How Does Resharper Know “Expression is always true”?

Question

Check out the following code:

private void Foo(object bar)
{
   Type type = bar.GetType();

    if (type != null) // Expression is always true
    {   
    }         


        

Answer 1

JetBrains perfectly explains how ReSharper does this in their features list.

Summary from link (this particular question is about NotNullAttribute):

We have analyzed a great share of .NET Framework Class Library, as well as NUnit Framework, and annotated it through external XML files, using a set of custom attributes from the JetBrains.Annotations namespace, specifically:

StringFormatMethodAttribute (for methods that take format strings as parameters)
InvokerParameterNameAttribute (for methods with string literal arguments that should match one of caller parameters)
AssertionMethodAttribute (for assertion methods)
AssertionConditionAttribute (for condition parameters of assertion methods)
TerminatesProgramAttribute (for methods that terminate control flow)
CanBeNullAttribute (for values that can be null)
NotNullAttribute (for values that can not be null)
UsedImplicitlyAttribute (for entities that should not be marked as unused)
MeansImplicitUseAttribute (for extending semantics of any other attribute to mean that the corresponding entity should not be marked as unused)