Это мое предложение выделить ячейки:
//find last cell if needed
int lastCell = xlWorkSheet.Cells.Find(
"*",
System.Reflection.Missing.Value,
Excel.XlFindLookIn.xlValues,
Excel.XlLookAt.xlWhole,
Excel.XlSearchOrder.xlByRows,
Excel.XlSearchDirection.xlPrevious,
false,
System.Reflection.Missing.Value,
System.Reflection.Missing.Value).Row;
Excel.Range rangeCheck = xlWorkSheet.Range["A1:A" + lastCell];
foreach (Excel.Range cell in rangeCheck.Cells)
{
string checkStrigng = Convert.ToString(cell.Value);//converts cell value to string for comparision
if (checkString == textbox.Text)
{
cell.Interior.Color = System.Drawing.Color.Yellow;
}
}