diff --git a/src/ObjCRuntime/Registrar.cs b/src/ObjCRuntime/Registrar.cs
index e5f22cd92d01..50ac0a48ee65 100644
--- a/src/ObjCRuntime/Registrar.cs
+++ b/src/ObjCRuntime/Registrar.cs
@@ -135,6 +135,7 @@ internal class ObjCType {
 			public CategoryAttribute CategoryAttribute;
 			public TType Type;
 			public ObjCType BaseType;
+			// This only contains the leaf protocols implemented by this type.
 			public ObjCType [] Protocols;
 			public string [] AdoptedProtocols;
 			public bool IsModel;
@@ -159,6 +160,44 @@ internal class ObjCType {
 
 			public bool IsCategory { get { return CategoryAttribute != null; } }
 
+#if MTOUCH || MMP
+			HashSet<ObjCType> all_protocols;
+			// This contains all protocols in the type hierarchy.
+			// Given a type T that implements a protocol with super protocols:
+			// class T : NSObject, IProtocol2 {}
+			// [Protocol]
+			// interface IP1 {}
+			// [Protocol]
+			// interface IP2 : IP1 {}
+			// This property will contain both IP1 and IP2. The Protocols property only contains IP2.
+			public IEnumerable<ObjCType> AllProtocols {
+				get {
+					if (Protocols == null || Protocols.Length == 0)
+						return null;
+
+					if (all_protocols == null) {
+						var queue = new Queue<ObjCType> (Protocols);
+						var rv = new HashSet<ObjCType> ();
+						while (queue.Count > 0) {
+							var type = queue.Dequeue ();
+							if (rv.Add (type)) {
+								foreach (var iface in type.Type.Resolve ().Interfaces) {
+									if (!Registrar.Types.TryGetValue (iface.InterfaceType, out var superIface)) {
+										// This is not an interface that corresponds to a protocol.
+										continue;
+									}
+									queue.Enqueue (superIface);
+								}
+							}
+						}
+						all_protocols = rv;
+					}
+
+					return all_protocols;
+				}
+			}
+#endif
+
 			public void VerifyRegisterAttribute (ref List<Exception> exceptions)
 			{
 				if (RegisterAttribute == null)
diff --git a/tests/monotouch-test/Foundation/UrlProtocolTest.cs b/tests/monotouch-test/Foundation/UrlProtocolTest.cs
index 1e4b3a3ad185..79cc22d2546d 100644
--- a/tests/monotouch-test/Foundation/UrlProtocolTest.cs
+++ b/tests/monotouch-test/Foundation/UrlProtocolTest.cs
@@ -9,6 +9,9 @@
 
 using System;
 using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
 #if XAMCORE_2_0
 using Foundation;
 #if MONOMAC
@@ -66,5 +69,140 @@ public void Task ()
 			}
 		}
 #endif
+
+#if !__WATCHOS__
+		[Test]
+		public void RegistrarTest ()
+		{
+			Exception ex = null;
+			var done = new ManualResetEvent (false);
+			var success = false;
+
+			Task.Run (async () => {
+				try {
+					var config = NSUrlSessionConfiguration.DefaultSessionConfiguration;
+					config.WeakProtocolClasses = NSArray.FromNSObjects (new Class (typeof (CustomUrlProtocol)));
+					var session = NSUrlSession.FromConfiguration (config);
+					var custom_url = new NSUrl ("foo://server");
+					using (var task = await session.CreateDownloadTaskAsync (custom_url)) {
+						success = true;
+					}
+				} catch (Exception e) {
+					ex = e;
+				} finally {
+					done.Set ();
+				}
+			});
+
+			Assert.IsTrue (TestRuntime.RunAsync (DateTime.Now.AddSeconds (10), () => { }, () => done.WaitOne (0)), "Timed out");
+			Assert.IsNull (ex, "Exception");
+			Assert.IsTrue (custom_url_protocol_instance.Called_DidCompleteWithError, "DidCompleteWithError");
+			// if DidReceiveChallenge is called or not seems to vary between test runs, so we can't assert it.
+			//Assert.IsFalse (custom_url_protocol_instance.Called_DidReceiveChallenge, "DidReceiveChallenge");
+			Assert.IsTrue (custom_url_protocol_instance.Called_DidReceiveData, "DidReceiveData");
+			Assert.IsTrue (custom_url_protocol_instance.Called_DidReceiveResponse, "DidReceiveResponse");
+			Assert.IsTrue (custom_url_protocol_instance.Called_StartLoading, "StartLoading");
+			Assert.IsTrue (custom_url_protocol_instance.Called_StopLoading, "StopLoading");
+			Assert.IsTrue (custom_url_protocol_instance.Called_WillPerformHttpRedirection, "WillPerformHttpRedirection");
+
+			Assert.IsTrue (CustomUrlProtocol.Called_CanInitWithRequest, "CanInitWithRequest");
+			Assert.IsTrue (CustomUrlProtocol.Called_GetCanonicalRequest, "GetCanonicalRequest");
+
+			Assert.IsTrue (success, "Success");
+		}
+
+		static CustomUrlProtocol custom_url_protocol_instance;
+
+		public class CustomUrlProtocol : NSUrlProtocol, INSUrlSessionDelegate, INSUrlSessionTaskDelegate, INSUrlSessionDataDelegate {
+			[Export ("canInitWithRequest:")]
+			public static new bool CanInitWithRequest (NSUrlRequest request)
+			{
+				Called_CanInitWithRequest = true;
+				return true;
+			}
+			public static bool Called_CanInitWithRequest;
+
+			[Export ("canonicalRequestForRequest:")]
+			public static new NSUrlRequest GetCanonicalRequest (NSUrlRequest request)
+			{
+				Called_GetCanonicalRequest = true;
+				return request;
+			}
+			public static bool Called_GetCanonicalRequest;
+
+			[Export ("initWithRequest:cachedResponse:client:")]
+			public CustomUrlProtocol (NSUrlRequest request, NSCachedUrlResponse cachedResponse, INSUrlProtocolClient client)
+			: base (request, cachedResponse, client)
+			{
+				custom_url_protocol_instance = this;
+			}
+
+			[Export ("startLoading")]
+			public override void StartLoading ()
+			{
+				Called_StartLoading = true;
+				var config = NSUrlSession.SharedSession.Configuration;
+				var session = NSUrlSession.FromConfiguration (config, this, new NSOperationQueue ());
+
+				var task = session.CreateDataTask (new NSUrlRequest (new NSUrl ("https://microsoft.com")));
+				task.Resume ();
+			}
+			public bool Called_StartLoading;
+
+			[Export ("stopLoading")]
+			public override void StopLoading ()
+			{
+				Called_StopLoading = true;
+			}
+			public bool Called_StopLoading;
+
+			//NSURLSessionTaskDelegate
+			[Export ("URLSession:task:willPerformHTTPRedirection:newRequest:completionHandler:")]
+			public virtual void WillPerformHttpRedirection (NSUrlSession session, NSUrlSessionTask task, NSHttpUrlResponse response, NSUrlRequest newRequest, Action<NSUrlRequest> completionHandler)
+			{
+				Called_WillPerformHttpRedirection = true;
+				completionHandler (newRequest);
+			}
+			public bool Called_WillPerformHttpRedirection;
+
+			[Export ("URLSession:task:didReceiveChallenge:completionHandler:")]
+			public virtual void DidReceiveChallenge (NSUrlSession session, NSUrlSessionTask task, NSUrlAuthenticationChallenge challenge, Action<NSUrlSessionAuthChallengeDisposition, NSUrlCredential> completionHandler)
+			{
+				Called_DidReceiveChallenge = true;
+				completionHandler (NSUrlSessionAuthChallengeDisposition.PerformDefaultHandling, null);
+			}
+			public bool Called_DidReceiveChallenge;
+
+			//NSURLSessionDataDelegate
+			[Export ("URLSession:dataTask:didReceiveResponse:completionHandler:")]
+			public virtual void DidReceiveResponse (NSUrlSession session, NSUrlSessionDataTask dataTask, NSUrlResponse response, Action<NSUrlSessionResponseDisposition> completionHandler)
+			{
+				Called_DidReceiveResponse = true;
+				completionHandler (NSUrlSessionResponseDisposition.Allow);
+				this.Client.ReceivedResponse (this, response, NSUrlCacheStoragePolicy.Allowed);
+			}
+			public bool Called_DidReceiveResponse;
+
+			[Export ("URLSession:dataTask:didReceiveData:")]
+			public virtual void DidReceiveData (NSUrlSession session, NSUrlSessionDataTask dataTask, NSData data)
+			{
+				Called_DidReceiveData = true;
+				this.Client.DataLoaded (this, data);
+			}
+			public bool Called_DidReceiveData;
+
+			[Export ("URLSession:task:didCompleteWithError:")]
+			public virtual void DidCompleteWithError (NSUrlSession session, NSUrlSessionTask task, NSError error)
+			{
+				Called_DidCompleteWithError = true;
+				if (error != null) {
+					this.Client.FailedWithError (this, error);
+				} else {
+					this.Client.FinishedLoading (this);
+				}
+			}
+			public bool Called_DidCompleteWithError;
+		}
+#endif // !__WATCHOS__
 	}
 }
diff --git a/tools/common/StaticRegistrar.cs b/tools/common/StaticRegistrar.cs
index 1f7e153943dd..4986d472be65 100644
--- a/tools/common/StaticRegistrar.cs
+++ b/tools/common/StaticRegistrar.cs
@@ -4129,10 +4129,11 @@ TypeDefinition GetDelegateProxyType (ObjCMethod obj_method)
 			}
 
 			// Might be an implementation of an optional protocol member.
-			if (obj_method.DeclaringType.Protocols != null) {
+			var allProtocols = obj_method.DeclaringType.AllProtocols;
+			if (allProtocols != null) {
 				string selector = null;
 
-				foreach (var proto in obj_method.DeclaringType.Protocols) {
+				foreach (var proto in allProtocols) {
 					// We store the DelegateProxy type in the ProtocolMemberAttribute, so check those.
 					if (selector == null)
 						selector = obj_method.Selector ?? string.Empty;
@@ -4175,10 +4176,11 @@ MethodDefinition GetBlockWrapperCreator (ObjCMethod obj_method, int parameter)
 			}
 
 			// Might be an implementation of an optional protocol member.
-			if (obj_method.DeclaringType.Protocols != null) {
+			var allProtocols = obj_method.DeclaringType.AllProtocols;
+			if (allProtocols != null) {
 				string selector = null;
 
-				foreach (var proto in obj_method.DeclaringType.Protocols) {
+				foreach (var proto in allProtocols) {
 					// We store the BlockProxy type in the ProtocolMemberAttribute, so check those.
 					// We may run into binding assemblies built with earlier versions of the generator,
 					// which means we can't rely on finding the BlockProxy attribute in the ProtocolMemberAttribute.