Hi y.alim,

I have created sample please take its reference.

I have used below article to generate the Excel. Refer below article for details.

Export DataTable to Excel in ASP.Net using C# and VB.Net

HTML

<table border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td>Name</td>
        <td><asp:TextBox ID="txtName" runat="server" /></td>
    </tr>
    <tr>
        <td>Age</td>
        <td><asp:TextBox ID="txtAge" runat="server" /></td>
    </tr>
    <tr>
        <td>Gender</td>
        <td><asp:TextBox ID="txtGender" runat="server" /></td>
    </tr>
    <tr>
        <td></td>
        <td><asp:Button ID="btnAdd" Text="Add Details" runat="server" OnClick="AddDetails" /></td>
    </tr>
</table>
<br /><br />
<asp:Button ID="btnExport" Text="Export to Excel" runat="server" OnClick="ExportToExcel" />

Namespaces

C#

using System.Data;
using System.IO;
using ClosedXML.Excel;

VB.Net

Imports System.Data
Imports System.IO
Imports ClosedXML.Excel

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Age", typeof(string));
        dt.Columns.Add("Gender", typeof(string));
        ViewState["Customers"] = dt;
    }
}

protected void AddDetails(object sender, EventArgs e)
{
    DataTable dt = (DataTable)ViewState["Customers"];
    dt.Rows.Add(txtName.Text.Trim(), txtAge.Text.Trim(), txtGender.Text.Trim());
    ViewState["Customers"] = dt;
    txtName.Text = ""; txtAge.Text = ""; txtGender.Text = "";
}

protected void ExportToExcel(object sender, EventArgs e)
{
    DataTable dt = (DataTable)ViewState["Customers"];
    using (XLWorkbook wb = new XLWorkbook())
    {
        wb.Worksheets.Add(dt, "Customers");
        wb.Worksheet(1).Columns().AdjustToContents();

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

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As DataTable = New DataTable()
        dt.Columns.Add("Name", GetType(String))
        dt.Columns.Add("Age", GetType(String))
        dt.Columns.Add("Gender", GetType(String))
        ViewState("Customers") = dt
    End If
End Sub

Protected Sub AddDetails(ByVal sender As Object, ByVal e As EventArgs)
    Dim dt As DataTable = CType(ViewState("Customers"), DataTable)
    dt.Rows.Add(txtName.Text.Trim(), txtAge.Text.Trim(), txtGender.Text.Trim())
    ViewState("Customers") = dt
    txtName.Text = ""
    txtAge.Text = ""
    txtGender.Text = ""
End Sub

Protected Sub ExportToExcel(ByVal sender As Object, ByVal e As EventArgs)
    Dim dt As DataTable = CType(ViewState("Customers"), DataTable)
    Using wb As XLWorkbook = New XLWorkbook()
        wb.Worksheets.Add(dt, "Customers")
        wb.Worksheet(1).Columns().AdjustToContents()
        Response.Clear()
        Response.Buffer = True
        Response.Charset = ""
        Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
        Response.AddHeader("content-disposition", "attachment;filename=Customers.xlsx")
        Using MyMemoryStream As MemoryStream = New MemoryStream()
            wb.SaveAs(MyMemoryStream)
            MyMemoryStream.WriteTo(Response.OutputStream)
            Response.Flush()
            Response.End()
        End Using
    End Using
End Sub

Screenshots

Exported Excel