try below code,
static void WriteContactsToExcel()
{
//Create Excel object
var excel = new Application();
if (excel == null)
throw new ApplicationException("Error occurred while instantiating Excel object");
//Set these to speed up the Excel application.
excel.Visible = false;
excel.ScreenUpdating = false;
excel.DisplayAlerts = false;
object misValue = System.Reflection.Missing.Value;
object missing = Type.Missing;
string destinationFilePath = @"C:\Excel\WriteExcelExample.xlsx";
Workbook workbook = excel.Workbooks.Add();
Worksheet worksheet = workbook.Sheets[1] as Worksheet;
//Create Excel Sheet Header Columns//mentioned as in FetchXML
worksheet.Cells[1, "A"] = "ContactId";
worksheet.Cells[1, "B"] = "FirstName";
worksheet.Cells[1, "C"] = "LastName";
worksheet.Cells[1, "D"] = "FullName";
worksheet.Cells[1, "E"] = "Phone";
worksheet.Cells[1, "F"] = "Fax";
//Get your CRM connection
IOrganizationService orgService = GetCRMConnection();
string fetchXML = @"<fetch version='1.0' output-format='xml-platform' mapping='logical' no-lock='true' distinct='false'>
<entity name='contact'>
<attribute name='contactid' />
<attribute name='firstname' />
<attribute name='lastname' />
<attribute name='fullname' />
<attribute name='telephone1' />
<attribute name='fax' />
</entity>
</fetch>";
var fetchExp = new FetchExpression(fetchXML);
var contactEntityCollection = orgService.RetrieveMultiple(fetchExp);
for (int i = 1; i < contactEntityCollection.Entities.Count; i++)
{
worksheet.Cells[i, "A"] = contactEntityCollection.Entities[i].Id.ToString();
worksheet.Cells[i, "B"] = contactEntityCollection.Entities[i].GetAttributeValue<string>("firstname");
worksheet.Cells[i, "C"] = contactEntityCollection.Entities[i].GetAttributeValue<string>("lastname");
worksheet.Cells[i, "D"] = contactEntityCollection.Entities[i].GetAttributeValue<string>("fullname");
worksheet.Cells[i, "E"] = contactEntityCollection.Entities[i].GetAttributeValue<string>("telephone1");
worksheet.Cells[i, "F"] = contactEntityCollection.Entities[i].GetAttributeValue<string>("fax");
Console.WriteLine("Writing " + i + " Contact " + contactEntityCollection.Entities[i].Id.ToString());
}
if (!string.IsNullOrEmpty(destinationFilePath))
{
//Save the Excel File
workbook.SaveAs(destinationFilePath, Microsoft.Office.Interop.Excel.XlFileFormat.xlOpenXMLWorkbook, misValue,
misValue, false, false, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange,
Microsoft.Office.Interop.Excel.XlSaveConflictResolution.xlUserResolution, true,
misValue, misValue, misValue);
workbook.Close(missing, missing, missing);
excel.Quit();
}
}
