ReSharper: how to remove “Possible 'System.NullReferenceException'” warning

Question

Here is a piece of code:

IUser user = managerUser.GetUserById(UserId);
if ( user==null ) 
    throw new Exception(...);

Quote quote = new Quote(user.FullNam         


        

Answer 1

A new answer in old post...

Here a little sample of my code regarding how to use CodeContract via ContractAnnotation with Resharper:

    [ContractAnnotation("value:null=>true")]
    public static bool IsNullOrEmpty(this string value)
    {
        return string.IsNullOrEmpty(value);
    }

It is very simple...if u find the breadcrumb in the wood. You can check other cases too.

Have a nice day

Answer 2

This is caused by the Resharper engine. These "possible NullReferenceException" happen because someone (probably at Resharper) has declared/configured somewhere an annotation on the method.

Here is how it works: ReSharper NullReferenceException Analysis and Its Contracts

Unfortunately, sometimes, these useful annotation are just wrong.

When you detect an error, you should report it to JetBrains and they will update the annotations on the next release. They're used to this.

Meanwhile, you can try to fix it by yourself. Read the article for more :)

Answer 3

Resharper only looks at the current method for its analysis, and does not recursively analyse other methods you call.

You can however direct Resharper a bit and give it meta-information about certain methods. It knows for example about "Assert.IsNotNull(a)", and will take that information into account for the analysis. It is possible to make an external annotations file for Resharper and give it extra information about a certain library to make its analysis better. Maybe this might offer a way to solve your problem.

More information can be found here.

An example showing how it's used for the library Microsoft.Contracts can be found here.

Answer 4

Q1: Because Resharper doesn't do path analysing. It just sees a possible null reference and flags that.

Q2: You can't without doing either of what you provided already.

Answer 5

You do know (or expect) that this code will throw an exception if there is a null reference:

ComponentException<MyUserManagerException>.FailIfTrue([...]);

However, since there is no contract specifying this, ReSharper has to assume that this is just a normal method call which may return without throwing any exception in any case.

Make this method implement the ReSharper contract, or as a simple workaround (which only affects debug mode, therefore no performance penalty for release mode), just after the FailIfTrue call:

Debug.Assert(user != null);

That will get rid of the warning, and as an added bonus do a runtime check in debug mode to ensure that the condition assumed by you after calling FailIfTrue is indeed met.

Answer 6

Please check if you have any user==null if check above the given code. If there is, then ReSharper thinks that the variable "can be null" so recommends you to use a check/assert before referencing it. In some cases, that's the only way ReSharper can guess whether a variable can or cannot be null.