Please modify your code a little bit like below...
string id = GridView1.DataKeys[e.RowIndex].Value.ToString();
int curview = GridView1.EditIndex;
GridViewRow row = GridView1.Rows[curview];
TextBox tquantity = row.FindControl("txt_mtype") as TextBox;
After this continue your coding.
Try it and let me know.
Thanks...