C#

using System;
using System.Collections;
using System.Runtime.InteropServices;
using System.Security;

namespace Example
{
  public class NetFiles
  {
    private const int ErrorFileNotFound = 2;
    private const int ErrorAccessDenied = 5;
    private const int ErrorMoreData = 234;
    private const int MaxPreferredLength = -1; // Get all records
    private const int BufferSize = 4000; // (sizeof(FileInfo) == 160) * 25 < 4096 (under 4 kb)

    [DllImport("netapi32.dll", CharSet=CharSet.Unicode)]
    private static extern int NetFileEnum
    (
      [MarshalAs(UnmanagedType.LPWStr)]string serverName,
      [MarshalAs(UnmanagedType.LPWStr)]string basePath,
      [MarshalAs(UnmanagedType.LPWStr)]string userName,
      [MarshalAs(UnmanagedType.U4)]int level,
      [MarshalAs(UnmanagedType.SysUInt), Out]out IntPtr buffer,
      [MarshalAs(UnmanagedType.U4)]int preferredMaxLength,
      [MarshalAs(UnmanagedType.U4), Out]out int entriesRead,
      [MarshalAs(UnmanagedType.U4), Out]out int totalEntries,
      [MarshalAs(UnmanagedType.U4), In, Out]ref int resumeHandle
    );

    [DllImport("netapi32.dll", CharSet=CharSet.Unicode)]
    private static extern int NetFileClose
    (
      [MarshalAs(UnmanagedType.LPWStr)]string serverName,
      [MarshalAs(UnmanagedType.U4)]int fileID
    );

    [DllImport("netapi32.dll")]
    private static extern int NetApiBufferFree
    (
      IntPtr buffer
    );

    private NetFiles()
    {
    }

    public static IList GetOpenFiles()
    {
      return GetOpenFiles(null, null, null);
    }

    public static IList GetOpenFilesOnServer(string server)
    {
      return GetOpenFiles(server, null, null);
    }

    public static IList GetOpenFilesWithPath(string basePath)
    {
      return GetOpenFiles(null, basePath, null);
    }

    public static IList GetOpenFilesForUsername(string username)
    {
      return GetOpenFiles(null, null, username);
    }

    public static IList GetOpenFiles(string server, string basePath,
      string username)
    {
      IntPtr buffer;
      int entriesRead;
      int totalEntries;
      int resumeHandle = 0;
      ArrayList list = new ArrayList();

      // Enum the files on the server with the path and username.
      int retVal = NetFileEnum(server, basePath, username, 3, out buffer,
        BufferSize, out entriesRead, out totalEntries, ref resumeHandle);
      while ((retVal == 0 || retVal == ErrorMoreData) &&
        buffer != IntPtr.Zero)
      {
        // For each address, marshal the struct and add to the list.
        for (int i=0; i<entriesRead; i++)
        {
          IntPtr address = new IntPtr(buffer.ToInt32() +
            i * Marshal.SizeOf(typeof(FileInfo)));
          if (address != IntPtr.Zero)
          {
            FileInfo info =
              (FileInfo)Marshal.PtrToStructure(address, typeof(FileInfo));
            list.Add(info);
          }
        }

        // Free the buffer.
        NetApiBufferFree(buffer);

        // If more data is available, get it and repeat.
        if (retVal == ErrorMoreData)
          retVal = NetFileEnum(server, basePath, username, 3, out buffer,
            MaxPreferredLength, out entriesRead, out totalEntries,
              ref resumeHandle);
        else break;
      }

      // Copy the ArrayList to an array and return it.
      if (retVal == 0) return list;
      else if (retVal == 5) throw new SecurityException("Access denied");
      else throw new System.ComponentModel.Win32Exception(retVal);
    }

    public static void CloseFile(int handle)
    {
      CloseFile(handle, null);
    }

    public static void CloseFile(int handle, string server)
    {
      int retVal = NetFileClose(server, handle);
      if (retVal == 0) return;
      else if (retVal == ErrorAccessDenied)
        throw new SecurityException("Access denied");
      else if (retVal == ErrorFileNotFound)
        throw new System.IO.FileNotFoundException("The file was not found.");
      else throw new System.ComponentModel.Win32Exception(retVal);
    }
  }

  [Serializable]
  [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Unicode)]
  public struct FileInfo
  {
    [MarshalAs(UnmanagedType.U4)]public int ID;
    [MarshalAs(UnmanagedType.U4)]public FilePermissions Permissions;
    [MarshalAs(UnmanagedType.U4)]public int Locks;
    [MarshalAs(UnmanagedType.LPWStr)]public string Path;
    [MarshalAs(UnmanagedType.LPWStr)]public string Username;
  }

  [Flags]
  public enum FilePermissions
  {
    Read = 1,

    Write = 2,

    Create = 4
  }
}

msComm1.OnComm += new EventHandler(this.OnComm);

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Can't manage to P/Invoke that Win32 API in .NET? Why not do interop the wiki way!