主题：如何判断某个字符串在集合内

集合为

Dim ftp1 As New FtpClient
Dim fls As List(of String)
ftp1.Host="196.128.143.28"
ftp1.Account = "foxuser"
ftp1.Password = "138238110"
fls = ftp1.GetFileList("\photo")
For Each fl As String In fls
    Output.Show(fl)
Next

如何如何判断某个字符串在集合内，也就是判断文件名和FTP目录的文件名是否有重复