Assign value to Excel.Range

[dudinissan@gmail.com]

Hello,
I am writing c# web application.
My code is:

Excel.Applicati on EXL;
EXL = new Excel.Applicati on();
Excel.Worksheet WSheet = new Excel.Worksheet Class();
string FileName = MapPath(".") + "\\..xls";
WSheet = (Excel.Workshee t)EXL.Workbooks .Open(FileName, Type.Missing,
Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
Type.Missing).W orksheets.get_I tem(1);
Excel.Range rng = WSheet.get_Rang e("H12", Type.Missing);
string H12 = "H12: " + rng.Value2.ToSt ring(); // Works fine

How can I assign values to cells ?
rng.Cells[6, 3] = "a"; --> retrun the following error:
error: cannot assign '"a"' into 'rng.Cells[6,3]'

Thank you.