Principal.cs

using System;
using System.Windows.Forms;
using QuartzTypeLib;

namespace R812ReproducirVideo
{
    public partial class Principal : Form
    {
        // Constante del estilo de ventana:
        private const int WS_CHILD = 0x40000000;
        private const int WS_CHIPCHILDREN = 0x000000;

        // Referencia a la interfaz de control multimedia>
        private IMediaControl mc = null;

        // Referncia a la ventana de vídeo sobre el formulario:
        private IVideoWindow ventanaVideo = null;

        public Principal()
        {
            InitializeComponent();
        }

        private void btnReproducirVideo_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Filter = "Media Files|*.mpg;*.avi;*.wma;*.mov;*.wav;*.mp3;*.mp4|All Files|*.*";

            if (DialogResult.OK == ofd.ShowDialog())
            {
                if (mc != null)
                {
                    mc.Stop();
                }

                FilgraphManager grapManager = new FilgraphManager();
                grapManager.RenderFile(ofd.FileName);

                try
                {
                    ventanaVideo = (IVideoWindow) grapManager;
                    ventanaVideo.Owner = (int) pbxVideo.Handle;
                    ventanaVideo.WindowStyle = WS_CHILD | WS_CHIPCHILDREN;

                    ventanaVideo.SetWindowPosition(
                        pbxVideo.ClientRectangle.Left,
                        pbxVideo.ClientRectangle.Top,
                        pbxVideo.ClientRectangle.Width,
                        pbxVideo.ClientRectangle.Height
                    );
                }
                catch
                {
                    // Gestión eventual de errores.
                }

                mc = (IMediaControl) grapManager;
                mc.Run();
            }
        }

        private void pbxVideo_SizeChanged(object sender, EventArgs e)
        {
            if (ventanaVideo != null)
            {
                try
                {
                    ventanaVideo.SetWindowPosition(
                        pbxVideo.ClientRectangle.Left,
                        pbxVideo.ClientRectangle.Top,
                        pbxVideo.ClientRectangle.Width,
                        pbxVideo.ClientRectangle.Height
                    );
                }
                catch
                {
                    // Gestión eventual de errores.
                }
            }
        }
    }
}