Constructor invocation returned null: what to do?

Question

I have code which looks like: ``` private static DirectiveNode CreateInstance(Type nodeType, DirectiveInfo info) { var ctor = nodeType.GetConstructor(new[] { typeof(DirectiveInfo) }); if(ctor == null) { throw new MissingMethodException(nodeType.FullName, "ctor"); } var node = ctor.Invoke(new[] { info }) as DirectiveNode; if(node == null) { // ???; } return node; } ``` I am looking for what to do (e.g. what type of exception to throw) when the `Invoke` method returns something which isn't a `DirectiveNode` or when it returns `null` (indicated by `// ???` above). (By the method's contract, `nodeType` will always describe a subclass of `DirectiveNode`.) I am not sure when calling a constructor would return `null`, so I am not sure if I should handle anything at all, but I still want to be on the safe side and throw an exception if something goes wrong.

Accepted Answer

You need to ensure that `nodeType` is a `DirectiveNode`:

```
if (!typeof(DirectiveNode).IsAssignableFrom(nodeType))
    throw new ArgumentException("The specified node type is not a 'DirectiveNode'");
```

Also, you can (should) use `Activator.CreateInstance` instead of manually finding the `ConstructorInfo` and invoking it. It's cleaner, more expressive, and more maintainable.

Constructor invocation returned null: what to do?

Solution

Problem