Handling file download in ASP.NET

Question

Suggestion in either in C# or VB.NET is acceptable.

I have a class to handle file download link ASP.NET project like the following:

Public Class AIUFileHandler    
    Public Shared Sub DownloadFileHandler(ByVal fileName As String, ByVal filePath As String)    
        Dim r As HttpResponse    
        r.ContentType = "application/vnd.openxmlformats-officedocument.presentationml.presentation"
        r.AddHeader("content-disposition", String.Format("attachment;filename={0}", fileName))
        r.TransmitFile(filePath)    
        r.[End]()    
    End Sub    
End Class

Then, I call that function from a code behind of an ASP.NET page like this:

Protected Sub btnGetForm_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnGetForm.Click
    Dim fileName = "test.docx"
    Dim filePath = Server.MapPath("~/pub/test.docx")  
    AIUFileHandler.DownloadFileHandler(fileName, filePath)
End Sub

I got this error message like this:

Object reference not set to an instance of an object.

r.ContentType = "application/vnd.openxmlformats-officedocument.presentationml.presentation"

But if I use it like this without making another class, it works:

Protected Sub btnGetForm_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnGetForm.Click
    Dim fileName = "test.docx"
    Dim filePath = Server.MapPath("~/pub/test.docx")  
    Response.ContentType = "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
    Response.AddHeader("content-disposition", String.Format("attachment;filename={0}", fileName))
    Response.TransmitFile(filePath)
    Response.[End]()  
End Sub

What's the problem with my class?

Thank you.

Answer 1

You need to initialize the the r variable inside the DownloadFileHandler method before using it:

Dim r As HttpResponse = HttpContext.Current.Response

or pass it as argument:

Public Class AIUFileHandler
    Public Shared Sub DownloadFileHandler(ByVal fileName As String, ByVal filePath As String, ByVal r as HttpResponse)
        r.ContentType = "application/vnd.openxmlformats-officedocument.presentationml.presentation"
        r.AddHeader("content-disposition", String.Format("attachment;filename={0}", fileName))
        r.TransmitFile(filePath)
        r.[End]()
    End Sub
End Class

and to call:

Protected Sub btnGetForm_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnGetForm.Click
    Dim fileName = "test.docx"
    Dim filePath = Server.MapPath("~/pub/test.docx")
    AIUFileHandler.DownloadFileHandler(fileName, filePath, Response)
End Sub

Answer 2

Replace

Dim r As HttpResponse

with

Dim r as HttpResponse  = HttpContext.Current.Response

in AIUFileHandler class

Answer 3

btw, I use next code to send file to a user as an attachment:

var info = new FileInfo(Server.MapPath(path));
Response.Clear();
Response.AppendHeader("Content-Disposition", String.Concat("attachment; filename=", info.Name));
Response.AppendHeader("Content-Length", info.Length.ToString(CultureInfo.InvariantCulture));
Response.ContentType = type;
Response.WriteFile(info.FullName, true);
Response.End();

Also you can wrap it into try-finally block if target file is programmatically generated:

var info = ..
try
{
    // do stuff
}
finally
{
    File.Delete(info.FullName);
}