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Uploading Files Usingthe File Field Control75 out of 104 rated this helpful
By Bill Evjen
Reuters
Feburary 2004
Applies to:
Microsoft ASP.NET
Summary: Learn how to use the Microsoft ASP.NET File
Field control to allow your end-users to upload one ormore files to your server. (16 printed pages)
Contents
Introduction
Looking Closely at the File Field Control
Working Around File Size Limitations
Controlling Uploaded File Types
Uploading Multiple Files at the Same Time
Conclusion
Related Books
IntroductionMicrosoft ASP.NET Web Forms are all about
communicating with an end-user to abstract what
information you need to from them depending on the
service administered. In many cases with Web Forms, it is
usually simply textual data that is collected and stored.
However, there are many cases where you are going to
need more than simple textual data; you may want your
users to upload files to the server.
For instance, your application can have a document library
that is shared with all users. There are plenty of examples,
but invariably you will need this ability to upload any type
of document for storage, or to have your application take
action upon the document once it is uploaded.
In any case, ASP.NET provides you with a simple ability to
upload documents to the server with just a little work on
your part. This article describes how to use the File Fieldcontrol for uploading documents to your Web server.
Looking Closely at the File Field
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The File Field control uses the HtmlInputFile class and
is a cool control simply because it does things that were
difficult to do before ASP.NET (that is, without buying a
third-party component to do it for you). The File Field
control uploads files to the server from the client's machine.
It is important to note that this control is called the File
Field control in all versions of Microsoft Visual Studio
.NET, while in the ASP.NET Web Matrix, this control is called
the FileUpload control. In both cases, you will find the
control in the HTML section of the Toolbox.
The File Field control allows access to program the HTML
tag. This tag is used to work with
file data within an HTML form. In the past when using
classic ASP (ASP 3.0 or earlier), many programmers worked
with third-party components to upload files from the client
to the server. Now, with .NET and this new control,uploading is taken care of for youand it couldn't be
simpler. Listing 1 shows you how to use the File Field
control to upload files to the server.
Note The sample code is provided in both
Microsoft Visual Basic (VB) and C#.
Listing 1. Uploading files to the server using the File
Field control
VB
Sub SubmitButton_Click(Source As Object, e As
EventArgs)
If Not (File1.PostedFile Is Nothing) Then
Try
File1.PostedFile.SaveAs("C:\Uploads\
uploadedfile.txt")Span1.InnerHtml = "Upload Successful
!"
Catch ex As Exception
Span1.InnerHtml = "Error saving file
C:\\" & _
File1.Value & "
" & ex.ToString()
End Try
End If
End Sub
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< o y>
Select a file to upload:
C#
void SubmitButton_Click(Object sender, EventA
rgs e) {
if (File1.PostedFile != null) {
try {
File1.PostedFile.SaveAs("C:\\Uploads
\\uploadedfile.txt");
Span1.InnerHtml = "Upload Successful
!";
}catch (Exception ex) {
Span1.InnerHtml = "Error saving file
C:\\" +
File1.Value + "
" + ex.ToString();
}
}
}
Select a file to upload:
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When the page from Listing 1 is run, you can select a file
and upload it to the server by clicking the Upload File
button on the page. This is a great feature that ASP 3.0
programmers always wished for. Now it is here in .NET!
With only a few lines of code, it is easy to upload any type
of files to the server.
There are some important items we should go over for this
example so you understand all the needed pieces to make
this work. First, for the example in Listing 1 to work, you
have to make the destination folder on the server writeable
for the account used by ASP.NET so the file can be saved to
the specified folder.
If you think your ASP.NET account is not enabled to write to
the folder you want, simply open up Microsoft WindowsExplorer and navigate to the folder to which you want to
add this permission. Right-click on the folder (in this case,
the Uploads folder), and then select Properties. In the
Properties dialog box, click on the Security tab and make
sure the ASP.NET Machine Account is included in the list
and has the proper permissions to write to disk (see Figure
1).
Figure 1. Looking at the Security tab of the Uploads
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If you don't see the ASP.NET Machine Account under the
Security tab, you can add it by clicking the Add button and
entering ASPNET (without the period) in the text area, as
illustrated in Figure 2.
Figure 2. Adding the ASP.NET Machine Account to the
folder security definition
Click OK to add the ASP.NET Machine Account to the list.
From here, make sure you give this account the proper
permissions; then click OK and you are ready to go.
Looking at the code from Listing 1, you might notice some
really important things right away. First, the tag
has been altered by the addition of another attribute. Toupload files to a server, your tag must have the
attribute enctype="multipart/form-data". Without
this attribute, the Web form cannot upload the file.
The Submit button on the page causes an
OnServerClick event to occur. This event uploads the
file and then displays a message telling you if the upload
was successful. If it was unsuccessful, the page displays an
error message describing why the upload failed.
By using the tag, the browser
automatically places a Browse button next to the text field
on the ASP.NET page. You don't have to program anything
else for this to occur. When the end user clicks the Browse
button, he can navigate through the local file system to find
the file to be uploaded to the server. This is shown in
Figure 3. Clicking Open will place that filename and the file's
path within the text field.
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Figure 3. Choosing a file
Working Around File Size Limitations
You may not realize it, but there is a limit to the size of a
file that can be uploaded using this technique. By default,
the maximum size of a file to be uploaded to the server
using the File Field control is around 4MB. You cannot
upload anything that is larger than this limit.
One of the great things about .NET, however, is that it
usually provides a way around limitations. You can usually
change the default settings that are in place. To change this
size limit, you make some changes in either the
machine.config or web.config file.
In the machine.config file, find a node called
that looks like the following:
A lot is going on in this single node, but the setting that
takes care of the size of the files to be uploaded is the
maxRequestLength attribute. By default, this is set to
4096 kilobytes (KB). Simply change this value to increase the
size of the files that you can upload to the server. If you
want to allow 10 megabyte (MB) files to be uploaded to the
server, set the maxRequestLength value to 11264,
meaning that the application allows files that are up to
11000 KB to be uploaded to the server.
Making this change in the machine.config file applies
this setting to all the applications that are on the server. Ifyou want to apply this to only the application you are
working with, then apply this node to the web.config file
of your application, overriding any setting that is in the
machine.config file. Make sure this node res ides
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between the nodes in the configuration
file.
Another setting involved in the size limitation of files to be
uploaded is the value given to the executionTimeout
attribute in the node.
The value given the executionTimeout attribute is the
number of seconds the upload is allowed to occur beforebeing shut down by ASP.NET. If you are going to allow
large files to be uploaded to the server, you are also going
to want to increase this value along with the
maxRequestLength value.
One negative with increasing the size of a file that can be
uploaded is that there are hackers out there who attack
servers by throwing a large number of requests at them. To
guard against this, you can actually decrease the size of the
files that are allowed to be uploaded; otherwise, you may
find hundreds or even thousands of 10 MB requests hitting
your server.
Controlling Uploaded File Types
There are a several methods you can use to control the
types of files that are uploaded to the server. Unfortunately,
there is no bullet-proof method to protect you from
someone uploading files that would be considered
malicious. You can take a few steps, however, to make this
process of allowing end users to upload files a little moremanageable.
One nice method you can employ is to use the ASP.NET
validation controls that are provided for free with ASP.NET.
These controls enable you to do a regular-expression
check upon the file that is being uploaded to see if the
extension of the file is one you permit to be uploaded.
This is ideal for browsers that allow client-side use of the
validation controls because it forces the checking to be
done on the client; the file is not uploaded to the server ifthe signature isn't one you allow. Listing 2 shows you an
example of using validation controls to accomplish this
task.
Note The use of validation controls is not
explained here. Take a look at Validating
ASP.NET Server Controls for a complete
explanation of validation controls and how to
use them in your ASP.NET pages.
Listing 2: Using validation controls to restrict the types
of files uploaded to the server
VB
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Sub SubmitButton_Click(Source As Object, e As
EventArgs)
If Not (File1.PostedFile Is Nothing) Then
Try
File1.PostedFile.SaveAs("C:\Uploads\
uploadedfile.txt")
Span1.InnerHtml = "Upload Successful
!"
Catch ex As Exception
Span1.InnerHtml = "Error saving file
C:\\" & _
File1.Value & "
" & ex.ToString()
End Try
End If
End Sub
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C#
void SubmitButton_Click(Object sender, EventA
rgs e) {
if (File1.PostedFile != null) {
try {
File1.PostedFile.SaveAs("C:\\Uploads
\\uploadedfile.txt");
Span1.InnerHtml = "Upload Successful
!";
}
catch (Exception ex) {
Span1.InnerHtml = "Error saving file
C:\\" +
File1.Value + "
" + ex.ToString();
}
}
}
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This simple ASP.NET page uses validation controls so that
the end user can only upload .mp3, .mpeg, or .m3u files
to the server. If the file type is not one these three choices,
a Validation control throws an exception onto the screen.
This is shown in Figure 4.
Figure 4. Validating the file type using validation
controls
Using validation controls is not a foolproof way of
controlling the files that are uploaded to the server. It
wouldn't be too hard for someone to change the file
extension of a file so it would be accepted and uploaded to
the server, thereby bypassing this simple security model.
Uploading Multiple Files at the Same Time
So far, you have seen some good examples of how to
upload a file to the server without much hassle. Now let's
take a look at how to upload multiple files to the server
from a single page.
No built-in capabilities in the Microsoft .NET Framework
enable you to upload multiple files from a single ASP.NETpage. With a little work, however, you can easily accomplish
this task.
One trick is to import the System.IO class into your
ASP.NET a e, and to then use the
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HttpFileCollection class to capture all the files that
are sent in with the Request object. This approach enables
you to upload as many files as you want from a single
page.
For this example, you can build an ASP.NET page that has
four file-input boxes and one Submit button. After the user
clicks the Submit button and the files are posted to the
server, the code behind takes the files and saves them to a
specific location on the server. After the files are saved, the
file information that was posted is displayed in the ASP.NET
page (see Listing 3).
Listing 3: Uploading multiple files to the server
VB
Sub SubmitButton_Click(Source As Object, e As
EventArgs)
Dim filepath As String = "C:\Uploads"
Dim uploadedFiles As HttpFileCollection =
Request.Files
Dim i As Integer = 0
Do Until i = uploadedFiles.CountDim userPostedFile As HttpPostedFile =
uploadedFiles(i)
Try
If (userPostedFile.ContentLength > 0
) Then
Span1.InnerHtml += "File #" &
(i+1) & "
"Span1.InnerHtml += "File Content
Type: " & _
userPostedFile.ContentType & "
"Span1.InnerHtml += "File Size: "
& _
userPostedFile.ContentLength &
"kb
"Span1.InnerHtml += "File Name: "
& _
userPostedFile.FileName & ""
userPostedFile.SaveAs(filepath &
"\" & _
Path.GetFileName(userPostedFil
e.FileName))
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Span1.InnerHtm += Locat on w er
e saved: " & _
filepath & "\" & _
Path.GetFileName(userPostedFil
e.FileName) & _
"
"
End If
Catch ex As Exception
Span1.InnerHtml += "Error:
" & ex.Message
End Try
i += 1
Loop
End Sub
Select File1:
Select File2:
Select File3:
Select File4:
C#
" "
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=
protected void SubmitButton_Click(Object send
er, EventArgs e){
string filepath = "C:\\Uploads";
HttpFileCollection uploadedFiles = Request
.Files;
for (int i = 0; i < uploadedFiles.Count; i
++){
HttpPostedFile userPostedFile = uploade
dFiles[i];
try
{
if (userPostedFile.ContentLength > 0
)
{
Span1.InnerHtml += "File #" +
(i+1) +"
";Span1.InnerHtml += "File Content
Type: " +
userPostedFile.ContentType + "
";Span1.InnerHtml += "File Size: "
+
userPostedFile.ContentLength +
"kb
";Span1.InnerHtml += "File Name: "
+
userPostedFile.FileName + "";
userPostedFile.SaveAs(filepath +
"\\" +
Path.GetFileName(userPostedFil
e.FileName));
Span1.InnerHtml += "Location wher
e saved: " +
filepath + "\\" +
Path.GetFileName(userPostedFil
e.FileName) +
"
";
}
}
catch (Exception Ex)
{
Span1.InnerText += "Error:
" + Ex.Message;
}
}
}
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Select File1:
Select File2:
Select File3:
Select File4:
The end user can select up to four files and click the
Upload Files button, which initializes the
SubmitButton_Click event. Using the
HttpFileCollection class with the Request.Files
property lets you gain control over all the files that are
uploaded from the page. When the files are in this state,
you can do whatever you want with them. In this case, thefiles' properties are examined and written to the screen. In
the end, the files are saved to the Uploads folder in the
root directory of the server. The result of this action is
illustrated in Figure 5.
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Figure 5. Uploading four files at once to the server from
a single ASP.NET page
As you may have noticed, one interesting point about this
example is that the states of the file input text boxes are
not saved with the postback. You can see this in Figure 5. In
ASP.NET, the state of the file-input text boxes cannot be
saved because doing so might pose a security risk.
ConclusionThe File Field control provided by ASP.NET is a powerful
control that was quite difficult to achieve in the days of
Active Server Pages 3.0. This new capability allows your
end-users to upload one or more files to your server.
Remember, you can control the size of the files that are
uploaded by working with settings in either the
machine.config or web.config file.
Related Books
ASP.NET Professional Secrets
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