Error Handling Strategy for Sandboxed SharePoint Solutions

Question

I am looking to develop an error handling strategy for a SharePoint solution that makes use of sandboxed webparts. I was initially looking at a general exception handling approach based on this article, but this doesn't work for sandboxed webparts. Once an unhandled exception has been thrown in the sandbox, the user code service appears to take control, so that the exception handling in the base web part isn't reached. Are there any established error handling approaches for sandboxed solutions?

Is anyone aware of a method of determining when an unhandled exception has been thrown in a sandboxed webpart, if only to change the displayed error message to a more user friendly message? I would like to replace the standard "Web Part Error: Unhandled exception was thrown by the sandboxed code wrapper's Execute method in the partial trust app domain: An unexpected error has occurred." message at very least.

Thanks, MagicAndi.

Answer 1

Actually, you can follow the approach suggested by the article you mentioned. You just have to provide safe overridables for all virtual properties and methods your descendant web parts are going to override. The patter can be described:

1. Override and seal every virtual property and method supposed to be overriden with code that can throw an exception.
2. Create a virtual counterpart of the overridable with the same prototype and call the base class from it if necessary. This is supposed to be overriden by your descendants.
3. Call the new overridable from the sealed member in a try&catch and remember the exception if caught there.
4. Rendering method either renders the usual content or the remembered error message.

This is a torso of the base class I use:

public class ErrorSafeWebPart : WebPart {

    #region Error remembering and rendering

    public Exception Error { get; private set; }

    // Can be used to skip some code later that needs not
    // be performed if the web part renders just the error.
    public bool HasFailed { get { return Error != null; } }

    // Remembers just the first error; following errors are
    // usually a consequence of the first one.
    public void RememberError(Exception error) {
        if (Error != null)
            Error = error;
    }

    // You can do much better error rendering than this code...
    protected virtual void RenderError(HtmlTextWriter writer) {
        writer.WriteEncodedText(Error.ToString());
    }

    #endregion

    #region Overriddables guarded against unhandled exceptions

    // Descendant classes are supposed to override the new DoXxx
    // methods instead of the original overridables They should
    // not catch exceptions and leave it on this class.

    protected override sealed void CreateChildControls() {
        if (!HasFailed)
            try {
                DoCreateChildControls();
            } catch (Exception exception) {
                RememberError(exception);
            }
    }

    protected virtual void DoCreateChildControls()
    {}

    protected override sealed void OnInit(EventArgs e) {
        if (!HasFailed)
            try {
                DoOnInit(e);
            } catch (Exception exception) {
                RememberError(exception);
            }
    }

    protected virtual void DoOnInit(EventArgs e) {
        base.OnInit(e);
    }

    // Continue similarly with OnInit, OnLoad, OnPreRender, OnUnload 
    // and/or others that are usually overridden and should be guarded.

    protected override sealed void RenderContents(HtmlTextWriter writer) {
        // Try to render the normal contents if there was no error.
        if (!HasFailed)
            try {
                DoRenderContents(writer);
            } catch (Exception exception) {
                RememberError(exception);
            }
        // If an error occurred in any phase render it now.
        if (HasFailed)
            RenderError(writer);
    }

    protected virtual void DoRenderContents(HtmlTextWriter writer) {
        base.RenderContents(writer);
    }

    #endregion
}

--- Ferda