How can I get resharper to know that my variable is not null after I have called an extension method on it?

Question

I have an extension method:

public static bool Exists(this object toCheck)
{
   return toCheck != null;
}

if I use it and then do something like this:

if (duplicate.Exists())
    throw new Exception(duplicate);

then resharper warns me that there is a possible null reference exception.

I know this is not possible, but how can I tell resharper that this is ok?

Answer 1

You can do it with contract annotations, but the way provided in another answer does not work for me (that is - still produces a warning). But this one works:

public static class Extensions {
    [ContractAnnotation("null => false; notnull => true")]
    public static bool Exists(this object toCheck) {
        return toCheck != null;
    }
}

To get ContractAnnotationAttribute - recommended way is to install JetBrains.Annotations nuget package. If you don't want to install package - go to Resharper > Options > Code Annotations and press "copy implementation to clipboard" button, then paste it anywhere in your project (ensure to not change namespace).

Answer 2

You can use "Contract Annotation Syntax" to indicate to Resharper that a method does not return normally under some circumstances, e.g. when a parameter is null.

For your example you can do something like this:

[ContractAnnotation(toCheck:notnull => true]
public static bool Exists(this object toCheck)
{
   return toCheck != null;
}

Where the toCheck:null => true tells Resharper that if toCheck is not null, the method will return true.

[EDIT] Updated link to point to the most recent Resharper documentation.