What's the point of “As” keyword in C#

问题:

From the docs:

The as operator is like a cast except that it yields null on conversion failure instead of raising an exception. More formally, an expression of the form:

   expression as type

is equivalent to:

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

except that expression is evaluated only once.

So why wouldn't you choose to either do it one way or the other. Why have two systems of casting?

回答1:

They aren't two system of casting. The two have similar actions but very different meanings. An "as" means "I think this object might actually be of this other type; give me null if it isn't." A cast means one of two things:

• I know for sure that this object actually is of this other type. Make it so, and if I'm wrong, crash the program.

• I know for sure that this object is not of this other type, but that there is a well-known way of converting the value of the current type to the desired type. (For example, casting int to short.) Make it so, and if the conversion doesn't actually work, crash the program.

See my article on the subject for more details.

http://blogs.msdn.com/b/ericlippert/archive/2009/10/08/what-s-the-difference-between-as-and-cast-operators.aspx

回答2:

Efficiency and Performance

Part of performing a cast is some integrated type-checking; so prefixing the actual cast with an explicit type-check is redundant (the type-check occurs twice). Using the as keyword ensures only one type-check will be performed. You might think "but it has to do a null check instead of a second type-check", but null-checking is very efficient and performant compared to type-checking.

if (x is SomeType )
{
  SomeType y = (SomeType )x;
  // Do something
}

makes 2x checks, whereas

SomeType y = x as SomeType;
if (y != null)
{
  // Do something
}

makes 1x -- the null check is very cheap compared to a type-check.

回答3:

Because sometimes you want things to fail if you can't cast like you expect, and other times you don't care and just want to discard a given object if it can't cast.

It's basically a faster version of a regular cast wrapped in a try block; but As is far more readable and also saves typing.

回答4:

It allows fast checks without try/cast overhead, which may be needed in some cases to handle message based inheritance trees.

I use it quite a lot (get in a message, react to specific subtypes). Try/cast wouuld be significantly slower (many try/catch frames on every message going through) - and we talk of handling 200.000 messages per second here.

回答5:

I generally choose one or the other based on the semantics of the code.

For example, if you have an object that you know that it must be an string then use (string) because this expresses that the person writing the code is sure that the object is a string and if it's not than we already have bigger problems than the runtime cast exception that will be thrown.

Use as if you are not sure that the object is of a specific type but want to have logic for when it is. You could use the is operator followed by a cast, but the as operator is more efficient.

回答6:

Let me give you real world scenarios of where you would use both.

public class Foo
{
  private int m_Member;

  public override bool Equals(object obj)
  {
    // We use 'as' because we are not certain of the type.
    var that = obj as Foo;
    if (that != null)
    {
      return this.m_Member == that.m_Member;
    }
    return false;   
  }
}

And...

public class Program
{
  public static void Main()
  {
    var form = new Form();
    form.Load += Form_Load;
    Application.Run(form);
  }

  private static void Form_Load(object sender, EventArgs args)
  {
    // We use an explicit cast here because we are certain of the type
    // and we want an exception to be thrown if someone attempts to use
    // this method in context where sender is not a Form.
    var form = (Form)sender;

  }
}

回答7:

Maybe examples will help:

// Regular casting
Class1 x = new Class1();
Class2 y = (Class2)x; // Throws exception if x doesn't implement or derive from Class2

// Casting with as
Class2 y = x as Class2; // Sets y to null if it can't be casted.  Does not work with int to short, for example.

if (y != null)
{
  // We can use y
}

// Casting but checking before.
// Only works when boxing/unboxing or casting to base classes/interfaces
if (x is Class2)
{
  y = (Class2)x; // Won't fail since we already checked it
  // Use y
}

// Casting with try/catch
// Works with int to short, for example.  Same as "as"
try
{
  y = (Class2)x;
  // Use y
}
catch (InvalidCastException ex)
{
  // Failed cast
}