Unhandled Exception running eBay C# SDK on Mono

I downloaded the eBay SDK for use in an internal application we are developing. The catch is we are developing on Mono. When I run the eBay Hello World sample on .NET, it displays the following output:

+++++++++++++++++++++++++++++++++++++++
+ Welcome to eBay SDK for .Net Sample +
+ - HelloWorld                        +
+++++++++++++++++++++++++++++++++++++++
Begin to call eBay API, please wait ...
End to call eBay API, show call result:
eBay official Time: 11/27/2013 3:02:19 PM

When I run it with Mono, I get the following:

Unhandled Exception: eBay.Service.Core.Sdk.ApiException: Exception has been thrown by the target of an invocation. 
---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
---> System.InvalidOperationException: WebServiceBindingAttribute is required on proxy class 'eBay.Service.Core.Sdk.eBayAPIInterfaceService2'.
at System.Web.Services.Protocols.SoapTypeStubInfo..ctor (System.Web.Services.Protocols.LogicalTypeInfo logicalTypeInfo) [0x00000]
at (wrapper managed-to-native) System.Reflection.MonoCMethod:InternalInvoke (object,object[],System.Exception&)
etc...etc...etc...

I found a workaround and will post it in an answer below.

Solution

I searched Xamarin's Bugzilla and found nothing, but I found this relevant bug posted in Novell's old Mono Bugzilla: https://bugzilla.novell.com/show_bug.cgi?id=693367

So I dug through the eBay SDK source and found that this was indeed the issue. eBay.Service.Core.Sdk.eBayAPIInterfaceService2 inherits from eBay.Service.Core.Sdk.eBayAPIInterfaceService, which is decorated with a System.Web.Services.WebServiceBindingAttribute.

So I changed eBay.Service.Core.Sdk.eBayAPIInterfaceService2, adding [System.Web.Services.WebServiceBindingAttribute(Name = "eBayAPISoapBinding", Namespace = "urn:ebay:apis:eBLBaseComponents")] so that it now looks like the following:

using System;
using System.Net;
using eBay.Service.Core.Soap;

namespace eBay.Service.Core.Sdk {
    /// <summary>
    /// Enhanced eBayAPIInterfaceService with GZIP compression support.
    /// </summary>
    [System.Web.Services.WebServiceBindingAttribute(Name = "eBayAPISoapBinding", Namespace = "urn:ebay:apis:eBLBaseComponents")]
    internal class eBayAPIInterfaceService2 : eBayAPIInterfaceService {
        ...
    }
}

I followed the instructions in the eBay SDK for building from the sources and the problem was solved.

Footnote: Incidentally, if this is your first time using Mono to connect to an HTTPS web service, you will likely immediately run into another exception. That one will look like this:

Unhandled Exception: System.Net.WebException: Error getting response stream (Write: The authentication or decryption has failed.): SendFailure ---> System.IO.IOException: The authentication or decryption has failed. ---> Mono.Security.Protocol.Tls.TlsException: Invalid certificate received from server. Error code: 0xffffffff800b010a
at Mono.Security.Protocol.Tls.Handshake.Client.TlsServerCertificate.validateCertificates (Mono.Security.X509.X509CertificateCollection certificates) [0x00000] in <filename unknown>:0 
at Mono.Security.Protocol.Tls.Handshake.Client.TlsServerCertificate.ProcessAsTls1 () [0x00000] in <filename unknown>:0 
at Mono.Security.Protocol.Tls.Handshake.HandshakeMessage.Process () [0x00000] in <filename unknown>:0 
at (wrapper remoting-invoke-with-check) Mono.Security.Protocol.Tls.Handshake.HandshakeMessage:Process ()
at Mono.Security.Protocol.Tls.ClientRecordProtocol.ProcessHandshakeMessage (Mono.Security.Protocol.Tls.TlsStream handMsg) [0x00000] in <filename unknown>:0 
at Mono.Security.Protocol.Tls.RecordProtocol.InternalReceiveRecordCallback (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 
--- End of inner exception stack trace ---
at Mono.Security.Protocol.Tls.SslStreamBase.AsyncHandshakeCallback (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 
--- End of inner exception stack trace ---
at System.Net.HttpWebRequest.EndGetResponse (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 
at System.Net.HttpWebRequest.GetResponse () [0x00000] in <filename unknown>:0

For the solution to that, go here: http://www.mono-project.com/FAQ:_Security