Export Data from Dataset to Excell in c# :

   private void btnprint_Click(object sender, EventArgs e)
        {
            try
            {
                if (dataGridView1.Rows.Count <= 0)
                {
                    return;
                }

                DataTable dt2 = new DataTable();
                dt2 = (DataTable)dataGridView1.DataSource;
                DataSet ds1 = new DataSet();
                ds1.Tables.Add(dt2.Copy());

                string filename = "";
                SaveFileDialog saveFileDialog1 = new SaveFileDialog();
                saveFileDialog1.DefaultExt = "txt";
                saveFileDialog1.Filter = "Text files (*.xls)|*.xls|All files (*.*)|*.*";
                if (saveFileDialog1.ShowDialog() == DialogResult.OK)
                {
                    filename = saveFileDialog1.FileName;
                }

                using (var workbook = SpreadsheetDocument.Create(filename, DocumentFormat.OpenXml.SpreadsheetDocumentType.Workbook))
                {
                    var workbookPart = workbook.AddWorkbookPart();
                    workbook.WorkbookPart.Workbook = new DocumentFormat.OpenXml.Spreadsheet.Workbook();
                    workbook.WorkbookPart.Workbook.Sheets = new DocumentFormat.OpenXml.Spreadsheet.Sheets();

                    uint sheetId = 1;

                    foreach (System.Data.DataTable table in ds1.Tables)
                    {
                        var sheetPart = workbook.WorkbookPart.AddNewPart<WorksheetPart>();
                        var sheetData = new DocumentFormat.OpenXml.Spreadsheet.SheetData();
                        sheetPart.Worksheet = new DocumentFormat.OpenXml.Spreadsheet.Worksheet(sheetData);

                        DocumentFormat.OpenXml.Spreadsheet.Sheets sheets = workbook.WorkbookPart.Workbook.GetFirstChild<DocumentFormat.OpenXml.Spreadsheet.Sheets>();
                        string relationshipId = workbook.WorkbookPart.GetIdOfPart(sheetPart);

                        if (sheets.Elements<DocumentFormat.OpenXml.Spreadsheet.Sheet>().Count() > 0)
                        {
                            sheetId =
                                sheets.Elements<DocumentFormat.OpenXml.Spreadsheet.Sheet>().Select(s => s.SheetId.Value).Max() + 1;
                        }

                        DocumentFormat.OpenXml.Spreadsheet.Sheet sheet = new DocumentFormat.OpenXml.Spreadsheet.Sheet() { Id = relationshipId, SheetId = sheetId, Name = table.TableName };
                        sheets.Append(sheet);

                        DocumentFormat.OpenXml.Spreadsheet.Row headerRow = new DocumentFormat.OpenXml.Spreadsheet.Row();

                        List<String> columns = new List<string>();
                        foreach (DataColumn column in table.Columns)
                        {
                            columns.Add(column.ColumnName);

                            DocumentFormat.OpenXml.Spreadsheet.Cell cell = new DocumentFormat.OpenXml.Spreadsheet.Cell();
                            cell.DataType = DocumentFormat.OpenXml.Spreadsheet.CellValues.String;
                            cell.CellValue = new DocumentFormat.OpenXml.Spreadsheet.CellValue(column.ColumnName);
                            headerRow.AppendChild(cell);
                        }

                        sheetData.AppendChild(headerRow);

                        foreach (DataRow dsrow in table.Rows)
                        {
                            DocumentFormat.OpenXml.Spreadsheet.Row newRow = new DocumentFormat.OpenXml.Spreadsheet.Row();
                            foreach (String col in columns)
                            {
                                DocumentFormat.OpenXml.Spreadsheet.Cell cell = new DocumentFormat.OpenXml.Spreadsheet.Cell();
                                cell.DataType = DocumentFormat.OpenXml.Spreadsheet.CellValues.String;
                                cell.CellValue = new DocumentFormat.OpenXml.Spreadsheet.CellValue(dsrow[col].ToString()); //
                                newRow.AppendChild(cell);
                            }

                            sheetData.AppendChild(newRow);
                        }
                    }
                }

                System.Diagnostics.Process.Start(filename);
                ds1.Clear();
            }
            catch
            {
            }
        }

Generic & it's Example For Beginners :
A small Notes On generics.

By   using generics We can create a class with placeholder which are used for type of class members like method , variable .

For Example:

class Program
    {
        static void Main(string[] args)
        {
            mygenerics<int> mint = new mygenerics<int>();
            Console.WriteLine(mint.add(5, 6));

            mygenerics<string> mstr = new mygenerics<string>();
            Console.WriteLine(mstr.add("Rk", "Ki"));

            mygenerics<float> mfloat = new mygenerics<float>();
            Console.WriteLine(mfloat.add(20,3));

            mygenerics<double> mdouble = new mygenerics<double>();
            Console.WriteLine(mdouble.add(20.5, 3.05));

            Console.ReadKey();
        }
    }

    class mygenerics<t>
    {
        public t add(t a, t b)
        {
            t c = (dynamic)a + (dynamic)b;
            return c;
        }
    }

Check assigning items of a list to a new list of the same type but with an added property from Stack Overflow

https://stackoverflow.com/questions/52074938/assigning-items-of-a-list-to-a-new-list-of-the-same-type-but-with-an-added-prope