It's known Excel problem. Microsoft Excel API is a COM library with bad integration with .NET Framework.

I've worked with Excel Jetcell .NET for a long time and can really recommend it.

At Excel Sample page you will find many useful C# VB.NET Excel codes, for instance:

// Create New Excel Workbook in C#
ExcelWorkbook Wbook = new ExcelWorkbook();
ExcelCellCollection Cells = Wbook.Worksheets.Add("Sheet1").Cells;
 
Cells["A1"].Value =  "Excel writer example (C#)";
Cells["A1"].Style.Font.Bold = true;
Cells["B1"].Value =  "=550 + 5";
 
// Write Excel XLS file
Wbook.WriteXLS("excel_net.xls");