How to prevent master page header getting cached?

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C#

ASP.NET

Hi,

My requirement was to download a file from my project folder.. i wrote the following code:

protected void btnOpenSampleExcel_Click(object sender, EventArgs e)
{
    try
    {
        string fileName = "SampleRetsServerCredentials.csv";
        string filePath = Server.MapPath("/SampleRetsCredential\\SampleRetsServerCredentials.csv");
        Response.AppendHeader("content-disposition", "filename=" + fileName);
        Response.ContentType = "application/octet-stream";
        Response.WriteFile(filePath, false);
    }

    catch (Exception ex)
    {
        _alertMessage = "Sorry, cant show the sample file now! Please try after some time.";
        radManager.RadAlert("message", 400, null, "title", null);
    }
}

Download is working correctly, but the Master page header portion is also shown in the csv after downloading.. How can i prevent such a behaviour.

Thanks,
Amrutha.