Export dataset in excel Asp.net

Question

I created a program to export a Dataset to excel at runtime via Response, but when opening the file a message of incompatibility between the file format and the file extension is displayed.

Follows code:

DataTable dt = new DataTable();
DataSet ds = new DataSet();            

int IdEmpresa = this.Master.UsuarioLogado.IdEmpresa;
UpQuesitoRespostaBLL uqr = new UpQuesitoRespostaBLL();
List<UpQuesitoCargoResposta> lstOpcoesRespostas = uqr.ListarOpcoesRespostasPorEmpresa(IdEmpresa);
int qtdeRespostas = lstOpcoesRespostas.FirstOrDefault().Respostas.Count();

dt.Columns.Add("Quesito");
dt.Columns.Add("Area");
dt.Columns.Add("Cargo");
dt.Columns.Add("Peso");
for (int i = 1; i <= qtdeRespostas; i++)
{
    dt.Columns.Add("Valor_" + i);
    dt.Columns.Add("Resposta_" + i);
}
foreach (var resp in lstOpcoesRespostas)
{
    DataRow row = dt.NewRow();
    row[0] = resp.DsQuesito.ToString();
    row[1] = resp.DsArea.ToString();
    row[2] = resp.DsCargo.ToString();
    row[3] = resp.Peso.ToString();
    int j = 3;
    for (int i = 0; i < qtdeRespostas; i++)
    {
        row[j + 1] = resp.Respostas[i].Valor.ToString();
        row[j + 2] = resp.Respostas[i].DsResposta.ToString();
        j += 2;
    }
    dt.Rows.Add(row);
}

ds.Tables.Add(dt);

StringWriter sw = DataSetToExcel(ds);

string date = DateTime.Now.ToShortDateString().Replace("/", "-").ToString();
HttpContext.Current.Response.Clear();
HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=RespostasPerformance" + date + ".xls");
HttpContext.Current.Response.Charset = "";
HttpContext.Current.Response.End();

I’ve already modified the contenttype and header extension, but no attempt worked.

The error is in the format or the way I am exporting ?