how i show null in data set while importing excel sheets in .net

    string SheetName = dtExcelSchema.Rows[0]["TABLE_NAME"].ToString();

    connExcel.Close();


    //Read Data from First Sheet
    connExcel.Open();
    cmdExcel.CommandText = "SELECT * From [" + SheetName + "]";

    oda.SelectCommand = cmdExcel;
    oda.Fill(dt);
    connExcel.Close();
    GridView1.Caption = Path.GetFileName(FilePath);
    GridView1.DataSource = dt;
    GridView1.DataBind();

i am importing an excel sheet but in place of null value data set showing nothing i want to show null