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.

question aside, use x is X, first one is just stupid and create extra casting — kirie, Dec 19 '13 at 16:44
@TimSchmelter `is` returns false if the left side is `null`. They're the same in that regard. — CodesInChaos, Dec 19 '13 at 16:45
Take a look at [this answer on another question](http://stackoverflow.com/a/496167/1324810). It's pretty extensive. — Joe, Dec 19 '13 at 16:47
@kirie: I beg to differ - the first one is just what is done in the `var typedX = x as X; if (typedX != null) { ...` pattern, and that is at least considered to be more performant by FxCop than a check with `is` and a subsequent extra cast. — O. R. Mapper, Dec 19 '13 at 16:47
possible duplicate of [Casting vs using the 'as' keyword in the CLR](http://stackoverflow.com/questions/496096/casting-vs-using-the-as-keyword-in-the-clr) — Joe, Dec 19 '13 at 16:47
@CodesInChaos: i've already recognized it. [Here's](http://stackoverflow.com/questions/7640043/why-does-the-is-operator-return-false-when-given-null) the related question. Thanks for pointing that out. Sorry for deleting my comment, i noticed later your comment. — Tim Schmelter, Dec 19 '13 at 16:48
@O.R. Mapper Isn't it better to use if(x != null && x is X), this one more readable to me — kirie, Dec 19 '13 at 16:51
@kirie: If you use `is` first and then, in the `if` block, perform a typecast, FxCop will complain about a duplicate cast that should be improved into a single `as` with a subsequent `null` comparison for performance reasons. It wouldn't create any noticeable difference in most scenarios, though. In any case, the `null` check in your statement is obsolete nonetheless, as it is basically included in the `is` check. cf. [CA1800](http://msdn.microsoft.com/en-us/library/ms182271.aspx). — O. R. Mapper, Dec 19 '13 at 16:53

score 6 · Answer 1 · answered Dec 19 '13 at 16:40

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

score 2 · Answer 2 · answered Dec 19 '13 at 16:44

2

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.

answered Dec 19 '13 at 16:44

Servy

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`null is ...` returns false. Even in the degenerate case `object o = null; Assert.IsFalse(o is object)` – CodesInChaos Dec 19 '13 at 16:45

Michael Liu · Accepted Answer · 2013-12-19T17:50:04.860

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.

Thanks for quoting the C# specification. Seems plausible! – aKzenT Dec 20 '13 at 11:09 — aKzenT, Dec 20 '13 at 11:09

score 0 · Answer 4 · answered Dec 19 '13 at 16:41

0

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.

answered Dec 19 '13 at 16:41

rdodev

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Which is equivalent in the cases where it compiles – CodesInChaos Dec 19 '13 at 16:43
Does not answer the question whether the if statements are functionally the same... – aKzenT Dec 19 '13 at 16:50
1

@aKzenT One does the casting and the other does not, how does not answer the question? – rdodev Dec 19 '13 at 16:51
1

Question is: Is there any situation where these two if statements will produce different results? – aKzenT Dec 19 '13 at 16:52
@aKzenT but the question title asks something else. Even the first line of your question: *Is there a difference between the following if statements?* Please edit that to make it clear if you're that specific. – nawfal Dec 19 '13 at 16:54

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

4 Answers4