Initialize variable with null casted to variable's datatype

Question

I ran across some code like this:

List<string> list = (List<string>)null;

Is there some reason the programmer didn't just initialize by:

List<string> list = null;

Is there a difference between the two?

Is this a habit that migrated from another programming language? Maybe C, C++, or Java?

possible duplicate of [Cast a null into something?](http://stackoverflow.com/questions/10633946/cast-a-null-into-something) — celerno, Oct 27 '14 at 18:12
As the answers state, there is no difference in your lines of code. I believe it could be the effect of a refactoring tool as well, an example could be that originally `var` was used instead of explicit typing, and the refactoring tool just replace `var`. — flindeberg, Oct 27 '14 at 19:16

score 4 · Accepted Answer · answered Oct 27 '14 at 18:12

4

Is there a difference between the two?

No there is no difference.

In ILSpy, This line List<string> list = (List<string>)null; changes into List<string> list = null;

Is this a habit that migrated from another programming language?

Can't say. May be, earlier there was something different than null and then it was changed to null.

List<string> list = (List<string>) Session["List"];

answered Oct 27 '14 at 18:12

Habib

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I was wondering if maybe C/C++ or Java required the casting of null. But I suppose it's possible that he replaced some code with the null. – Walter Stabosz Oct 27 '14 at 18:28

score 2 · Answer 2 · 2014-10-27T19:13:23.367

In this instance, there is no practical difference, and both assignments will compile down to exactly the same MSIL opcodes. However, there is one case where casting a null does make a difference, and that's when calling an overloaded method.

class A { }
class B { }

class C
{
    public static void Foo( A value );
    public static void Foo( B value );
}

Simply calling C.Foo( null ); is ambiguous, and the compiler can't reason about which you intend to invoke, but if you cast the null first: C.Foo( (A)null );, it's now clear that you mean to call the first overload, but pass it a null instead of an instance of A.

score 1 · Answer 3 · answered Oct 27 '14 at 18:14

1

There's no difference between those two lines of code. It's matter of taste I think. Although if you use casting, you can remove the type from your variable, like this:

var list = (List<string>)null;

Without casting you can't do it.

answered Oct 27 '14 at 18:14

evilone

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I don't get it. Why would you use the `var` keyword, and then cast to the type you want to define in the first place? – Walter Stabosz Oct 27 '14 at 18:40
Well it's one possibility :) Matter of taste too I guess. I would prefer List list = null; myself. – evilone Oct 27 '14 at 18:42

score 0 · Answer 4 · answered Oct 27 '14 at 18:14

0

No, in the above case, you don't need the cast. You only need it in an ternary expression like this:

return somecondition ? new List<string>() : (List<string>)null;

answered Oct 27 '14 at 18:14

Thomas Weller

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I think the cast is redundant, you can do it like this: return somecondition ? new List() : null; – evilone Oct 27 '14 at 18:16
I can't test it now, but it seems that even in your sample cast is useless – Dennis Oct 27 '14 at 18:18
1

Commenters are correct, you don't need to cast, you can return `null` from a method that returns List. – Walter Stabosz Oct 27 '14 at 18:21

Initialize variable with null casted to variable's datatype

4 Answers4