Best way to handle null properties on a method parameter [duplicate]

Question

When checking a method's parameter, I throw an ArgumentNullException if it's null. See the first line in the method below. But what about properties on the parameter that shouldn't be null? If I try to handle them the same way, I get a code analysis error:

CA2208 Instantiate argument exceptions correctly Method 'PriorityDeratingComponentLogic.CreateItem(IvSimulation)' passes 'ivSimulation.SolarPanel' as the 'paramName' argument to a 'ArgumentNullException' constructor. Replace this argument with one of the method's parameter names. Note that the provided parameter name should have the exact casing as declared on the method.

public DeratingComponentBase CreateItem(IvSimulation ivSimulation)
{
    if (ivSimulation == null) { throw new ArgumentNullException("ivSimulation"); }
    if (ivSimulation.SolarPanel == null) { throw new ArgumentNullException("ivSimulation.SolarPanel"); }
    if (ivSimulation.GlobalEquipment == null) { throw new ArgumentNullException("ivSimulation.GlobalEquipment"); }

    // ... method body here
}

Is the CA error something I should suppress, or is there a generally-accepted way to better handle this? Perhaps the issue is upstream and we shouldn't even have to check for those properties being null at this point?

Answer 1

Throwing an ArgumentNullException is intended to indicate that the argument itself is null. However, when one of argument components is null, but the argument itself isn't, the proper exception is ArgumentException with the name of the argument as parameter:

if (ivSimulation.GlobalEquipment == null) {
    throw new ArgumentException("GlobalEquipment cannot be null", "ivSimulation");
 }

This provides the information about the error in the message, while specifying ivSimulation as the parameter name.

Note: I assume that you have no choice of validating GlobalEquipment in the constructor of IvSimulation, because throwing ArgumentNullException would be a perfectly valid choice in that constructor.

Answer 2

It can be argued that a null property on a non-null parameter variable isn't really a null argument, so the ArgumentNullException is not actually appropriate in this case. (Clearly, others may disagree, which is fine.) This feels to me like a qualitatively different situation. The argument isn't missing, but it is in an invalid state. I think I would use an InvalidOperationException for cases like this, where an object is in an invalid state.