The extern keyword also can define an external assembly alias, making it possible to reference different versions of the same component from within a single assembly. For more information, see extern alias (C# Reference).

It is an error to use the abstract (C# Reference) and extern modifiers together to modify the same member. Using the extern modifier means that the method is implemented outside the C# code, while using the abstract modifier means that the method implementation is not provided in the class.

The extern keyword is more limited in use than in C++. To compare with the C++ keyword, see Using extern to Specify Linkage in the C++ Language Reference.

using System;
using System.Runtime.InteropServices;
class MainClass 
{
    [DllImport("User32.dll")]
    public static extern int MessageBox(int h, string m, string c, int type);

    static int Main() 
    {
        string myString; 
        Console.Write("Enter your message: ");
        myString = Console.ReadLine();
        return MessageBox(0, myString, "My Message Box", 0);
    }
}

Example 2

// cmdll.c
// compile with: /LD /MD
int __declspec(dllexport) SampleMethod(int i)
{
    return i*10;
}

Example 3

// cm.cs
using System;
using System.Runtime.InteropServices;
public class MainClass 
{
    [DllImport("Cmdll.dll")]
    public static extern int SampleMethod(int x);
    static void Main() 
    {
        Console.WriteLine("SampleMethod() returns {0}.", 
                SampleMethod(5));
    }
}

Output

SampleMethod() returns 50.

Remarks

C# Language Specification

说的真是太好了，这个我就不说了。

            while (true)
            {
                Console.WriteLine("Please enter Frequency of the sounds:");
                int freq = Convert.ToInt32(Console.ReadLine());
                Console.WriteLine("Please duration of the sound, in milliseconds:");
                int dur = Convert.ToInt32(Console.ReadLine());
                Beep(freq, dur);
            }

去感受一下声音吧。

其实 .net里面Console下面已经有了Beep函数了哈，嘿嘿 ， 没有早告诉点， 不然

不会有这么多麻烦事。

using System;
using System.Collections.Generic;
using System.Text;

namespace SoundPlaying
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Beep();
            while (true)
            {
                Console.WriteLine("Please enter Frequency of the sounds:");
                int freq = Convert.ToInt32(Console.ReadLine());
                Console.WriteLine("Please duration of the sound, in milliseconds:");
                int dur = Convert.ToInt32(Console.ReadLine());
                Console.Beep(freq, dur);
            }
        }
    }
}

和上面的效果一样了哈。

最后使用的API 就是 PlaySound

同样的，我去摘MSDN了（不要砸我啊）

PlaySound

The PlaySound function plays a sound specified by the given filename, resource, or system event. (A system event may be associated with a sound in the registry or in the WIN.INI file.)

BOOL PlaySound(
  LPCSTR pszSound,  
  HMODULE hmod,     
  DWORD fdwSound    
);

Parameters

pszSound

A string that specifies the sound to play. The maximum length, including the null terminator, is 256 characters. If this parameter is NULL, any currently playing waveform sound is stopped. To stop a non-waveform sound, specify SND_PURGE in the fdwSound parameter.

Three flags in fdwSound (SND_ALIAS, SND_FILENAME, and SND_RESOURCE) determine whether the name is interpreted as an alias for a system event, a filename, or a resource identifier. If none of these flags are specified, PlaySound searches the registry or the WIN.INI file for an association with the specified sound name. If an association is found, the sound event is played. If no association is found in the registry, the name is interpreted as a filename.

hmod

Handle to the executable file that contains the resource to be loaded. This parameter must be NULL unless SND_RESOURCE is specified in fdwSound.

fdwSound

Flags for playing the sound. The following values are defined.

Value	Meaning
SND_APPLICATION	The sound is played using an application-specific association.
SND_ALIAS	The pszSound parameter is a system-event alias in the registry or the WIN.INI file. Do not use with either SND_FILENAME or SND_RESOURCE.
SND_ALIAS_ID	The pszSound parameter is a predefined sound identifier.
SND_ASYNC	The sound is played asynchronously and PlaySound returns immediately after beginning the sound. To terminate an asynchronously played waveform sound, call PlaySound with pszSound set to NULL.
SND_FILENAME	The pszSound parameter is a filename.
SND_LOOP	The sound plays repeatedly until PlaySound is called again with the pszSound parameter set to NULL. You must also specify the SND_ASYNC flag to indicate an asynchronous sound event.
SND_MEMORY	A sound event's file is loaded in RAM. The parameter specified by pszSound must point to an image of a sound in memory.
SND_NODEFAULT	No default sound event is used. If the sound cannot be found, PlaySound returns silently without playing the default sound.
SND_NOSTOP	The specified sound event will yield to another sound event that is already playing. If a sound cannot be played because the resource needed to generate that sound is busy playing another sound, the function immediately returns FALSE without playing the requested sound. If this flag is not specified, PlaySound attempts to stop the currently playing sound so that the device can be used to play the new sound.
SND_NOWAIT	If the driver is busy, return immediately without playing the sound.
SND_PURGE	Sounds are to be stopped for the calling task. If pszSound is not NULL, all instances of the specified sound are stopped. If pszSound is NULL, all sounds that are playing on behalf of the calling task are stopped. You must also specify the instance handle to stop SND_RESOURCE events.
SND_RESOURCE	The pszSound parameter is a resource identifier; hmod must identify the instance that contains the resource.
SND_SYNC	Synchronous playback of a sound event. PlaySound returns after the sound event completes.

Return Values

Returns TRUE if successful or FALSE otherwise.

Remarks

The sound specified by pszSound must fit into available physical memory and be playable by an installed waveform-audio device driver. PlaySound searches the following directories for sound files: the current directory; the Windows directory; the Windows system directory; directories listed in the PATH environment variable; and the list of directories mapped in a network. For more information about the directory search order, see the documentation for the OpenFile function.

If it cannot find the specified sound, PlaySound uses the default system event sound entry instead. If the function can find neither the system default entry nor the default sound, it makes no sound and returns FALSE.

Windows 95/98/Me: PlaySoundW is supported by the Microsoft Layer for Unicode. To use this, you must add certain files to your application, as outlined in Microsoft Layer for Unicode on Windows 95/98/Me Systems.

Requirements

  Windows NT/2000/XP: Included in Windows NT 3.1 and later.
  Windows 95/98/Me: Included in Windows 95 and later.
  Header: Declared in Mmsystem.h; include Windows.h.
  Library: Use Winmm.lib.
  Unicode: Implemented as Unicode and ANSI versions on Windows NT/2000/XP. Also supported by Microsoft Layer for Unicode.

有件很麻烦的事就是这些宏， 其实他们是一些数字了， 但是要在C#

里面用， 可不能再这样了， 用一个const表示下吧，可是到底这些

宏代表什么呢？我当时是建了一个VC的Win32 Application 项目

然后写出宏， 把鼠标移上去看它代表的数字。

唉， 真够烦的阿， 不知道有什么好的方法， 还望过路大下侠指导一下。

        public const int SND_FILENAME = 0x00020000;
        public const int SND_ASYNC = 0x0001;

[DllImport("winmm.dll")]
        public static extern bool PlaySound(string pszSound, int hmod, int fdwSound);

PlaySound("clock.wav", 0, SND_FILENAME | SND_ASYNC);

Ok， 就可以听到了音乐了阿。