isnumeric

http://silverlight.net/forums/t/89404.aspx

you can use int.TryParse() or float.TryParse() like

int dummy;
if (TryParse(someString, out dummy))
{
     // it is numeric
}

you can do the same with float or double, or any other numeric type.
Indeed TryParse will convert the string to a numeric value (dummy). If you just want to test if it is numeric, you have to use a dummy variable.

double Dummy = new double();
shall be just:
double Dummy;

My apologies - you are right!