GraphicsDeviceHelpers.cs

    public static class GraphicsDeviceHelpers
    {
        struct TargetInfo
        {
            public RenderTargetBinding[] Targets;
            public int TargetCount;
        }

        private static int _current;
        private static readonly TargetInfo[] _lastTargets = new TargetInfo[10];

        static public void ResetRenderTargetStack(this GraphicsDevice device)
        {
            for (var i = 0; i < _current; i++)
            {
#if MONOGAME
                Array.Clear(_lastTargets[i].Targets, 0, 4);
#else
                _lastTargets[i].Targets = null;
#endif         
            }

            _current = 0;
        }

        /// <summary>
        /// Save the currently set render targets.
        /// </summary>
        static public void PushRenderTargets(this GraphicsDevice device)
        {
#if MONOGAME
            if (_lastTargets[_current].Targets == null)
                _lastTargets[_current].Targets = new RenderTargetBinding[4];
            device.GetRenderTargets(_lastTargets[_current].Targets, out _lastTargets[_current].TargetCount);
#else
            _lastTargets[_current].Targets = device.GetRenderTargets();
#endif
            ++_current;
        }

        /// <summary>
        /// Restore the last stored render targets.
        /// </summary>
        static public void PopRenderTargets(this GraphicsDevice device)
        {
            --_current;
            if (_current < 0)
                throw new Exception("The render target stack was unbalanced!");

#if MONOGAME
            device.SetRenderTargets(_lastTargets[_current].Targets, _lastTargets[_current].TargetCount);
            Array.Clear(_lastTargets[_current].Targets, 0, 4);
#else
            device.SetRenderTargets(_lastTargets[_current].Targets);
            _lastTargets[_current].Targets = null;
#endif
        }
    }