Question

ITool GetTool(Guid tool)
{
    if (tool == Hammer.Id)
        return new Hammer();
    else if (tool == Drill.Id)
        return new Drill();

    else
        throw new ....?
}

What’s the most appropriate exception type to throw here? NotSupportedException is the closest I’ve found but I don’t think that’s quite right.

Answer 1

By process of elimination:

• making your own implies that the exception is catchable and could be handled. It should never be caught, you can’t recover from bugs without recompiling.
• NotSupportedException implies that your code is at fault for not supporting the requested tool. Not likely, a Guid has trillions of invalid tool values.
• ArgumentOutOfRange implies that the Guid is too small or too large. Doesn’t make sense, they don’t have a range.
• ApplicationException is now considered not best-practice and is not specific enough.

ArgumentException(“Unknown tool requested”)