C#做的webservice获取目录下文件，失败

写了段获取目录下的文件信息，结果调用时，报错

public Int32 GetserverImages(string imgFileKind, ref Fileinfos[] strFileinfos, ref string strMsg, ref Int32 irtn)
{
List str_fileinfos = new List(strFileinfos);
string path = ConfigurationManager.ConnectionStrings["FilePath"].ConnectionString;
DirectoryInfo folder = new DirectoryInfo(path);
foreach (FileInfo file in folder.GetFiles())
{
str_fileinfos.Add(new Fileinfos(file.Name,
file.Name,
file.Name,
file.FullName,
file.CreationTime.ToString(),
file.LastAccessTime.ToString(),
file.LastWriteTime.ToString()));
}
strFileinfos = str_fileinfos.ToArray();
irtn = 1;
strMsg = "获取文件资料成功！";
return 1;
}

 List<Fileinfos> str_fileinfos = new List<Fileinfos>(strFileinfos);
strFileinfos为空。