string与char一起出现显示不全

char[] cha = new char[4] ;
            for (int i = 0; i < 4; i++)
            {
                cha[i] = tbInput.Text[i];
            }
            String str1 = new String(cha);
            string str2 = tbChange.Text;

           // str1 = "被赋值";
            labShowA.Text = "A为:" + str1 + "   B为:" + str2;
当不执行注释部分时，labShowA.Text显示只有str1,
而执行注释部分时，labShowA.Text显示全部。