How does the C# visual studio compiler treat struct/NULL comparisons?

Question

We just ran across some bad code like this in our c#.net 4 codebase

DateTime myDate = someValue;
If (myDate==Null)
    Do Something

It occurred to us that this condition will never occur.

How does the compiler handle these non-nullable struct comparisons?

Originally we were surprised that it would compile... but rationalized it on the point that you could certainly have a constant comparison like:

If(1==2)

Which would also never resolve true... but in that case the compiler can easily tell they are constants. Does it optimize or rollup non-nullable comparisons?

Which language is this? What do you mean by ".NET compiler"? — John Saunders, Feb 22 '12 at 23:59
This is a: language dependent, and b: compiler-dependent (heuristic code removal is usuallt an optimisation - in fact it changed between different MS c# versions, and depends on what operators are defined!) — Marc Gravell, Feb 23 '12 at 00:03
@JohnSaunders C# in Vis Studio. Sorry for the ambiguity. Edited. — Matthew, Feb 23 '12 at 00:04
Hmm, something a-miss here. I don't see what implicit conversion could prevent the compiler from generating an error. — Hans Passant, Feb 23 '12 at 00:24
@HansPassant: `(Nullable)myDate ==(Nullable)null` works, but only if `myDate` has implemented the `==` operator. — StriplingWarrior, Feb 23 '12 at 14:57

score 7 · Accepted Answer · edited Jun 20 '20 at 09:12

I punched this into LinqPad:

var t = new DateTime();
t.Dump();
(t == null).Dump();

And got this:

IL_0000:  ldloca.s    00 
IL_0002:  initobj     System.DateTime
IL_0008:  ldloc.0     
IL_0009:  call        LINQPad.Extensions.Dump
IL_000E:  pop         
IL_000F:  ldc.i4.0    
IL_0010:  call        LINQPad.Extensions.Dump

So yes, the compiler compiles it to the same as:

var t = new DateTime();
t.Dump();
(false).Dump();

Interestingly, if I create my own struct (TestStruct) and try this:

TestStruct t;
(t == null).Dump();

... the compiler complains that I can't do an equals comparison between TestSruct and null.

Update

In a comment, Paolo points to another StackOverflow post reporting this last phenomenon. Apparently by overloading the == and != operators, a value type becomes subject to an automatic conversion from t == null to (Nullable<TestClass>)t == (Nullable<TestClass>)null. If you haven't overloaded those operators, this implicit conversion doesn't make sense, so you get an error.

I think you mean that the compiler complains that you can't do an equals comparison between TestStruct and null. — John Pick, Feb 23 '12 at 00:11
Check out this answer if you want to know why your second case is not compiling http://stackoverflow.com/a/2022669/63011 — Paolo Moretti, Feb 23 '12 at 00:20

How does the C# visual studio compiler treat struct/NULL comparisons?

1 Answers1

Update