How to Capture File Upload Errors with RadAsyncUpload Custom Handler

Problem

How to throw errors that will be recognized by the "OnClientFileUploadFailed" handler when using a custom handler.

Description

The server returns a status code of 500 and a message similar to "HTTP Error code is: 500" or something like [HttpWebRequest_WebException_RemoteServer] in args.get_message(). This happens because uncaught exceptions trigger the standard ASP.NET error page mechanism and bury any exception info (if passed) in an HTML page in a response that expects JSON.

Solution

You can work around this by following these steps. You can find attached a small example below the list.

1. Create a custom upload result class that has a field for the exception.

CustomErrorAsyncUploadResult.cs

C#

public class CustomErrorAsyncUploadResult : IAsyncUploadResult
{
    public CustomErrorAsyncUploadResult()
    {
 
    }
 
    public string CustomError
    {
        get;
        set;
    }
 
    public long ContentLength
    {
        get;
        set;
    }
 
    public string ContentType
    {
        get;
        set;
    }
 
    public string FileName
    {
        get;
        set;
    }
}

2. Catch the exception in the handler and populate the custom field.

myHandler.ashx.cs

C#

public class myHandler : AsyncUploadHandler
{
    protected override IAsyncUploadResult Process(UploadedFile file, HttpContext context,  IAsyncUploadConfiguration configuration, string tempFileName)
    {
        CustomErrorAsyncUploadResult result =  CreateDefaultUploadResult<CustomErrorAsyncUploadResult>(file);
        try
        {
            throw new Exception("file did not validate/upload for <insert reason here>");
        }
        catch (Exception ex)
        {
            //for example, remove some information, make sure to add the custom field for  the error
            result.ContentLength = 0;
            result.ContentType = "";
            result.CustomError = ex.Message;
        }
        return result;
    }
}

3. Return an imitation of success to the client and handle the OnClientFileUploaded event.

ASPX

<telerik:RadAsyncUpload runat="server" ID="RadAsyncUpload1" HttpHandlerUrl="~/myHandler.ashx" OnClientFileUploaded="OnClientFileUploaded" OnClientFileUploadFailed="OnClientFileUploadFailed"></telerik:RadAsyncUpload>

4. Check for the presence of an error and if so - invalidate the file row.

JavaScript

function OnClientFileUploaded(sender, args) {
    var info = args.get_fileInfo();
    if (info.CustomError != null) {
        var row = args.get_row();
        sender._updateRowImage(row, 'fail');
        sender._updateCancelButton(row);
        $telerik.$(".ruFileProgressWrap", row).remove();
 
        alert(info.CustomError);
    }
}

5. Handle the OnClientFileUploadFailed event just in case to capture any errors.

JavaScript

function OnClientFileUploadFailed(sender, args) {
    args.set_handled(true);
}

See Also

• How to Create a Custom Handler for RadAsyncUpload

• Ensure Security - AllowedCustomMetaDataTypes