How to detect null comparisons with non-nullable variable types

Question

Is there any way to detect this code snippet?

DateTime dt = DateTime.Now;
if (dt == null) {
    MessageBox.Show("Imposible with datetime!!!");
}

In our project we have detected that in Release compilation, the whole if gets removed because a comparison between a DateTime and null will be always false. I assume the optimizer translates it as:

if (false) { 
    ...
}

Is there any way to make the analyzer detect this code in any way? An error in compile time would be fine too, even if we have to create an FXCop rule, but we don't know how to track this case.

DateTime dt = null; // This code raises a compile time error, but the comparison with null is fine?

If you are open to solving this through a different IDE, IntelliJ should have this check available (among many other static checks). — Sergey Kalinichenko, Apr 13 '16 at 19:23
See Eric Lipert's response to a similar question: http://stackoverflow.com/a/1972317/871910 — zmbq, Apr 13 '16 at 19:25
@juharr I see. Swapping `DateTime` with `int` or any primitive throws a warning. I thought all structs would do but no. I guess when you've overloaded `==` compiler will shut its mouth. — Sriram Sakthivel, Apr 13 '16 at 19:38
In 4.5 exists Compiler Warning (level 2) CS0472, but don't work with DateTime... only Byte/Int... https://msdn.microsoft.com/en-us/library/ms228621.aspx — Victor Sanchez, May 04 '16 at 07:16

score 0 · Answer 1 · answered Apr 13 '16 at 20:56

Similar to the IDE vein, ReSharper has support for catching this type of issue. It is a plugin to Visual Studio and we have found it very helpful over the years. It is still ahead of Microsoft in terms for refactoring features.

https://www.jetbrains.com/resharper/

How to detect null comparisons with non-nullable variable types

1 Answers1