How to Append data coloms in a worksheet from a data table in Excel Interop c#

@RAMAN RAGAVAN , Welcome to Microsoft Q&A, Based on my further test, you could try to use the following code to append data coloms in a worksheet from dataset.

static void Main(string[] args)  
        {  
           DataSet ds = GetDataSet();  
           string excelfile = @"t1.xlsx";  
            for (int i = 0; i < ds.Tables.Count; i++)  
            {  
                ExportToExcel(ds.Tables[i], excelfile);  
            }  
             
  
  
        }  
        public static void ExportToExcel(DataTable tbl, string excelFilePath = null)  
        {  
            var excelApp = new Excel.Application();  
            try  
            {  
              
                if (!File.Exists(excelFilePath))  
                {  
                      
                    excelApp.Workbooks.Add();  
  
                    // single worksheet  
                    Excel._Worksheet workSheet = excelApp.ActiveSheet;  
  
                    // column headings  
                    for (var i = 0; i < tbl.Columns.Count; i++)  
                    {  
                        workSheet.Cells[1, i + 1] = tbl.Columns[i].ColumnName;  
                    }  
  
                    // rows  
                    for (var i = 0; i < tbl.Rows.Count; i++)  
                    {  
                        // to do: format datetime values before printing  
                        for (var j = 0; j < tbl.Columns.Count; j++)  
                        {  
                            workSheet.Cells[i + 2, j + 1] = tbl.Rows[i][j];  
                        }  
                    }  
                    workSheet.SaveAs(excelFilePath);  
                }  
                else  
                {  
                      
                    var workbook = excelApp.Workbooks.Open(excelFilePath);  
                    Excel._Worksheet workSheet = workbook.Sheets[1];  
                    var row = workSheet.UsedRange.Rows.Count;  
                    var col= workSheet.UsedRange.Columns.Count;  
                    for (var i = 0; i < tbl.Columns.Count; i++)  
                    {  
                        workSheet.Cells[1, i + 1+col] = tbl.Columns[i].ColumnName;  
                    }  
  
                    // rows  
                    for (var i = 0; i < tbl.Rows.Count; i++)  
                    {  
                        // to do: format datetime values before printing  
                        for (var j = 0; j < tbl.Columns.Count; j++)  
                        {  
                            workSheet.Cells[i + 2, j + 1+col] = tbl.Rows[i][j];  
                        }  
                    }  
                    workbook.Save();  
  
                }  
  
                // load excel, and create a new workbook  
                
            }  
            catch (Exception ex)  
            {  
                throw new Exception("ExportToExcel: \n" + ex.Message);  
            }  
            finally  
            {  
                excelApp.Quit();  
            }  
        }  
        static DataSet GetDataSet()  
        {  
            var ds = new DataSet();  
            DataTable dt1 = new DataTable();  
            dt1.Columns.Add("Name", typeof(string));  
            dt1.Columns.Add("Age", typeof(int));  
            // Create a DataRow, add Name and Age data, and add to the DataTable  
            DataRow dr1 = dt1.NewRow();  
            dr1["Name"] = "Mohammadsd"; // or dr[0]="Mohammad";  
            dr1["Age"] = 24; // or dr[1]=24;  
            dt1.Rows.Add(dr1);  
            ds.Tables.Add(dt1);  
            DataTable dt2 = new DataTable();  
            dt2.Columns.Add("Name", typeof(string));  
            dt2.Columns.Add("Age", typeof(int));  
  
            // Create a DataRow, add Name and Age data, and add to the DataTable  
            DataRow dr2 = dt2.NewRow();  
            dr2["Name"] = "Mohammad"; // or dr[0]="Mohammad";  
            dr2["Age"] = 24; // or dr[1]=24;  
            dt2.Rows.Add(dr2);  
            dt2.Rows.Add("test2", 26);  
            ds.Tables.Add(dt2);  
            DataTable dt3 = new DataTable();  
            dt3.Columns.Add("Name", typeof(string));  
            dt3.Columns.Add("sex", typeof(int));  
            // Create a DataRow, add Name and Age data, and add to the DataTable  
            DataRow dr3 = dt3.NewRow();  
            dr3["Name"] = "last"; // or dr[0]="Mohammad";  
            dr3["sex"] = 24; // or dr[1]=24;  
            dt3.Rows.Add(dr3);  
              
            ds.Tables.Add(dt3);  
            return ds;  
        }  

Result:

Best Regards,
Jack

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