c# - 如何使用 WPF 中的 MediaPlayer 编写一个方法来打开、开始播放，然后返回音频文件的持续时间？

Question

下面很明显我想做什么，但是我得到以下异常:

Unable to return a TimeSpan property value for a Duration value of 'Automatic'.

看到那篇我很气馁

NaturalDuration cannot be determined until after MediaOpened has occurred. (link)

这是否意味着我必须想出一个人为的方法来打开文件，在单独的线程中等待媒体打开事件，然后仅在事件触发后才返回持续时间？

    public static int PlayAudio(string fileName)
    {
        try
        {
            myMediaPlayer.Stop();
            myMediaPlayer.Close();
            myMediaPlayer.Open(new Uri(filename));
            myMediaPlayer.Play();
            return myMediaPlayer.NaturalDuration.TimeSpan.Milliseconds;
        }
        catch (Exception e)
        {
            MessageBox.Show(e.Message);
            return -1;
        }
    }

Answer 1

I have to come up with a contrived method to open the file, wait for the media opened event in a separate thread, then return the duration only after the event has fired

不幸的是，这正是它的意思。以下是我为此想到的:

using System;
using System.Threading;
using System.Windows.Media;
using System.Windows.Threading;

class Program
{
    static void Main(string[] args)
    {
        var mediaFile = @"c:\path\to\mediafile.mp3";
        var duration = GetMediaDuration(mediaFile, TimeSpan.FromMilliseconds(500));
        Console.WriteLine(duration.ToString());
        Console.ReadKey();
    }

    static TimeSpan GetMediaDuration(string mediaFile)
    {
        return GetMediaDuration(mediaFile, TimeSpan.Zero);
    }

    static TimeSpan GetMediaDuration(string mediaFile, TimeSpan maxTimeToWait)
    {
        var mediaData = new MediaData() {MediaUri = new Uri(mediaFile)};

        var thread = new Thread(GetMediaDurationThreadStart);
        DateTime deadline = DateTime.Now.Add(maxTimeToWait);
        thread.Start(mediaData);

        while (!mediaData.Done && ((TimeSpan.Zero == maxTimeToWait) || (DateTime.Now < deadline)))
            Thread.Sleep(100);

        Dispatcher.FromThread(thread).InvokeShutdown();

        if (!mediaData.Done)
            throw new Exception(string.Format("GetMediaDuration timed out after {0}", maxTimeToWait));
        if (mediaData.Failure)
            throw new Exception(string.Format("MediaFailed {0}", mediaFile));

        return mediaData.Duration;
    }

    static void GetMediaDurationThreadStart(object context)
    {
        var mediaData = (MediaData) context;
        var mediaPlayer = new MediaPlayer();

        mediaPlayer.MediaOpened += 
            delegate 
                {
                    if (mediaPlayer.NaturalDuration.HasTimeSpan)
                        mediaData.Duration = mediaPlayer.NaturalDuration.TimeSpan; 
                    mediaData.Success = true; 
                    mediaPlayer.Close();
                };

        mediaPlayer.MediaFailed += 
            delegate
                {
                    mediaData.Failure = true;
                    mediaPlayer.Close();
                };

        mediaPlayer.Open(mediaData.MediaUri);

        Dispatcher.Run();
    }
}

class MediaData
{
    public Uri MediaUri;
    public TimeSpan Duration = TimeSpan.Zero;
    public bool Success;
    public bool Failure;
    public bool Done { get { return (Success || Failure); } }
}