Export .xlsx from Gridview in vb.net using asp.net

Question

hi i am trying to export gridview data in xlsx file using below code, but when i open file its showing error, 

Response.Clear()

Response.AddHeader("content-disposition", "attachment;filename=Report.xlsx")

Response.Charset = ""

Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"

Dim stringWrite As System.IO.StringWriter = New System.IO.StringWriter()

Dim htmlWrite As System.Web.UI.HtmlTextWriter = New HtmlTextWriter(stringWrite)

For Each r As GridViewRow In GridView1.Rows

If r.RowType = DataControlRowType.DataRow Then

For columnIndex As Integer = 0 To r.Cells.Count - 1

r.Cells(columnIndex).Attributes.Add("class", "text")

Next

End If

Next

GridView1.RenderControl(htmlWrite)

Dim style As String = "&lt;style> .text { mso-number-format:\@; } </style> "

Response.Write(style)

Response.Write(stringWrite.ToString())

Response.[End]()