ASP.NET - File Uploading

ASP.Net has two controls that allow the users to upload files to the web server. Once the server receives the posted file data, the application can save it, check it or ignore it. The following controls allow the file uploading:

HtmlInputFile - an HTML server control
FileUpload - and ASP.Net web control

Both the controls allow file uploading, but the FileUpload control automatically sets the encoding of the form, whereas the HtmlInputFile does not do so.

In this tutorial, we will use the FileUpload control. The FileUpload control allows the user to browse for and select the file to be uploaded, providing a Browse button and a text box for entering the filename.

Once, the user has entered the filename in the text box, by typing the name or browsing, the SaveAs method of the FileUpload control can be called to save the file to the disk.

The basic syntax for using the FileUpload is:

<asp:FileUpload ID= "Uploader" runat = "server" />

The FileUpload class is derived from the WebControl class, and inherits all its members. Apart from those, the FileUpload class has the following read-only properties:

Properties	Description
FileBytes	Returns an array of the bytes in a file to be uploaded..
FileContent	Returns the stream object pointing to the file to be uploaded.
FileName	Returns the name of the file to be uploaded.
HasFile	Specifies whether the control has a file to upload.
PostedFile	Returns a reference to the uploaded file.

The posted file is encapsulated in an object of type HttpPostedFile, which could be accessed through the PostedFile property of the FileUpload class.

The HttpPostedFile class has the following important properties, which are much used:

Properties	Description
ContentLength	Returns the size of the uploaded file in bytes.
ContentType	Returns the MIME type of the uploaded file
FileName	Returns the full filename.
InputStream	Returns a stream object pointing to the uploaded file.

Example:

The following example demonstrates the FileUpload control and its properties. The form has a FileUpload control along with a save button and a label control for displaying the file name, file type and file length.

In the design view, the form looks like:

The content file:

<body>
<form id="form1" runat="server">
<div>
<h3> File Upload:</h3>
<br />
   <asp:FileUpload ID="FileUpload1" runat="server" />
   <br /><br />
   <asp:Button ID="btnsave" runat="server" 
               onclick="btnsave_Click" Text="Save" 
               style="width:85px" />
   <br /><br />
   <asp:Label ID="lblmessage" runat="server" />
</div>
</form>
</body>

The code behind the save button:

protected void btnsave_Click(object sender, EventArgs e)
{
   StringBuilder sb = new StringBuilder();
   if (FileUpload1.HasFile)
   {
      try
      {
      sb.AppendFormat(" Uploading file: {0}", 
                                  FileUpload1.FileName);
      //saving the file
      FileUpload1.SaveAs("<c:\\SaveDirectory>" + 
                                  FileUpload1.FileName);
      //Showing the file information
      sb.AppendFormat("<br/> Save As: {0}", 
                         FileUpload1.PostedFile.FileName);
      sb.AppendFormat("<br/> File type: {0}",   
                         FileUpload1.PostedFile.ContentType);
      sb.AppendFormat("<br/> File length: {0}", 
                         FileUpload1.PostedFile.ContentLength);
      sb.AppendFormat("<br/> File name: {0}", 
                         FileUpload1.PostedFile.FileName);
      }
      catch (Exception ex)
      {
      sb.Append("<br/> Error <br/>");
      sb.AppendFormat("Unable to save file <br/> {0}", 
                         ex.Message);
      }
   }
   else
   {
      lblmessage.Text = sb.ToString();
   }
}

Note the following:

The StringBuilder class is derived from System.IO namespace, so it should be included.
The try and catch blocks are used for catching errors, and display the error message.

Wednesday, 29 January 2014