求大神解答，为什么datalist中处理radiobutton的checked时string无法赋值？

代码如下，在最后if (strarm == tmp)判定时编译器老是提醒我tmp的值为空，不能比较，这是为什么呢？
string tmp;
RadioButton A = (RadioButton)DataList1.Items[i].FindControl("RA");
RadioButton B = (RadioButton)DataList1.Items[i].FindControl("RB");
RadioButton C = (RadioButton)DataList1.Items[i].FindControl("RC");
RadioButton D = (RadioButton)DataList1.Items[i].FindControl("RD");
Label showre = ((Label)DataList1.Items[i].FindControl("Label4"));
string strarm = ((Label)DataList1.Items[i].FindControl("Label3")).Text.Trim();
if (A.Checked)
{
tmp = "A";

        }
        else if (B.Checked)
        {
            tmp = "B";
        }
        else if (C.Checked)
        {
            tmp = "C";
        }
        else if (D.Checked)
        {
            tmp = "D";
        }
        if (strarm == tmp)//问题所在，提示tmp是null
        {
            score = score + 1;
        }

你有DEBUG过你的代码么?