using uCalcSoftware;

var uc = new uCalc();

static void MyArea(uCalc.Callback cb) {
   var Length = cb.ArgDbl(1); // Same as cb.Arg(1);
   var Width = cb.ArgDbl(2); // Same as cb.Arg(2);
   cb.Return(Length * Width);
}

uc.DefineFunction("Area(x, y)", MyArea);
Console.WriteLine(uc.Eval("Area(3, 4)"));