C# 提示不是所有路径都有返回值

Question

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

Answer 1

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;    
        }

Answer 2

没有进入foreach的意思是之前出错？

Answer 3

受教了  正准备发个 switch 里面return报的这个错呢