empty cell to be background color in excel i tried not working

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C#

My Code as follows;

sql ="select * FROM Tb_SCh_TIme_Table P PIVOT ";
                  sql = sql + "(MAX(Faculty_Code) FOR Session IN ([1],[2],[3] ,[4])) AS PVT ";
                  sql = sql + " where course = '" + dr[0].ToString().Trim() + "' order by date";
                  Xldr = GFun.ReadSql(sql);
                  while (Xldr.Read())
                  {
                      xlWorkSheet.Cells[xlRow, 1] = Xldr[1].ToString();
                      xlWorkSheet.Cells[xlRow, 2] = Xldr[3].ToString();
                      xlWorkSheet.Cells[xlRow, 3] = Xldr[4].ToString();
                      xlWorkSheet.Cells[xlRow, 4] = Xldr[5].ToString();
                      xlWorkSheet.Cells[xlRow, 5] = Xldr[6].ToString();
                      if (Xldr[3].ToString().Trim() == "")
                      {
                          //xlWorkSheet.Cells.Interior.ColorIndex = Color.Red;
                          xlWorkSheet.Cells.Interior.Color = Color.Red;
                      }
                  }

i tried the above code. in excel if its empty cell that cell to be filled with some back ground color i tried. but it is not working.

Below code is correct ;

if (Xldr[3].ToString().Trim() == " ")
 {
xlWorkSheet.Cells.Interior.Color = Color.Red;
 }

i tried but is not working .how can i do. from my above code.help me.