C# 提示不是所有路径都有返回值

public string GetFileType(string FileDirectory)
        {
            DirectoryInfo DI = new DirectoryInfo(FileDirectory);
            FileSystemInfo[] f = DI.GetFileSystemInfos();
            string Ex="";
            foreach (FileSystemInfo i in f)
            {
                if (i is DirectoryInfo)
                {
                    if ((i.Attributes & FileAttributes.Hidden) == 0)
                    {
                        Search(i.FullName);
                        return null;
                    }

                }
                else
                {
                    if (Ex != "")
                    {
                        Ex = i.Extension;
                        return Ex;
                    }
                    else
                    {
                        Ex = Ex + "," + i.Extension;
                        return Ex;
                    }


                }
            }
            
        }

这段代码还哪里需要返回值啊
------解决方案--------------------

最后加一句
return "";
[解决办法]
最外面
public string GetFileType(string FileDirectory)
{
DirectoryInfo DI = new DirectoryInfo(FileDirectory);
FileSystemInfo[] f = DI.GetFileSystemInfos();
string Ex="";
foreach (FileSystemInfo i in f)
{
if (i is DirectoryInfo)
{
if ((i.Attributes & FileAttributes.Hidden) == 0)
{
Search(i.FullName);
return null;
}

}
else
{
if (Ex != "")
{
Ex = i.Extension;
return Ex;
}
else
{
Ex = Ex + "," + i.Extension;
return Ex;
}


}
}
return string.Empty;

}
[解决办法]
也许没有进入foreach呢