[MidiThruConnection] Numerous other code updates (#13008)

* Enable nullability and fix code accordingly.
* Use 'is' and 'is not' instead of '==' and '!=' for object identity.
* Use CFString.CreateNative/ReleaseNative instead of other means to create
  native strings (the fastest and least memory hungry option).

src/CoreMidi/MidiThruConnection.cs

@@ -6,6 +6,8 @@
 // Copyright 2016 Xamarin Inc.
 //
 
+#nullable enable
+
 using System;
 using System.Runtime.InteropServices;
 
@@ -59,14 +61,17 @@ namespace CoreMidi {
 			/* CFDataRef */ IntPtr inConnectionParams,
 			/* MIDIThruConnectionRef* */ out MidiThruConnectionRef outConnection);
 
-		public static MidiThruConnection Create (string persistentOwnerID, MidiThruConnectionParams connectionParams, out MidiError error)
+		public static MidiThruConnection? Create (string persistentOwnerID, MidiThruConnectionParams connectionParams, out MidiError error)
 		{
 			MidiThruConnectionRef ret;
 
 			using (var data = connectionParams.WriteStruct ()) {
-				var retStr = NSString.CreateNative (persistentOwnerID);
-				error = MIDIThruConnectionCreate (retStr, data.Handle, out ret);
-				NSString.ReleaseNative (retStr);
+				var retStr = CFString.CreateNative (persistentOwnerID);
+				try {
+					error = MIDIThruConnectionCreate (retStr, data.Handle, out ret);
+				} finally {
+					CFString.ReleaseNative (retStr);
+				}
 			}
 
 			if (error != MidiError.Ok)
@@ -75,7 +80,7 @@ namespace CoreMidi {
 			return new MidiThruConnection (ret);
 		}
 
-		public static MidiThruConnection Create (string persistentOwnerID, MidiThruConnectionParams connectionParams)
+		public static MidiThruConnection? Create (string persistentOwnerID, MidiThruConnectionParams connectionParams)
 		{
 			MidiError error;
 			return Create (persistentOwnerID, connectionParams, out error);
@@ -86,7 +91,7 @@ namespace CoreMidi {
 			/* MIDIThruConnectionRef* */ MidiThruConnectionRef connection,
 			/* CFDataRef */ out IntPtr outConnectionParams);
 
-		public MidiThruConnectionParams GetParams (out MidiError error)
+		public MidiThruConnectionParams? GetParams (out MidiError error)
 		{
 			if (Handle == InvalidRef)
 				throw new ObjectDisposedException ("MidiThruConnection");
@@ -96,7 +101,7 @@ namespace CoreMidi {
 			if (error != MidiError.Ok || ret == IntPtr.Zero)
 				return null;
 			using (var data = Runtime.GetNSObject<NSData> (ret, true)) {
-				if (data == null)
+				if (data is null)
 					return null;
 				var cnnParams = new MidiThruConnectionParams ();
 				cnnParams.ReadStruct (data);
@@ -104,7 +109,7 @@ namespace CoreMidi {
 			}
 		}
 
-		public MidiThruConnectionParams GetParams ()
+		public MidiThruConnectionParams? GetParams ()
 		{
 			MidiError error;
 			return GetParams (out error);
@@ -119,7 +124,7 @@ namespace CoreMidi {
 		{
 			if (Handle == InvalidRef)
 				throw new ObjectDisposedException ("MidiThruConnection");
-			if (connectionParams == null)
+			if (connectionParams is null)
 				throw new ArgumentNullException (nameof (connectionParams));
 
 			using (var data = connectionParams.WriteStruct ()) {
@@ -133,31 +138,39 @@ namespace CoreMidi {
 			/* CFStringRef* */ IntPtr inPersistentOwnerID,
 			/* CFDataRef */ out IntPtr outConnectionList);
 
-		public static MidiThruConnection[] Find (string persistentOwnerID, out MidiError error)
+		public static MidiThruConnection[]? Find (string persistentOwnerID, out MidiError error)
 		{
-			if (persistentOwnerID == null)
+			if (persistentOwnerID is null)
 				throw new ArgumentNullException (nameof (persistentOwnerID));
 
 			IntPtr ret;
-			using (var nssstr = new NSString (persistentOwnerID)) {
-				error = MIDIThruConnectionFind (nssstr.Handle, out ret);
+			var persistentOwnerIDHandle = CFString.CreateNative (persistentOwnerID);
+			try {
+				error = MIDIThruConnectionFind (persistentOwnerIDHandle, out ret);
+			} finally {
+				CFString.ReleaseNative (persistentOwnerIDHandle);
 			}
 			using (var data = Runtime.GetNSObject<NSData> (ret)) {
+				if (data is null)
+					return null;
 				var typeSize = Marshal.SizeOf (typeof (MidiThruConnectionRef));
 				var totalObjs = (int) data.Length / typeSize;
 				if (totalObjs == 0)
 					return null;
 
-				var basePtr = (int) data.Bytes;
 				var connections = new MidiThruConnection[totalObjs];
-				for (int i = 0; i < totalObjs; i++)
-					connections[i] = new MidiThruConnection ((uint)(basePtr + i * typeSize));
+				unsafe {
+					uint* handles = (uint*) (IntPtr) data.Bytes;
+					for (int i = 0; i < totalObjs; i++) {
+						connections [i] = new MidiThruConnection (handles [i]);
+					}
+				}
 				
 				return connections;
 			}
 		}
 
-		public static MidiThruConnection[] Find (string persistentOwnerID)
+		public static MidiThruConnection[]? Find (string persistentOwnerID)
 		{
 			MidiError error;
 			return Find (persistentOwnerID, out error);