Type or namespace XLWorkbook Could not be found

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C#

ASP.NET4

Dear Frnds,

I need to export the collapse gridview to excel. i have the problem in this code. Type or namespace 'XLWorkbook' could not be found. what is the problem here.

DataTable dt = new DataTable("GridView_Data");
GridView gvOrders = (GridView)gvCustomers.Rows[1].FindControl("gvOrders");
foreach (TableCell cell in gvCustomers.HeaderRow.Cells)
{
dt.Columns.Add(cell.Text);
}
foreach (TableCell cell in gvOrders.HeaderRow.Cells)
{
dt.Columns.Add(cell.Text);
}
dt.Columns.RemoveAt(0);
foreach (GridViewRow row in gvCustomers.Rows)
{
GridView gvOrderscell = (row.FindControl("gvOrders") as GridView);
for (int j = 0; j < gvOrderscell.Rows.Count; j++)
{
dt.Rows.Add(row.Cells[1].Text, row.Cells[2].Text, gvOrderscell.Rows[j].Cells[0].Text, gvOrderscell.Rows[j].Cells[1].Text);
}
}
using (XLWorkbook wb = new XLWorkbook())
{
wb.Worksheets.Add(dt);

Response.Clear();
Response.Buffer = true;
Response.Charset = "";
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.AddHeader("content-disposition", "attachment;filename=GridView.xlsx");
using (MemoryStream MyMemoryStream = new MemoryStream())
{
wb.SaveAs(MyMemoryStream);
MyMemoryStream.WriteTo(Response.OutputStream);
Response.Flush();
Response.End();
}
}