UI doesn’t like to be informed about anything from a different thread: you’ll get “System.InvalidOperationException: Cross-thread operation not valid: Control ‘xxx’ accessed from a thread other than the thread it was created on.” exception from WinForms control, if you try:

public void ShowStatus(ApplicationStatus status)
{
    this._lblServiceAddress.Text = "Connected to: "
        + status.WebServiceAddress;
    this._lblUserId.Text = "Working as: "
        + status.UserId;
}

The easiest sollution is to use BeginInvoke method on the control Control or Form:

public void ShowStatus(ApplicationStatus status)
{
    this.BeginInvoke((MethodInvoker)(() =>
        {
            this._lblServiceAddress.Text = "Connected to: "
                + status.WebServiceAddress;
            this._lblUserId.Text = "Working as: "
                + status.UserId;
        }));
}

Well, it’s fun to write this once, but if you have many operations done in background sooner or later you’d like to have something nicer. Like an attribute for example:

[ThreadAccessibleUI]
public void ShowStatus(ApplicationStatus status)
{
    this._lblServiceAddress.Text = "Connected to: " + status.WebServiceAddress;
    this._lblUserId.Text = "Working as: " + status.UserId;
}

Here’s the attribute implementation of such attribute using PostSharp 1.5:

/// <summary>
/// PostSharp attribute.
/// Use it to mark Control's methods that
/// are invoked from background thread.
/// </summary>
/// <remarks>
/// Be careful as BeginInvoke uses the message queue.
/// This means that the interface will be refreshed
/// when application has a chance to process its messages.
/// </remarks>
[AttributeUsage(AttributeTargets.Method)]
[Serializable] // required by PostSharp
public class ThreadAccessibleUIAttribute : OnMethodInvocationAspect
{
    public override void OnInvocation(
        MethodInvocationEventArgs eventArgs)
    {
        Control control = eventArgs.Instance as Control;
        if (control == null)
            throw new ApplicationException(
                "ThreadAccessibleUIAttribute" +
                "can be applied only to methods on Control class");

        // The form may be closed before
        // this method is called from another thread.
        if (control.Created == false)
            return;

        control.BeginInvoke((MethodInvoker)eventArgs.Proceed);
    }
};