为什么不创建扩展方法？更具可读性。

public static IEnumerable<FileInfo> GetFilesByExtensions(this DirectoryInfo dir, params string[] extensions)
{
    if (extensions == null) 
         throw new ArgumentNullException("extensions");
    IEnumerable<FileInfo> files = Enumerable.Empty<FileInfo>();
    foreach(string ext in extensions)
    {
       files = files.Concat(dir.GetFiles(ext));
    }
    return files;
}

编辑：一个更有效的版本：

public static IEnumerable<FileInfo> GetFilesByExtensions(this DirectoryInfo dir, params string[] extensions)
{
    if (extensions == null) 
         throw new ArgumentNullException("extensions");
    IEnumerable<FileInfo> files = dir.EnumerateFiles();
    return files.Where(f => extensions.Contains(f.Extension));
}

用法：

DirectoryInfo dInfo = new DirectoryInfo(@"c:\MyDir");
dInfo.GetFilesByExtensions(".jpg",".exe",".gif");