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C# - Server cannot set content type after HTTP headers have been sent

I trying to download SSRS report in excel format from my MVC application.
Response.Clear();
Response.Buffer = true;
Response.Charset = "";
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.ContentType = "application/vnd.ms-excel";
Response.AppendHeader("Content-Disposition", "attachment; filename=" + reportName + "." + extension);
Response.BinaryWrite(result);
Response.Flush();
Response.End();
But i am getting this error as -
System.Web.HttpException (0x80004005): Server cannot set content type after HTTP headers have been sent.
I tried changing Response.Buffer to BufferOutput but still getting the same error.
What am I missing here?

In the code below, i tried to download an excel file as -
Response.Clear();
Response.Buffer = true;
Response.Charset = "";
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.AddHeader($"content-disposition", "attachment;filename=" + "{reportFileName}" + ".xlsx");
using (System.IO.MemoryStream MyMemoryStream = new MemoryStream())
{
MyMemoryStream.Write(result, 0 , result.Length);
MyMemoryStream.WriteTo(Response.OutputStream);
Response.Flush();
Response.End();
return File(MyMemoryStream.ToArray(), "application/octet-stream");
}

Whenever i click on view online button my file doesn't show proper pdf in asp.net

when I click on it my file doesn't open properly enter image description here
string filePath = (sender as LinkButton).CommandArgument;
Response.ContentType = ContentType;
// Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(filePath));
Response.AddHeader("Content-Disposition", "inline; filename"+Path.GetFileName(filePath)+".pdf");
Response.WriteFile(filePath);
Response.End();
}

You can try this code to get your work done,
string pdfPath = Server.MapPath("~/SomePDFFile.pdf");
WebClient client = new WebClient();
Byte[] buffer = client.DownloadData(pdfPath);
Response.ContentType = "application/pdf";
Response.AddHeader("content-length", buffer.Length.ToString());
Response.BinaryWrite(buffer);

Creating PDF using C#

When I try to convert binary file to PDF the pdf has damaged.
byte[] stream = presenter.getItemTable();
Response.Clear();
Response.ClearHeaders();
Response.ContentType = "application/pdf";
Response.AddHeader ("Content-Disposition", "attachment; filename=PressRelease.pdf");
Response.BinaryWrite(stream);//Entities.EDRSearchResult.ByteStream);
Response.Flush();
Response.End();
However when I convert same binary to Excel it is working fine and can open without any error.
byte[] stream = presenter.getItemTable();
Response.Clear();
Response.ClearHeaders();
Response.ContentType = "application/vnd.openxmlformats - officedocument.spreadsheetml.sheet";
Response.AddHeader("content-disposition","attachment;filename=DataTable.xls");
Response.BinaryWrite(stream);
Response.Flush();
Response.End();
Please help me.

when you try to convert binary file to PDF in this case you write
Response.AddHeader ("Content-Disposition", "attachment; filename=PressRelease.pdf");
Try
Response.AddHeader ("content-disposition", "attachment; filename=PressRelease.pdf");

Downloaded excel file not opening

I am trying to download excel file to the location in local system but on opening the file, i am getting error:
The file format or extension does not match.
Although the file at backend is of same extension but i am getting the error
PFB the code:
string ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.ContentType = ContentType;
Response.AppendHeader("Content-Disposition", "attachment; filename="+downloadedFileName);
downloadedFileName= "Myfile.xlsx"

It will save as details.xls
Response.ClearContent();
Response.Buffer = true;
Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "Details.xls"));
Response.ContentType = "application/ms-excel";

You have this error because you're not using Response.TransmitFile(). Your code should be:
string ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.ContentType = ContentType;
Response.AppendHeader("Content-Disposition", "attachment; filename="+downloadedFileName);
Response.TransmitFile(FilePath); // full path here

ASP.NET file download from server

After a user clicks a button, I want a file to be downloaded. I've tried the following which seems to work, but not without throwing an exception (ThreadAbort) which is not acceptable.
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
response.ClearContent();
response.Clear();
response.ContentType = "text/plain";
response.AddHeader("Content-Disposition", "attachment; filename=" + fileName + ";");
response.TransmitFile(Server.MapPath("FileDownload.csv"));
response.Flush();
response.End();

You can use an HTTP Handler (.ashx) to download a file, like this:
DownloadFile.ashx:
public class DownloadFile : IHttpHandler
{
public void ProcessRequest(HttpContext context)
{
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
response.ClearContent();
response.Clear();
response.ContentType = "text/plain";
response.AddHeader("Content-Disposition",
"attachment; filename=" + fileName + ";");
response.TransmitFile(Server.MapPath("FileDownload.csv"));
response.Flush();
response.End();
}
public bool IsReusable
{
get
{
return false;
}
}
}
Then you can call the HTTP Handler from the button click event handler, like this:
Markup:
<asp:Button ID="btnDownload" runat="server" Text="Download File"
OnClick="btnDownload_Click"/>
Code-Behind:
protected void btnDownload_Click(object sender, EventArgs e)
{
Response.Redirect("PathToHttpHandler/DownloadFile.ashx");
}
Passing a parameter to the HTTP Handler:
You can simply append a query string variable to the Response.Redirect(), like this:
Response.Redirect("PathToHttpHandler/DownloadFile.ashx?yourVariable=yourValue");
Then in the actual handler code you can use the Request object in the HttpContext to grab the query string variable value, like this:
System.Web.HttpRequest request = System.Web.HttpContext.Current.Request;
string yourVariableValue = request.QueryString["yourVariable"];
// Use the yourVariableValue here
Note - it is common to pass a filename as a query string parameter to suggest to the user what the file actually is, in which case they can override that name value with Save As...

Try this set of code to download a CSV file from the server.
byte[] Content= File.ReadAllBytes(FilePath); //missing ;
Response.ContentType = "text/csv";
Response.AddHeader("content-disposition", "attachment; filename=" + fileName + ".csv");
Response.BufferOutput = true;
Response.OutputStream.Write(Content, 0, Content.Length);
Response.End();

Making changes as below and redeploying on server content type as
Response.ContentType = "application/octet-stream";
This worked for me.
Response.Clear();
Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name);
Response.AddHeader("Content-Length", file.Length.ToString());
Response.ContentType = "application/octet-stream";
Response.WriteFile(file.FullName);
Response.End();

Further to Karl Anderson solution, you could put your parameters into session information and then clear them after response.TransmitFile(Server.MapPath( Session(currentSessionItemName)));.
See MSDN page HttpSessionState.Add Method (String, Object) for more information on sessions.

protected void DescargarArchivo(string strRuta, string strFile)
{
FileInfo ObjArchivo = new System.IO.FileInfo(strRuta);
Response.Clear();
Response.AddHeader("Content-Disposition", "attachment; filename=" + strFile);
Response.AddHeader("Content-Length", ObjArchivo.Length.ToString());
Response.ContentType = "application/pdf";
Response.WriteFile(ObjArchivo.FullName);
Response.End();
}

Simple solution for downloading a file from the server:
protected void btnDownload_Click(object sender, EventArgs e)
{
string FileName = "Durgesh.jpg"; // It's a file name displayed on downloaded file on client side.
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
response.ClearContent();
response.Clear();
response.ContentType = "image/jpeg";
response.AddHeader("Content-Disposition", "attachment; filename=" + FileName + ";");
response.TransmitFile(Server.MapPath("~/File/001.jpg"));
response.Flush();
response.End();
}