Export data from DataSet to Microsoft Excel using windows forms

you are doing it wrong.
Use Excel.Application app = new Excel.Application();
Sometimes there are errors when using Excel, so it would be good to insert this code at the begining:

Excel.Application app = null;
System.Globalization.CultureInfo oldCI = System.Threading.Thread.CurrentThread.CurrentCulture;
    System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("en-US");

app = new Excel.Application();