Cannot implicitly convert type 'object' to 'Microsoft.Office.Interop.Excel.Worksheet'. An explicit conversion exists (are you missing a cast?)

you have to create a new Worksheet with Sheets array by providing WorkSheet Name. and also please Cast The Newly Created WorkSheet.

Replace this :

Excel.Worksheet sh = wb.Sheets.Add();

with following

 Excel.Worksheet sh  = (Microsoft.Office.Interop.Excel.Worksheet)wb.Sheets["Sheet1"];