c# "==" operator : compiler behaviour with different structs

Question

Code to illustrate :

 public struct MyStruct { public int SomeNumber; } public string DoSomethingWithMyStruct(MyStruct s) { if (s == null) return "this can't happen"; else return "ok"; } private string DoSomethingWithDateTime(DateTime s) { if (s == null) return "this can't happen"; // XX else return "ok"; }

Now, "DoSomethingWithStruct" fails to compile with : "Operator '==' cannot be applied to operands of type 'MyStruct' and '<null>'". This makes sense, since it doesn't make sense to try a reference comparison with a struct, which is a value type.

OTOH, "DoSomethingWithDateTime" compiles, but with compiler warning : "Unreachable code detected" at line marked "XX". Now, I'm assuming that there is no compiler error here, because the DateTime struct overloads the "==" operator. But how does the compiler know that the code is unreachable ? e.g. Does it look inside the code which overloads the "==" operator ? (This is using Visual Studio 2005 in case that makes a difference).

Note : I'm more curious than anything about the above. I don't usually try to use "==" to compare structs and nulls.

EDIT : I'll try to simplify my question - why does "DoSomethingWithDateTime" compile, when "DoSomethingWithMyStruct" does not. Both arguments are structs.

Marc Gravell · Accepted Answer · 2010-04-19 06:05:46Z

It knows that a struct is never null (Nullable<T> aside); that is enough to emit the warning.

There is a known compiler issue in this area, that arose between the C# 2.0 compiler and the C# 3.0 compiler (and remains in the C# 4.0 compiler at the moment) [I'm not sure why you are seeing it on VS2005, though]. The equality test does not raise an unreachable-code warning for custom structs with == / != operators. DateTime has these operators; your struct doesn't - hence the difference.

This issue is logged on connect, and has been looked at fairly recently by the compiler team (who are keen to fix it when opportunity arises).

Not sure I believe that's the reason. System.Int32 doesn't have an explicit == operator, and yet you can legally compare it to null...
@Aaronaught I did say "custom structs"... I'm excluding primitives here, since they have separate rules baked into the language itself.

leppie · Accepted Answer · 2010-04-19 05:48:01Z

As Hun1Ahpu said, it can never be null.

However, you could provide your own == operator, that could take an object as a parameter type that would allow the above code to compile.

Obviously, you would need it to do something logical.

Hun1Ahpu · Accepted Answer · 2010-04-19 05:51:48Z

Because DateTime is a structure it cannot be null. And there is no way to override == operator for structure in such way that the second parameter will be null.

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