test the result for null.

    • 链接地址: http://www.djcxy.com/p/12806.html

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