c# - Returning string is not successful -

i have problem may easy can't solve it.
have function returns string , has 2 arguments:

public string myfunc(string id, string targetid)   // id 9999999995 , targetid 9999999998 {     return id + targetid;    // gives me id only(9999999995). } 

when try put text instead of id, works, not number. i've tried use stringbuilder same result.

i use this:

messagebox.show(myfunc(settings.default.id, combobox1.text)); 

the function working intended. check incoming parameters id , targetid contain values , values expect.