How can I determine the length (i.e. duration) of a .wav file in C#?

You may consider using the mciSendString(...) function (error checking is omitted for clarity):

using System;
using System.Text;
using System.Runtime.InteropServices;

namespace Sound
{
    public static class SoundInfo
    {
        [DllImport("winmm.dll")]
        private static extern uint mciSendString(
            string command,
            StringBuilder returnValue,
            int returnLength,
            IntPtr winHandle);

        public static int GetSoundLength(string fileName)
        {
            StringBuilder lengthBuf = new StringBuilder(32);

            mciSendString(string.Format("open \"{0}\" type waveaudio alias wave", fileName), null, 0, IntPtr.Zero);
            mciSendString("status wave length", lengthBuf, lengthBuf.Capacity, IntPtr.Zero);
            mciSendString("close wave", null, 0, IntPtr.Zero);

            int length = 0;
            int.TryParse(lengthBuf.ToString(), out length);

            return length;
        }
    }
}

Not to take anything away from the answer already accepted, but I was able to get the duration of an audio file (several different formats, including AC3, which is what I needed at the time) using the Microsoft.DirectX.AudioVideoPlayBack namespace. This is part of DirectX 9.0 for Managed Code. Adding a reference to that made my code as simple as this...

Public Shared Function GetDuration(ByVal Path As String) As Integer
    If File.Exists(Path) Then
        Return CInt(New Audio(Path, False).Duration)
    Else
        Throw New FileNotFoundException("Audio File Not Found: " & Path)
    End If
End Function

And it's pretty fast, too! Here's a reference for the Audio class.

Download NAudio.dll from the link https://www.dll-files.com/naudio.dll.html

and then use this function

public static TimeSpan GetWavFileDuration(string fileName)       
{     
    WaveFileReader wf = new WaveFileReader(fileName);
    return wf.TotalTime; 
}

you will get the Duration

I had difficulties with the example of the MediaPlayer-class above. It could take some time, before the player has opened the file. In the "real world" you have to register for the MediaOpened-event, after that has fired, the NaturalDuration is valid. In a console-app you just have to wait a few seconds after the open.

using System;
using System.Text;
using System.Windows.Media;
using System.Windows;

namespace ConsoleApplication2
{
  class Program
  {
    static void Main(string[] args)
    {
      if (args.Length == 0)
        return;
      Console.Write(args[0] + ": ");
      MediaPlayer player = new MediaPlayer();
      Uri path = new Uri(args[0]);
      player.Open(path);
      TimeSpan maxWaitTime = TimeSpan.FromSeconds(10);
      DateTime end = DateTime.Now + maxWaitTime;
      while (DateTime.Now < end)
      {
        System.Threading.Thread.Sleep(100);
        Duration duration = player.NaturalDuration;
        if (duration.HasTimeSpan)
        {
          Console.WriteLine(duration.TimeSpan.ToString());
          break;
        }
      }
      player.Close();
    }
  }
}