AssignNullToNotNullAttribute inspection and TryGetValue pattern

Permanently deleted user

Created September 14, 2014 13:41

When I use Dictionary.TryGetValue() to check, whether there is particular item in dictionary and handle false result with assigning new value to out variable - AssignNullToNotNullAttribute inspection (like the other inspections of this kind) treats result variable sa NotNull, and it is correct behaviour.
Here is a sample:

As we see, "value" usage fires inspection in TestMethod2 and does not in TestMethod1 - this means that ReSharper "knows" convention of TryGetValue method. But when I use such a pattern in my one code (creating TryGetSomething methods in my one classes) it results with false positive higlightings in my code. Is there a way to mark my own TryGetSomething methods by some kind of attribute or extend ReSharper knowledge with pluging to fix such false positive higlightings?

4 comments

Richard Deeming

Created September 16, 2014 11:57

It's nothing to do with the TryGetValue method.

In TestMethod1, you're assigning the value variable within the body of the "if" statement. In TestMethod2, you are not; if the key is not found in the dictionary, value will be null.

Permanently deleted user

Created September 16, 2014 12:21

Thank you for answer, but my question was quite different - there is correct behaviour of inspection in that picture. But it works so only for Distionary.TryGetValue() method and my question is "How to achive such a behaviour for my SomeMyClass.TryGetValue() method?". I am sorry if my picture distorts my question.

Richard Deeming

Created September 16, 2014 19:34

Actually, I think the Dictionary.TryGetValue example is a bug. It's quite possible for the key to exist within the dictionary, but for the associated value to be null, in which case a null value will be passed to your [NotNull] parameter.

var dictionary = new Dictionary<string, string> { { "Test", null } };

string value;
if (!dictionary.TryGetValue("Test", out value)) value = "42";

Console.WriteLine(value == null ? "-NULL-" : value);
// Output: -NULL-

If you want to reproduce the (incorrect) behaviour with your own method, I suspect you'll need to use the ContractAnnotationAttribute.

Permanently deleted user

Created September 16, 2014 20:22

Thanks a lot! It's just the answer I need. Regarding to the case of storing nulls in dictionary - it's possible, but it's not so in my code, thus it's completely works for me.

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