Error shows as Use of unassigned local variable 'roc'

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C#

DateTime roc;  
       DateTime arpa; 

        for (int i = 0; i < objDT.Rows.Count; i++)
        {
            if (objDT.Rows[i][0].ToString().Contains("+") == true)
                Session["course"] = "Package";
            else
                Session["course"] = "Course";

            if (objDT.Rows[i][0].ToString().Trim() == "ARPA")
            {
                arpa = Convert.ToDateTime(objDT.Rows[i][4].ToString().Trim()); //added
                CheckArpa = true;
            }

            if (objDT.Rows[i][0].ToString().Trim() == "ROC")
            {
                roc = Convert.ToDateTime(objDT.Rows[i][5].ToString().Trim()); //added
                CheckRoc = true;
            }

                 
               if (roc < arpa) 
               {
                   ScriptManager.RegisterStartupScript(this, GetType(), "Invalid date", "alert('First Book Roc than the ARPA Course')", true);
                   return;
                }           

        }

When i run the error shows as follows;

Use of unassigned local variable 'roc'
Use of unassigned local variable 'arpa'

From my code i am declaring in the first DateTime roc; DateTime arpa;

why this above error shows what is the problem in my above code.

Please help me.

Regards,
Narasiman P