##C# - 128 108 104
C# - 128 108 104
Thanks to Compass, BMac and Shawn for suggesting improvements.
First try at Code Golf, and using C# seems to be a handicap here...
By using .Compute() you can use the string values and sum them directly. As a bonus this works for other operators aside from just "+".
Golfed:
static void t(string a,string b){System.Console.Write(new System.Data.DataTable().Compute(a+"+"+b,""));} Ungolfed:
static void t(string a, string b) { System.Console.Write(new System.Data.DataTable().Compute(a+"+"+b,"")); } Calling t("123","456"); gets you 579.