using System;
using System.Data;
using Microsoft.Office.Interop.Excel;
using Application = Microsoft.Office.Interop.Excel.Application;
using DataTable = System.Data.DataTable;


[STAThread]
static void Main()
{
    Application xlApp = null;

    try
    {
        xlApp = (Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
    }
    catch (Exception)//Excel not open
    {
        return;
    }

    xlApp.Visible = true;

    var wb = xlApp.ActiveWorkbook;

    Worksheet ws = null;

    try
    {
        ws = (Worksheet)wb.Worksheets["SomeSheet"];
    }
    catch (Exception e)
    {
        return;
    }

    if (ws == null)
    {
        return;
    }

    var dt = new DataTable();

    dt = new DataTable();
    dt.Clear();
    dt.Columns.Add("Col1");
    dt.Columns.Add("Col2");
    for (int ii = 0; ii < 20; ii++)
    {
        DataRow row = dt.NewRow();
        row["Col1"] = "xxxx";
        row["Col2"] = "yyyy";
        dt.Rows.Add(row);
    }

    //Header
    for (int i = 0; i < dt.Columns.Count; i++)
    {
        ws.Cells[1, i + 1] = dt.Columns[i].ColumnName;

    }

    //Data
    for (int i = 0; i < dt.Rows.Count; i++)
    {
        for (int j = 0; j < dt.Columns.Count; j++)
        {
            ws.Cells[i + 2, j + 1] = dt.Rows[i][j];
        }
    }

}