Get System.Timers.Timer to fire on a WPF Window's thread using the Timer's SynchronizingObject

You need to create an ISynchronizeInvoke for the dispatcher. Example.

    public class SynchronizeInvoke : ISynchronizeInvoke
    {
        private readonly System.Windows.Threading.Dispatcher dispatcher;

        public SynchronizeInvoke(System.Windows.Threading.DispatcherObject dObj)
            : this(dObj.Dispatcher)
        { }

        public SynchronizeInvoke()
            : this(Application.Current.Dispatcher)
        { }

        public SynchronizeInvoke(System.Windows.Threading.Dispatcher dispatcher)
        {
            this.dispatcher = dispatcher;
        }

        public bool InvokeRequired => !dispatcher.CheckAccess();

        private class AsyncResult : IAsyncResult
        {
            public readonly System.Windows.Threading.DispatcherOperation operation;
            public readonly IAsyncResult asyncResult;

            public AsyncResult(System.Windows.Threading.DispatcherOperation operation)
            {
                this.operation = operation;
                asyncResult = operation.Task;
            }

            public object? AsyncState => asyncResult.AsyncState;

            public WaitHandle AsyncWaitHandle => asyncResult.AsyncWaitHandle;

            public bool CompletedSynchronously => asyncResult.CompletedSynchronously;

            public bool IsCompleted => asyncResult.IsCompleted;
        }

        public IAsyncResult BeginInvoke(Delegate method, object?[]? args) => new AsyncResult(dispatcher.BeginInvoke(method, args));

        public object? EndInvoke(IAsyncResult result)
        {
            AsyncResult asyncResult = (AsyncResult)result;
            asyncResult.operation.Task.Wait();
            return asyncResult.operation.Result;
        }

        public object? Invoke(Delegate method, object?[]? args)
            => dispatcher.Invoke(method, args);
    }

Usage:

timer.SynchronizingObject = new SynchronizeInvoke();
// or
timer.SynchronizingObject = new SynchronizeInvoke(this.Dispatcher);
// or
timer.SynchronizingObject = new SynchronizeInvoke(this);

P.S. You can simplify the implementation of SynchronizeInvoke by using undocumented behavior. In practice, this implementation works flawlessly.

    public class SynchronizeInvoke : ISynchronizeInvoke
    {
        private readonly System.Windows.Threading.Dispatcher dispatcher;

        public SynchronizeInvoke(System.Windows.Threading.DispatcherObject dObj)
            : this(dObj.Dispatcher)
        { }

        public SynchronizeInvoke()
            : this(Application.Current.Dispatcher)
        { }

        public SynchronizeInvoke(System.Windows.Threading.Dispatcher dispatcher)
        {
            this.dispatcher = dispatcher;
        }

        public bool InvokeRequired => !dispatcher.CheckAccess();

        public IAsyncResult BeginInvoke(Delegate method, object?[]? args) => dispatcher.BeginInvoke(method, args).Task; 

        public object? EndInvoke(IAsyncResult result)
        {
            Task<object> task = (Task<object>)result;
            task.Wait();
            return task.Result;
        }

        public object? Invoke(Delegate method, object?[]? args)
            => dispatcher.Invoke(method, args);
    }