##C# - 128 characters ---> now 108
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.WriteLine(new System.Data.DataTable().Compute(a+"+"+b,""));}

Ungolfed:
 
 static void t(string a, string b)
 {
 System.Console.WriteLine(new System.Data.DataTable().Compute(a+"+"+b,""));
 }

Calling `t("123","456");` gets you 579.