How to have the cancel button enabled during an upload process

Actually my aim to enable a cancel button while this upload takes place.I tried raising an event which makes the cancel button enabled while the upload process is going on.

But I am not able to click on the cancel button, i dont get the access to click it.Once the process is completed i am able to click the button to generate the event.

Does any one know how this can be done in C#(Windows application)

Sample code is :

Code:

while (bytesRead > 0) 
{ 

if (iteration == 1) 
this._startTime = DateTime.Now; 

bytesWritten = this._workFlowProxy.UploadChunk(this._taskId, buffer, sentBytes, bytesRead); 
if (iteration == LoadManagerConstants.AverageSample + 1) //AverageSample = 5 
{ 
timeForInitialChunks = (long)DateTime.Now.Subtract(this._startTime).TotalMilliseconds; 
averageChunkTime = System.Math.Max 
(1, timeForInitialChunks / LoadManagerConstants.AverageSample); 

//Maximum size that can be uploaded at a time cannot be more than 4KB 
chunkSize = (int)System.Math.Min(LoadManagerConstants.MaxFileSize, 
chunkSize * preferredTransferDuration / averageChunkTime); 

buffer = new byte[chunkSize]; 
} 
sentBytes += bytesRead; 
percentageOfTotalSize = (int)(((decimal)sentBytes / (decimal)fileSize) * 100); 
this.SetProgressStatus(percentageOfTotalSize); 
bytesRead = fileToUpLoad.Read(buffer, LoadManagerConstants.Offset, chunkSize); 
iteration++; 
}