Question

I was looking through the “Domain Oriented N-Layered .NET 4.0 Sample App” project and ran across some code that I do not understand. In this project they often use syntax like the following to check arguments for null:

public GenericRepository(IQueryableContext context,ITraceManager traceManager)
{
    if (context == (IQueryableContext)null)
            throw new ArgumentNullException("context", Resources.Messages.exception_ContainerCannotBeNull);

Why would you cast null to the type of the object you are checking for null?

Answer 1

It’s pointless in the example given.

While not applicable in this case, there is sometimes a need to cast null (or at least there was before default(T) was added. Consider the following:

void DoSomething(string x) {
    ...
}

void DoSomething(object x) {
    ...
}

DoSomething(null);            // compiler can't infer the type
DoSomething((string)null);    // string type is now explicit
DoSomething(default(string)); // same as previous

EDIT

Just thought of another case where you would have to do the cast when testing equality. If you had an object that had an overloaded == operator that allowed comparison with two reference types, comparing against null would be ambiguous. However because IQueryableContext is most likely an interface and interfaces cannot overload the == operator, I still don’t see any valid reason to do it in the example you gave.

class CustomObject {

    private string _id;

    public CustomObject(string id) {
        _id=id;
    }

    public static bool operator ==(CustomObject lhs, CustomObject rhs) {
        if (ReferenceEquals(lhs, rhs)) { return true; }
        if (ReferenceEquals(lhs, null)) { return false; }
        if (ReferenceEquals(rhs, null)) { return false; }
        return lhs._id == rhs._id;
    }

    public static bool operator !=(CustomObject lhs, CustomObject rhs) {
        return !(lhs == rhs);
    }

    public static bool operator ==(CustomObject lhs, string rhs) {
        if (ReferenceEquals(lhs, rhs)) { return true; }
        if (ReferenceEquals(lhs, null)) { return false; }
        if (ReferenceEquals(rhs, null)) { return false; }
        return lhs._id == rhs;
    }

    public static bool operator !=(CustomObject lhs, string rhs) {
        return !(lhs==rhs);
    }

}

CustomObject o = null;
if (o == null) {
    Console.WriteLine("I don't compile.");
}