.NET Gmail API. Input is not a valid base64

Question

Although this seems very easy to fix, my .net application simply doesn't see this as a valid base64 string for some reason.

I'm using GMail API to fetch the messages and on the last part where I try to retrieve the body, I hit the wall with the following error message:

Exception thrown: 'System.FormatException' in mscorlib.dll
System.FormatException: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
at System.Convert.FromBase64_ComputeResultLength(Char* inputPtr, Int32 inputLength)
at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength)
at System.Convert.FromBase64String(String s)
at wForm.Form1.<Form1_Load>d__9.MoveNext() in `E:\Programs\Programming\wForms\wForm\wForm\Form1.cs:line 127`

and this is the encoded string (which decodes fine in other apps, such as the utilities-info website) : https://pastebin.ca/3893521

My code for loading the windows form and fetching those messages:

private async void Form1_Load(object sender, EventArgs e)
{
    txtEmail_ID.Focus();
    try
    {
        requestLabel = service.Users.Labels.List("me");
        requestMessage = service.Users.Messages.List("me");
        //requestMessage.LabelIds = "INBOX";
        //requestMessage.IncludeSpamTrash = false;
        // requestMessage.Q = "is:unread";

        var messageResponse = await requestMessage.ExecuteAsync();
        if (messageResponse != null && messageResponse.Messages != null)
        {
            foreach (var message in messageResponse.Messages)
            {
                var currentMessage = new messageInfo();
                var requestNestedMessage = service.Users.Messages.Get("me", message.Id);
                var nestedMessageResponse = await requestNestedMessage.ExecuteAsync();

                if (nestedMessageResponse != null) // Check if we have nested params
                {
                    foreach (var mParts in nestedMessageResponse.Payload.Headers) // Loop msg headers
                    {
                        if (mParts.Name == "Date")
                        {
                            currentMessage.msgDate = mParts.Value; // Get msg date
                        }
                        else if (mParts.Name == "From")
                        {
                            currentMessage.msgSender = mParts.Value; // Get msg sender
                        }
                        else if (mParts.Name == "Subject")
                        {
                            currentMessage.msgTitle = mParts.Value; // Get msg subject
                        }
                    }
                    if (nestedMessageResponse.Payload.Parts == null && nestedMessageResponse.Payload.Body != null)
                    {
                        currentMessage.msgBody = nestedMessageResponse.Payload.Body.Data;
                    }
                    else
                    {
                        currentMessage.msgBody = returnNestedParts(nestedMessageResponse.Payload.Parts, "");
                    }

                    Console.WriteLine(currentMessage.msgSender);
                    String codedString = currentMessage.msgBody.Replace("-", "+");
                    codedString = codedString.Replace("_", "/");

                    byte[] data = Convert.FromBase64String(codedString);
                    currentMessage.msgBody = Encoding.UTF8.GetString(data);
                    mInfo.Add(currentMessage);
                }
            }
        }
    } catch (Exception ex)
    {
        Console.WriteLine(ex);
    }
    //comboFolders.SelectedIndex = 0;
}

The program throws the exception on every message it fetches and the "Line 127" is on "byte[] data"

I have tried searching around in this forum for similar questions, however, none of their resolutions seem to have worked as most of them offered just to change the - and _ symbols to appropriate ones for base64url type.

Answer 1

I have resolved this, narrowed it down to the payload parts. The error appears when I want to join two payload parts of a message together, leaving trailing '=' signs un-handled and therefore forcing an exception on myself.