LastLogon Time

i am trying ot get userlast logon from active directory:

code is

if (objRead.HasRows)
            {
                while (objRead.Read())
                {
                    int ll = objRead.GetOrdinal("lastLogon");

                   
                    Label1.Text = objRead.GetString(ll); //.GetValue(ll).ToString(); //.GetString(1);
                    Label1.Visible = true;


                }
            }

exception is 

Exception System.InvalidCastException: Unable to cast COM object of type 'System.__ComObject' to class type 'System.String'. Instances of types that represent COM components cannot be cast to types that do not represent COM components; however they can be cast to interfaces as long as the underlying COM component supports QueryInterface calls for the IID of the interface. at System.Data.OleDb.ColumnBinding.ValueString() at System.Data.OleDb.OleDbDataReader.GetString(Int32 ordinal) at _Default.Page_Load(Object sender, EventArgs e) .

Exception only occurs at Lastlogin, for other items it works perfectly.

Is there any other way to get lastlogin time?