Asking a Generic Method to Throw Specific Exception Type on FAIL

Question

Right, I know I am totally going to look an idiot with this one, but my brain is just not kicking in to gear this morning.

I want to have a method where I can sa

Answer 1

You can almost do it like this:

static void TestException<E>(string message) where E : Exception, new()
{
    var e = new E();
    e.Message = message;
    throw e;
}

However, that doesn't compile because Exception.Message is read only. It can only be assigned by passing it to the constructor, and there's no way to constrain a generic type with something other than a default constructor.

I think you'd have to use reflection (Activator.CreateInstance) to "new up" the custom exception type with the message parameter, like this:

static void TestException<E>(string message) where E : Exception
{
    throw Activator.CreateInstance(typeof(E), message) as E;
}

Edit Oops just realised you're wanting to return the exception, not throw it. The same principle applies, so I'll leave my answer as-is with the throw statements.

Answer 2

I think seeing as all exceptions should have a parameterless constructor, and have the Message property, so the following should work:

static ExType TestException<ExType>(string message) where ExType:Exception
{
    ExType ex = new ExType();
    ex.Message = message;
    return ex;
}

Edit: OK, Message is read only, so you'll have to hope the class implements the Exception(string) constructor instead.

static ExType TestException<ExType>(string message) where ExType:Exception
{
    return new ExType(message);
}

Answer 3

The only issue with the solution is that it is possible to create a subclass of Exception which does not implement a constructor with a single string parameter, so the MethodMissingException might be thrown.

static void TestException<E>(string message) where E : Exception, new()
{
    try 
    {
      return Activator.CreateInstance(typeof(E), message) as E;
    } 
    catch(MissingMethodException ex) 
    {
      return new E();
    }
}

Answer 4

Have you tried, instead:

static T TestException<Exception>(string message)
{}

because I have a feeling that putting in the generic constraint is not necessary as all throwable exceptions must inherit from System.Exception anyway.

Remember that generics do accept inherited types.

Answer 5

I have been instantiating inline the type of exception I want to throw, like this:

if (ItemNameIsValid(ItemName, out errorMessage))
    throw new KeyNotFoundException("Invalid name '" + ItemName + "': " + errorMessage);
if (null == MyArgument)
    throw new ArgumentNullException("MyArgument is null");