从（特定）服务器下载FTP文件适用于.NET 4+，但不适用于.NET 2.0

Question

Update (the reason of this issue):

Okay, I managed - with the help of @MitchelSellers's comment - to figure out where the problem exactly is, and I'm trying to find a solution for it.

• Obviously this server doesn't allow the command CWD (change working directory).
• Now downloading a file using .NET 2.0 -for unknown reasons- it sends this command followed by / after logging in, to change the working directory to the current working directory!! (as shown below, and I also confirmed that using FileZilla with another server).
• Also as shown below, that's not the case with the recent versions of .NET , which is why it works on .NET 4.x .

So, I'm trying to find a way to make it work on .NET 2.0 .

---

The original question:

Here's my code:

Private Sub DownloadTestFile()
    Dim filePath As String = "ftp://ftp.kohlandfrisch.com/testfile"
    Dim request As FtpWebRequest

    Dim buffer(1023) As Byte
    Dim bytesIn As Integer

    request = DirectCast(WebRequest.Create(filePath), FtpWebRequest)
    request.Method = WebRequestMethods.Ftp.DownloadFile
    request.Credentials = New NetworkCredential("username", "password")
    request.UseBinary = False
    request.UsePassive = True
    request.Proxy = Nothing

    Using stream As IO.Stream = request.GetResponse.GetResponseStream
        Using output = IO.File.Create(localFilePath)
            bytesIn = 1
            Do Until bytesIn < 1
                bytesIn = stream.Read(buffer, 0, 1024)
                If bytesIn > 0 Then output.Write(buffer, 0, bytesIn)
            Loop
        End Using
    End Using
End Sub

When running that code on .NET 4 or 4.x , it works perfectly fine. However when running it on .NET 2.0 (I have to use .NET 2.0) , it throws an exception when calling request.GetResponse . Here's the exception message is:

The remote server returned an error: (501) Syntax error in parameters or arguments.

I figured out that there must be something wrong with the request sent from .NET 2.0 , so I decided to capture requests and responses using Wireshark, and my assumption was correct , but I still don't understand what the problem exactly is since I'm using the same code on both .NET versions .

Wireshark results

.NET 4.5.2

.NET 2.0

Any ideas?

Side note: Although my code is in VB, any answer with C# code is welcomed.

Answer 1

UPDATE according to comment:

.NET 2 does not honour that flag :-( It contains a method called BuildCommandsList() - part the source in .NET 2 looks like this:

if (m_PreviousServerPath != newServerPath) { 
    if (!m_IsRootPath
        && m_LoginState == FtpLoginState.LoggedIn
        && m_LoginDirectory != null)
    { 
        newServerPath = m_LoginDirectory+newServerPath;
    } 
    m_NewServerPath = newServerPath; 

    commandList.Add(new PipelineEntry(FormatFtpCommand("CWD", newServerPath), PipelineEntryFlags.UserCommand)); 
}

So basically the CWD is hardcoded in .NET 2. This leaves you with 2 undesirable "options": either make the FTP server RFC compliant (which it currently is not!) or user some other library/way to access it.

A very unusual way might be to call/automate the DOS-command ftp from your .NET application...

Left for reference:

MS has documented this change of behaviour between .NET 2 and .NET 4 here: https://support.microsoft.com/en-au/help/2134299/system.net.ftpwebrequest-class-behaves-differently-in-.net-framework-4-vs-.net-framework-3.5

I suspect that it is worth a try to remove the described flag in .NET 2.0 - something similar to this:

private static void SetMethodRequiresCWD()
{
        Type requestType = typeof(FtpWebRequest);
        FieldInfo methodInfoField = requestType.GetField("m_MethodInfo", BindingFlags.NonPublic | BindingFlags.Instance);
        Type methodInfoType = methodInfoField.FieldType;


        FieldInfo knownMethodsField = methodInfoType.GetField("KnownMethodInfo", BindingFlags.Static | BindingFlags.NonPublic);
        Array knownMethodsArray = (Array)knownMethodsField.GetValue(null);

        FieldInfo flagsField = methodInfoType.GetField("Flags", BindingFlags.NonPublic | BindingFlags.Instance);

        int MustChangeWorkingDirectoryToPath = 0x100;
        foreach (object knownMethod in knownMethodsArray)
        {
            int flags = (int)flagsField.GetValue(knownMethod);
            flags &= (~MustChangeWorkingDirectoryToPath);
            flagsField.SetValue(knownMethod, flags);
        }
}

Sorry - I can't try this right now so this more like a suggestion...

EDIT: Alternatively you could use some ftp library which behaves similar to what .NET 4 does.

Answer 2

Have you tried making the filePath an absolute path? ie

Dim filePath As String = "ftp://ftp.kohlandfrisch.com/%2ftestfile"

%2f represents the / character.

This might be a workaround.