Since this is .NET 2008, search for that topic in the .NET forum. The question gets answered about once a week or so, and there are a variety of solutions. Pick the one you prefer.

One option that isn't very good, but is similar to an earlier suggestion here would be to use Double.TryParse (or Integer.TryParse if decimals are not allowed). That would return True if the string was a double, and False if it was not. However, it wouldn't restrict the textbox entries to just numeric characters. It seems that there were a series of answers based on regular expressions, but I haven't dealt with this problem, so I haven't been looking for a solution.