WPF Controls has no ‘InvokeRequired’/’Invoke’ like WinForms, so making accessing a WPF Window/Control threadsafe can be accomplished by using the System.Windows.Threading.Dispatcher. Here are some examples. First a property: 

public int Progress
{
	get
	{
		if (!Dispatcher.CheckAccess())
		{
			Func<int> f = delegate () { return Progress; };
			return Dispatcher.Invoke(f);
		}

		...
	}
	set
	{
		if (!Dispatcher.CheckAccess())
		{
			Action<int> a = delegate (int progress) { Progress = progress; };
			Dispatcher.Invoke(a);
			return;
		}

		...
	}
}

  A void method

public void SetProgress(object obj, int progress)
{
	if (!Dispatcher.CheckAccess())
	{
		Action<object, int> a = new Action<object, int>(SetProgress);
		Dispatcher.Invoke(a, DispatcherPriority.Normal, obj, progress);
		return;
	}

	...
}

And lastly a method with a return value

public bool AllProcessesDone()
{
	if (!Dispatcher.CheckAccess())
	{
		Func<bool> f = new Func<bool>(AllProcessesDone);
		return Dispatcher.Invoke(f, DispatcherPriority.Normal);
	}

	...
}

As always, feel free to comment, or ask.