Restrict custom assertion to a specific (reference) type while keeping existing object assertions

[andreivanea]

Hi there,

I've trying to write a custom assertion for a class. I've tried following this discussion about extending existing assertions but I didn't get the results that I've hoped for. Consider the following code.

public class MyClass { }

public static class MyClassExtensions
{
    public static AndConstraint<ObjectAssertions> BeTheBestClassEver(this ObjectAssertions parent, 
         string because = "", params object[] becauseArgs)
    {
         // ...
    }
}

This kind of works. I don't lose existing object assertions on my class (such as BeEquivalentTo) and I'm able to use the assertion like this:

new MyClass().should().BeTheBestClassEver();

The problem is that I can also do this:

new { }.should().BeTheBestClassEver();

How can I restrict this custom assertion to my specific type, while keeping existing assertion? I've looked into using the generic version ObjectAssertions<TSubject, TAssertions> and specifying my type as TSubject but I'm not sure what do with TAssertions as I get the following error: The type 'FluentAssertions.Primitives.ObjectAssertions' cannot be used as type parameter 'TAssertions' in the generic type or method 'ObjectAssertions<TSubject, TAssertions>'. There is no implicit reference conversion from 'FluentAssertions.Primitives.ObjectAssertions' to 'FluentAssertions.Primitives.ObjectAssertions<MyClass, FluentAssertions.Primitives.ObjectAssertions>'.

What's the correct way to implement a custom assertion that can be applied on a specific type?

[andreivanea]

This might have been the wrong approach here. I only "explored" alternative solutions because following the official documentation on creating custom assertions did not yield the expected result. What I had written initially was:

public class MyClassAssertions : ReferenceTypeAssertions<MyClass, MyClassAssertions>
{
    public MyClassAssertions(MyClass instance)  : base(instance) { }

    public AndConstraint<MyClassAssertions> BeTheBestClassEver(string because = "", params object[] becauseArgs)
    {
        // ...
    }
}

This meant that all assertions defined in ObjectAssertions were basically removed from my class. To get keep existing assertions and get my new, custom assertion for my type only, I've replaced ReferenceTypeAssertions with ObjectAssertions:

public class MyClassAssertions : ObjectAssertions<MyClass, MyClassAssertions>
{
}

My conclusion is that extending ObjectAssertions instead of ReferenceTypeAssertions would be preferred in most cases.

[dennisdoomen]

This is also the correct approach. And you obviously need an extension method named Should on MyClass that returns an instance of MyClassAssertions