Do "x as X != null" and "x is X" always return the same result?

Question

Is there any situation where these two if statements will produce different results?

if(x as X != null)
{
  // Do something
}

if(x is X)
{
  // Do something
}

Edit: To clarify: I know what is the difference between the operators (in general) and what they mean. The question is if there is any situation where these two would produce different results.

Answer 1

Is there any situation where these two if statements will produce different results?

First, note that there are more restrictions on the use of as than of is, so x is X may compile in cases where (x as X) != null may not. For example, as requires that the specified type be a reference type or a nullable type; is works with non-nullable value types as well.

Assume now that both x is X and (x as X) != null are valid expressions. According to §7.10.11 of the C# 4.0 specification, "the operation E as T produces the same result as E is T ? (T)(E) : (T)null". If we plug that latter expression into (x as X) != null, we get

    (x as X) != null
==  ((x is X) ? (X)x : null) != null  // by the definition of "as"
==  (x is X) ? ((X)x != null) : (null != null)  // distribute
==  (x is X) ? true : false  // because (x is X) implies (x != null)
==  x is X

This proves that x is X and (x as X) != null are equivalent if both are valid expressions.

Answer 2

There functionally is no difference between the 2 statements. In general the as version is used to avoid a double type test because you can do the following

var local = x as X;
if (local != null) { 
  // Sweet I have local! 
}

vs.

if (x is X) { 
  // This runs the type check yet again 
  var local = (X)x; 
}

If you aren't actually using the value after the type test then just use the is version

Answer 3

If x is a non-nullable type (or a generic type not constrained to be a nullable type) then the first option won't compile; the second is the only option.

Beyond that, they're the same.

Answer 4

In this case:

if(x as X != null)
{
  // Do something
}

You are first attempting to cast x to type X before comparing to null (i.e. whether the cast succeeded or not).

In this other case:

if(x is X)
{
  // Do something
}

You are simply asking whether x is of type X and no casting is done.