Question

I have an Asset object that has a property AssignedSoftware, which is a collection.

I want to make sure that the same piece of Software is not assigned to an Asset more than once. In Add method I check to see if the Software already exist, and if it does, I want to throw an exception.

Is there a standard .NET exception that I should be throwing? Or does best practices dictate I create my own custom exception?

Answer 1

From the Class Library design guidelines for errors (http://msdn.microsoft.com/en-us/library/8ey5ey87(VS.71).aspx):

In most cases, use the predefined exception types. Only define new exception types for programmatic scenarios, where you expect users of your class library to catch exceptions of this new type and perform a programmatic action based on the exception type itself. This is in lieu of parsing the exception string, which would negatively impact performance and maintenance.

…

Throw an ArgumentException or create an exception derived from this class if invalid parameters are passed or detected.

Throw the InvalidOperationException exception if a call to a property set accessor or method is not appropriate given the object’s current state.

This seems like an "Object state invalid" scenario to me, so I’d pick InvalidOperationException over ArgumentException: The parameters are valid, but not at this point in the objects life.