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by Jiří {x2} Činčura

Easier ArgumentException messages

31 Jul 2012 .NET, C#, Programming in general

It’s a good practice to include parameter name when throwing ArgumentException (or similar). But I hate typing the parameter name as string. It’s A) boring and B) dumb, because when you refactor the parameter name you have to manually also change this string.

But why not let the language constructs help us? 8-) If we capture the parameter as expression we’ll be able to use static typing and extract the name from the expression node.

Shouldn’t be that hard, I thought one afternoon, and here’s the result (it really isn’t).

static string ArgumentExceptionMessage<T>(Expression<Func<T>> argument)
{
	var me = argument.Body as MemberExpression;
	if (me == null)
		throw new ArgumentException("Cannot extract argument name.", ArgumentExceptionMessage(() => argument));
	return me.Member.Name;
}

And you can use it (normally you would throw exception in all cases, I just wanted to show the result, hence Console.WriteLine).

class Foo
{
	public int MyProperty { get; set; }
}

static void Main(string[] args)
{
	var f = new Foo();
	Console.WriteLine(ArgumentExceptionMessage(() => args));
	Console.WriteLine(ArgumentExceptionMessage(() => f.MyProperty));

	throw new ArgumentException("This argument is not correct.", ArgumentExceptionMessage(() => args));
}

There’s never enough precondition checks. And understandable messages/parameters of exceptions is a valued help for developers. You can even wrap it to method returning ArgumentException directly to have more succinct code.