I tried searching for a simple file upload control for silverlight 4 and came across some excellent multi-file upload controls specially the one from codeplex "http://silverlightfileupld.codeplex.com/ ", but that is not what I needed due to complexity or the difficulty of code editing or changing to adjust according to my project .So I began in my personal file uploader as the following,

1 - Open file dialogue to get the file

SQL
public OpenFileDialog fileDialog = null;
fileDialog.Multiselect = false; // one file at a timefileDialog.Filter = ""; // Add file filter

2 - WCF web service to write the file to the server with the following function

Code
[OperationContract]
Code
public void SaveFile(UploadFile UploadFile)
{
       	FileStream FileStream = new FileStream("[WhateverPathWithPermission]" + UploadFile.FileName, FileMode.Append);
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    	FileStream.Write(UploadFile.File, 0, UploadFile.File.Length);
      	FileStream.Close();
      	FileStream.Dispose();
}

3 - Add to WCF service a class contract to handle segment blocks transfer between client and server.

Code
[DataContract]
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public class UploadFile
{
	[DataMember]
      	public string FileName;
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       	[DataMember]
       	public byte[] File;
}

4 - Handle file blocks server transfer.

Code
 proxy.SaveFileCompleted += new EventHandler<System.ComponentModel.AsyncCompletedEventArgs>(proxy_SaveFileCompleted);
if (fileDialog.File != null)
{
 Stream strm = fileDialog.File.OpenRead(); // the file dialog you created in step 1
 Buffer = new byte[strm.Length];
 strm.Read(Buffer, 0, (int)strm.Length);
 strm.Dispose();
 strm.Close();
 file = new MainService.UploadFile(); // use service client proxy
 file.FileName = fileDialog.File.Name;
 byte[] bufferSegment = new byte[15000];
 segments = (Buffer.Length / bufferSegment.Length);
 remainder = (Buffer.Length % bufferSegment.Length);
 segIndex = 0;
 if (Buffer.Length > 15000)
 {
 saveFileSegment(segIndex);
 segIndex++; // for the index of the segment to transfer.
 fileIsFinished = false; // to detect if all file transfer segments is finished or not.
 }
 else
 {
 file.File = Buffer;
 proxy.SaveFileAsync(file, false);
 fileIsFinished = true;
 }
}
//On complete of segment transfer
void proxy_SaveFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
{
 // check if end of file.
 if (fileIsFinished == false)
 {
 if (segIndex < segments)
 {
 saveFileSegment(segIndex);
 segIndex++;
 }
 else
 {
 byte[] bufferSegment = new byte[remainder];
 for (int i = 0; i < remainder; i++)
 {
 bufferSegment[i] = Buffer[15000 * segments + i];
 }
 file.File = bufferSegment;
 proxy = new MainService.MainServiceClient();
 proxy.SaveFileCompleted += new EventHandler<System.ComponentModel.AsyncCompletedEventArgs>(proxy_SaveFileCompleted);
 proxy.SaveFileAsync(file);
 fileIsFinished = true;
 }
 }
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}
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/// function to safe file segment part.
C#
public void saveFileSegment(int segmentIndex)
{
    byte[] bufferSegment = new byte[15000];
    for (int i = 0; i < 15000; i++)
    {
        bufferSegment[i] = Buffer[15000 * segmentIndex + i];
    }
    file.File = bufferSegment;
proxy = new MainService.MainServiceClient();
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proxy.SaveFileCompleted += new EventHandler<System.ComponentModel.AsyncCompletedEventArgs>(proxy_SaveFileCompleted);
proxy.SaveFileAsync(file);
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}