diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/HttpTextWriter.cs b/tests/common/Touch.Unit/Touch.Client/Runner/HttpTextWriter.cs
index 2e97df0bbd96..6d4e40142955 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/HttpTextWriter.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/HttpTextWriter.cs
@@ -16,7 +16,7 @@
 
 namespace MonoTouch.NUnit {
 	class HttpTextWriter : TextWriter {
-		public string HostName;
+		public string? HostName;
 		public int Port;
 
 		TaskCompletionSource<bool> finished = new TaskCompletionSource<bool> ();
@@ -52,7 +52,7 @@ Task<bool> SendData (NSUrl url, string uploadData)
 			var tcs = new TaskCompletionSource<bool> ();
 			var request = new NSMutableUrlRequest (url);
 			request.HttpMethod = "POST";
-			var rv = NSUrlSession.SharedSession.CreateUploadTask (request, NSData.FromString (uploadData), (NSData data, NSUrlResponse response, NSError error) => {
+			var rv = NSUrlSession.SharedSession.CreateUploadTask (request, NSData.FromString (uploadData), (NSData? data, NSUrlResponse? response, NSError? error) => {
 				if (error is not null) {
 					Console.WriteLine ("Failed to send data to {0}: {1}", url.AbsoluteString, error);
 					tcs.SetResult (false);
@@ -67,7 +67,12 @@ Task<bool> SendData (NSUrl url, string uploadData)
 
 		async Task SendData (string action, string uploadData)
 		{
+			if (string.IsNullOrEmpty (HostName))
+				throw new InvalidOperationException ("No host name specified.");
+
 			var url = NSUrl.FromString ("http://" + HostName + ":" + Port + "/" + action);
+			if (url is null)
+				throw new InvalidOperationException ("Failed to create the reporting url.");
 
 			int attempts_left = 10;
 			while (!await SendData (url, uploadData)) {
@@ -105,13 +110,14 @@ public override void Write (char value)
 			log.Append (value);
 		}
 
-		public override void Write (char [] buffer)
+		public override void Write (char []? buffer)
 		{
 			Console.Out.Write (buffer);
-			log.Append (buffer);
+			if (buffer is not null)
+				log.Append (buffer);
 		}
 
-		public override void WriteLine (string value)
+		public override void WriteLine (string? value)
 		{
 			Console.Out.WriteLine (value);
 			log.AppendLine (value);
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/MacRunner.cs b/tests/common/Touch.Unit/Touch.Client/Runner/MacRunner.cs
index 4f1eaf0e0443..2cbaf024afe4 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/MacRunner.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/MacRunner.cs
@@ -67,7 +67,7 @@ static async Task<bool> RunTestsAsync (TouchOptions options, Assembly [] assembl
 
 			await runner.RunAsync ();
 
-			return !runner.Result.IsFailure ();
+			return runner.Result is not null && !runner.Result.IsFailure ();
 		}
 
 		protected override void WriteDeviceInformation (TextWriter writer)
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/NUnitOutputTextWriter.cs b/tests/common/Touch.Unit/Touch.Client/Runner/NUnitOutputTextWriter.cs
index 78da78b69896..cfb459fe7fbc 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/NUnitOutputTextWriter.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/NUnitOutputTextWriter.cs
@@ -37,12 +37,12 @@ public class NUnitOutputTextWriter : TextWriter {
 		StringBuilder extra_data = new StringBuilder ();
 		XmlMode mode;
 
-		public NUnitOutputTextWriter (BaseTouchRunner runner, TextWriter baseWriter, OutputWriter xmlWriter)
+		public NUnitOutputTextWriter (BaseTouchRunner runner, TextWriter? baseWriter, OutputWriter? xmlWriter)
 			: this (runner, baseWriter, xmlWriter, XmlMode.Default)
 		{
 		}
 
-		public NUnitOutputTextWriter (BaseTouchRunner runner, TextWriter baseWriter, OutputWriter xmlWriter, XmlMode xmlMode)
+		public NUnitOutputTextWriter (BaseTouchRunner runner, TextWriter? baseWriter, OutputWriter? xmlWriter, XmlMode xmlMode)
 		{
 			Runner = runner;
 			BaseWriter = baseWriter ?? Console.Out;
@@ -60,7 +60,7 @@ public override Encoding Encoding {
 
 		public BaseTouchRunner Runner { get; private set; }
 
-		public OutputWriter XmlOutputWriter { get; private set; }
+		public OutputWriter? XmlOutputWriter { get; private set; }
 
 		public override void Write (char value)
 		{
@@ -70,7 +70,7 @@ public override void Write (char value)
 				extra_data.Append (value);
 		}
 
-		public override void Write (string value)
+		public override void Write (string? value)
 		{
 			if (real_time_reporting)
 				BaseWriter.Write (value);
@@ -81,6 +81,7 @@ public override void Write (string value)
 		public override void Close ()
 		{
 			if (XmlOutputWriter is not null) {
+				var result = Runner.Result;
 				// now we want the XML report to write
 				real_time_reporting = true;
 				// write to a temporary string, because NUnit2XmlOutputWriter.WriteResultFile closes the stream,
@@ -88,7 +89,8 @@ public override void Close ()
 				var wrapped = mode == XmlMode.Wrapped;
 
 				if (!wrapped) {
-					XmlOutputWriter.WriteResultFile (Runner.Result, BaseWriter);
+					if (result is not null)
+						XmlOutputWriter.WriteResultFile (result, BaseWriter);
 					if (extra_data.Length > 0) {
 						BaseWriter.Write ("<!--");
 						extra_data.Replace ("--", "- - ");
@@ -100,7 +102,8 @@ public override void Close ()
 					BaseWriter.WriteLine ("<NUnitOutput>");
 
 					using (var textWriter = new StringWriter ()) {
-						XmlOutputWriter.WriteResultFile (Runner.Result, textWriter);
+						if (result is not null)
+							XmlOutputWriter.WriteResultFile (result, textWriter);
 						var str = textWriter.ToString ();
 						// Remove any xml declarations, since we're embedding this inside a different xml document.
 						if (str.StartsWith ("<?xml", StringComparison.Ordinal)) {
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TcpTextWriter.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TcpTextWriter.cs
index 1109b6bd9563..d76d34277ef4 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TcpTextWriter.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TcpTextWriter.cs
@@ -16,7 +16,7 @@ namespace MonoTouch.NUnit {
 	public class TcpTextWriter : TextWriter {
 
 		private TcpClient client;
-		TcpListener server;
+		TcpListener? server;
 		private StreamWriter writer;
 
 		public TcpTextWriter (string hostName, int port, bool isTunnel = false)
@@ -50,7 +50,7 @@ public TcpTextWriter (string hostName, int port, bool isTunnel = false)
 							break;
 					}
 				} else {
-					client = new TcpClient (HostName, port);
+					client = new TcpClient (hostName, port);
 				}
 				writer = new StreamWriter (client.GetStream ());
 			} catch {
@@ -61,7 +61,7 @@ public TcpTextWriter (string hostName, int port, bool isTunnel = false)
 			}
 		}
 
-		public string HostName { get; private set; }
+		public string? HostName { get; private set; }
 
 		public int Port { get; private set; }
 
@@ -96,7 +96,7 @@ public override void Write (char value)
 			writer.Write (value);
 		}
 
-		public override void Write (char [] buffer)
+		public override void Write (char []? buffer)
 		{
 			writer.Write (buffer);
 		}
@@ -106,7 +106,7 @@ public override void Write (char [] buffer, int index, int count)
 			writer.Write (buffer, index, count);
 		}
 
-		public override void Write (string value)
+		public override void Write (string? value)
 		{
 			writer.Write (value);
 		}
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TestCaseElement.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TestCaseElement.cs
index dc0c5ccbb0e0..69fe9624e2ab 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TestCaseElement.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TestCaseElement.cs
@@ -51,13 +51,16 @@ public TestCaseElement (TestMethod testCase, TouchRunner runner)
 #if NUNITLITE_NUGET
 				Run ();
 #else
-				var suite = (testCase.Parent as TestSuite);
-				var context = TestExecutionContext.CurrentContext;
-				context.TestObject = Reflect.Construct (testCase.Method.ReflectedType, null);
+				if (testCase.Parent is TestSuite suite) {
+					var context = TestExecutionContext.CurrentContext;
+					context.TestObject = Reflect.Construct (testCase.Method.ReflectedType, null);
 
-				suite.GetOneTimeSetUpCommand ().Execute (context);
-				Run ();
-				suite.GetOneTimeTearDownCommand ().Execute (context);
+					suite.GetOneTimeSetUpCommand ().Execute (context);
+					Run ();
+					suite.GetOneTimeTearDownCommand ().Execute (context);
+				} else {
+					Run ();
+				}
 #endif
 
 				Runner.CloseWriter ();
@@ -66,7 +69,7 @@ public TestCaseElement (TestMethod testCase, TouchRunner runner)
 		}
 
 		public TestMethod TestCase {
-			get { return Test as TestMethod; }
+			get { return (TestMethod) Test; }
 		}
 
 		public void Run ()
@@ -108,7 +111,8 @@ public override void TestFinished ()
 				// we still need to update our current element
 				if (GetContainerTableView () is not null) {
 					var root = GetImmediateRootElement ();
-					root.Reload (this, UITableViewRowAnimation.Fade);
+					if (root is not null)
+						root.Reload (this, UITableViewRowAnimation.Fade);
 				}
 				tapped = false;
 			}
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TestElement.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TestElement.cs
index cdb4eb0238ed..783faf8fbbef 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TestElement.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TestElement.cs
@@ -38,7 +38,7 @@ abstract class TestElement : StyledMultilineElement {
 		static internal UIColor Orange = UIDevice.CurrentDevice.CheckSystemVersion (13, 0) ? UIColor.SystemOrange : UIColor.Orange;
 		static internal UIColor Red = UIDevice.CurrentDevice.CheckSystemVersion (13, 0) ? UIColor.SystemRed : UIColor.Red;
 
-		private TestResult result;
+		private TestResult? result;
 
 		public TestElement (ITest test, TouchRunner runner)
 			: base ("?", "?", UITableViewCellStyle.Subtitle)
@@ -58,7 +58,13 @@ public TestElement (ITest test, TouchRunner runner)
 		protected TouchRunner Runner { get; private set; }
 
 		public TestResult Result {
-			get { return result ?? new TestCaseResult (Test as TestMethod); }
+			get {
+				if (result is not null)
+					return result;
+				if (Test is TestMethod testMethod)
+					return new TestCaseResult (testMethod);
+				return new TestSuiteResult ((TestSuite) Test);
+			}
 			set { result = value; }
 		}
 
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TestSuiteElement.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TestSuiteElement.cs
index 944b16c7be62..7fd934a35de5 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TestSuiteElement.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TestSuiteElement.cs
@@ -48,14 +48,14 @@ public TestSuiteElement (TestSuite test, TouchRunner runner)
 				DetailColor = Orange;
 				var skip = test.Properties ["_SKIPREASON"];
 				if (skip.Count > 0)
-					Value = skip [0].ToString ();
+					Value = skip [0]?.ToString () ?? "No test was found inside this suite";
 				else
 					Value = "No test was found inside this suite";
 			}
 		}
 
 		public TestSuite Suite {
-			get { return Test as TestSuite; }
+			get { return (TestSuite) Test; }
 		}
 
 		public void Run ()
@@ -91,7 +91,8 @@ public override void TestFinished ()
 
 			if (GetContainerTableView () is not null) {
 				var root = GetImmediateRootElement ();
-				root.Reload (this, UITableViewRowAnimation.Fade);
+				if (root is not null)
+					root.Reload (this, UITableViewRowAnimation.Fade);
 			}
 		}
 	}
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TouchOptions.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TouchOptions.cs
index 954158a401f2..e090e876d832 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TouchOptions.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TouchOptions.cs
@@ -46,7 +46,7 @@ public enum XmlVersion {
 
 	public class TouchOptions {
 
-		static TouchOptions current;
+		static TouchOptions? current;
 		static public TouchOptions Current {
 			get {
 				if (current is null)
@@ -68,7 +68,9 @@ public TouchOptions (IList<string> arguments)
 			HostName = defaults.StringForKey ("network.host.name");
 			HostPort = (int) defaults.IntForKey ("network.host.port");
 			UseTcpTunnel = defaults.BoolForKey ("execution.usetcptunnel");
-			Transport = defaults.StringForKey ("network.transport");
+			var transport = defaults.StringForKey ("network.transport");
+			if (!string.IsNullOrEmpty (transport))
+				Transport = transport;
 			SortNames = defaults.BoolForKey ("display.sort");
 			LogFile = defaults.StringForKey ("log.file");
 			TestName = defaults.StringForKey ("test.name");
@@ -89,8 +91,9 @@ public TouchOptions (IList<string> arguments)
 				HostPort = i;
 			if (bool.TryParse (Environment.GetEnvironmentVariable ("NUNIT_SORTNAMES"), out b))
 				SortNames = b;
-			if (!string.IsNullOrEmpty (Environment.GetEnvironmentVariable ("NUNIT_TRANSPORT")))
-				Transport = Environment.GetEnvironmentVariable ("NUNIT_TRANSPORT");
+			var envTransport = Environment.GetEnvironmentVariable ("NUNIT_TRANSPORT");
+			if (!string.IsNullOrEmpty (envTransport))
+				Transport = envTransport;
 			if (bool.TryParse (Environment.GetEnvironmentVariable ("NUNIT_ENABLE_XML_OUTPUT"), out b))
 				EnableXml = b;
 			var xml_mode = Environment.GetEnvironmentVariable ("NUNIT_ENABLE_XML_MODE");
@@ -110,7 +113,7 @@ public TouchOptions (IList<string> arguments)
 				{ "hostport=", "HTTP/TCP port to connect to.", v => HostPort = int.Parse (v) },
 				{ "use-tcp-tunnel", "Use a TCP tunnel to connect to the host.", v => UseTcpTunnel = true },
 				{ "enablenetwork", "Enable the network reporter.", v => EnableNetwork = true },
-				{ "transport=", "Select transport method. Either TCP (default), HTTP or FILE.", v => Transport = v },
+				{ "transport=", "Select transport method. Either TCP (default), HTTP or FILE.", v => { if (v is not null) Transport = v; } },
 				{ "enablexml:", "Enable the xml reported.", v => EnableXml = string.IsNullOrEmpty (v) ? true : bool.Parse (v) },
 				{ "xmlmode=", "The xml mode.", v => XmlMode = (XmlMode) Enum.Parse (typeof (XmlMode), v, true) },
 				{ "xmlversion=", "The xml version.", v => XmlVersion = (XmlVersion) Enum.Parse (typeof (XmlVersion), v, true) },
@@ -138,7 +141,7 @@ public TouchOptions ()
 
 		public bool EnableXml { get; set; }
 
-		public string HostName { get; private set; }
+		public string? HostName { get; private set; }
 
 		public int HostPort { get; private set; }
 
@@ -150,7 +153,7 @@ public TouchOptions ()
 
 		public string Transport { get; set; } = "TCP";
 
-		public string LogFile { get; set; }
+		public string? LogFile { get; set; }
 
 		public bool ShowUseNetworkLogger {
 			get { return (EnableNetwork && !String.IsNullOrWhiteSpace (HostName) && (HostPort > 0)) || Transport == "FILE"; }
@@ -158,7 +161,7 @@ public bool ShowUseNetworkLogger {
 
 		public bool SortNames { get; set; }
 
-		public string TestName { get; set; }
+		public string? TestName { get; set; }
 
 #if !__MACOS__
 		public UIViewController GetViewController ()
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TouchRunner.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TouchRunner.cs
index 7637016d6186..a2a873e133e8 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TouchRunner.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TouchRunner.cs
@@ -94,7 +94,7 @@ public ITestFilter Filter {
 			set { filter = value; }
 		}
 
-		public HashSet<string> ExcludedCategories { get; set; }
+		public HashSet<string>? ExcludedCategories { get; set; }
 
 		public bool TerminateAfterExecution {
 			get { return TouchOptions.Current.TerminateAfterExecution && !connection_failure; }
@@ -102,7 +102,7 @@ public bool TerminateAfterExecution {
 		}
 
 		List<Assembly> assemblies = new List<Assembly> ();
-		List<string> fixtures;
+		List<string>? fixtures;
 
 		public void Add (Assembly assembly)
 		{
@@ -188,7 +188,7 @@ public void SelectLastTestSuite ()
 			}
 		}
 
-		ITest Find (ITest parent, string fullname, List<ITest> hierarchy)
+		ITest? Find (ITest parent, string fullname, List<ITest> hierarchy)
 		{
 			if (parent.FullName == fullname) {
 				hierarchy.Add (parent);
@@ -274,13 +274,13 @@ public void Run ()
 
 		#region writer
 
-		public TestResult Result { get; set; }
+		public TestResult? Result { get; set; }
 
-		public TextWriter Writer { get; set; }
+		public TextWriter? Writer { get; set; }
 
-		Task WriterFinishedTask { get; set; }
+		Task? WriterFinishedTask { get; set; }
 
-		static string SelectHostName (string [] names, int port)
+		static string? SelectHostName (string [] names, int port)
 		{
 			if (names.Length == 0)
 				return null;
@@ -289,7 +289,7 @@ static string SelectHostName (string [] names, int port)
 				return names [0];
 
 			object lock_obj = new object ();
-			string result = null;
+			string? result = null;
 			int failures = 0;
 
 			using (var evt = new ManualResetEvent (false)) {
@@ -333,17 +333,21 @@ public bool OpenWriter (string message)
 				var writers = new List<TextWriter> ();
 				if (options.ShowUseNetworkLogger) {
 					try {
-						string hostname = null;
+						string? hostname = null;
 						WriterFinishedTask = null;
-						TextWriter defaultWriter = null;
+						TextWriter? defaultWriter = null;
 						switch (options.Transport) {
 						case "FILE":
+							if (string.IsNullOrEmpty (options.LogFile))
+								throw new InvalidOperationException ("The FILE transport requires a log file path.");
 							Console.WriteLine ("[{0}] Sending '{1}' results to the file {2}", now, message, options.LogFile);
 							defaultWriter = new StreamWriter (options.LogFile, true, System.Text.Encoding.UTF8) {
 								AutoFlush = true,
 							};
 							break;
 						case "HTTP":
+							if (string.IsNullOrWhiteSpace (options.HostName))
+								throw new InvalidOperationException ("No host name specified.");
 							var hostnames = options.HostName.Split (',');
 							hostname = hostnames [0];
 							if (hostnames.Length > 1)
@@ -361,6 +365,8 @@ public bool OpenWriter (string message)
 							Console.WriteLine ("Unknown transport '{0}': switching to default (TCP)", options.Transport);
 							goto case "TCP";
 						case "TCP":
+							if (string.IsNullOrWhiteSpace (options.HostName))
+								throw new InvalidOperationException ("No host name specified.");
 							if (!options.UseTcpTunnel)
 								hostname = SelectHostName (options.HostName.Split (','), options.HostPort);
 							else
@@ -411,18 +417,21 @@ public bool OpenWriter (string message)
 				Writer = new MultiplexedTextWriter (writers);
 			}
 
-			if (Writer is null)
-				Writer = Console.Out;
+			var writer = Writer;
+			if (writer is null) {
+				writer = Console.Out;
+				Writer = writer;
+			}
 
-			Writer.WriteLine ("[Runner executing:\t{0}]", message);
-			Writer.WriteLine ("[MonoTouch Version:\t{0}]", Constants.Version);
-			Writer.WriteLine ("[Assembly:\t{0}.dll ({1} bits)]", typeof (NSObject).Assembly.GetName ().Name, IntPtr.Size * 8);
-			Writer.WriteLine ("[GC:\t{0}]", GC.MaxGeneration == 0 ? "Boehm" : "sgen");
-			WriteDeviceInformation (Writer);
-			Writer.WriteLine ("[Device Locale:\t{0}]", NSLocale.CurrentLocale.Identifier);
-			Writer.WriteLine ("[Device Date/Time:\t{0}]", now); // to match earlier C.WL output
+			writer.WriteLine ("[Runner executing:\t{0}]", message);
+			writer.WriteLine ("[MonoTouch Version:\t{0}]", Constants.Version);
+			writer.WriteLine ("[Assembly:\t{0}.dll ({1} bits)]", typeof (NSObject).Assembly.GetName ().Name, IntPtr.Size * 8);
+			writer.WriteLine ("[GC:\t{0}]", GC.MaxGeneration == 0 ? "Boehm" : "sgen");
+			WriteDeviceInformation (writer);
+			writer.WriteLine ("[Device Locale:\t{0}]", NSLocale.CurrentLocale.Identifier);
+			writer.WriteLine ("[Device Date/Time:\t{0}]", now); // to match earlier C.WL output
 
-			Writer.WriteLine ("[Bundle:\t{0}]", NSBundle.MainBundle.BundleIdentifier);
+			writer.WriteLine ("[Bundle:\t{0}]", NSBundle.MainBundle.BundleIdentifier);
 			// FIXME: add data about how the app was compiled (e.g. ARMvX, LLVM, GC and Linker options)
 			PassedCount = 0;
 			IgnoredCount = 0;
@@ -432,7 +441,7 @@ public bool OpenWriter (string message)
 		}
 
 		// returns true if test run should still start
-		bool ShowConnectionErrorAlert (string hostname, int port, Exception ex)
+		bool ShowConnectionErrorAlert (string? hostname, int port, Exception ex)
 		{
 #if __TVOS__ || __MACOS__
 			return true;
@@ -446,11 +455,12 @@ bool ShowConnectionErrorAlert (string hostname, int port, Exception ex)
 				return true;
 
 			Console.WriteLine ("Network error: Cannot connect to {0}:{1}: {2}.", hostname, port, ex);
+			var alertDelegate = new UIAlertViewDelegate ();
 			UIAlertView alert = new UIAlertView ("Network Error",
 				String.Format ("Cannot connect to {0}:{1}: {2}. Continue on console ?", hostname, port, ex.Message),
-				(IUIAlertViewDelegate) null, "Cancel", "Continue");
+				alertDelegate, "Cancel", "Continue");
 			int button = -1;
-			alert.Clicked += delegate (object sender, UIButtonEventArgs e)
+			alert.Clicked += delegate (object? sender, UIButtonEventArgs e)
 			{
 				button = (int) e.ButtonIndex;
 			};
@@ -471,13 +481,17 @@ public virtual void Show (TestSuite suite)
 
 		public void CloseWriter ()
 		{
+			var writer = Writer;
+			if (writer is null)
+				return;
+
 			int total = PassedCount + InconclusiveCount + FailedCount; // ignored are *not* run
-			Writer.WriteLine ("Tests run: {0} Passed: {1} Inconclusive: {2} Failed: {3} Ignored: {4}", total, PassedCount, InconclusiveCount, FailedCount, IgnoredCount);
+			writer.WriteLine ("Tests run: {0} Passed: {1} Inconclusive: {2} Failed: {3} Ignored: {4}", total, PassedCount, InconclusiveCount, FailedCount, IgnoredCount);
 
 			// In some cases, the close is not correctly implemented and we might get a InvalidOperationException, we try to close and then null the obj for it to be
 			// GC.
 			try {
-				Writer.Close ();
+				writer.Close ();
 			} finally {
 				Writer = null;
 			}
@@ -487,26 +501,30 @@ public void CloseWriter ()
 
 		public void TestStarted (ITest test)
 		{
+			var writer = Writer ?? Console.Out;
 			if (test is TestSuite) {
-				Writer.WriteLine ();
+				writer.WriteLine ();
 #if NUNITLITE_NUGET
-				Writer.WriteLine (test.FullName);
+				writer.WriteLine (test.FullName);
 #else
-				Writer.WriteLine (test.Name);
+				writer.WriteLine (test.Name);
 #endif
 			}
 		}
 
 		public virtual void TestFinished (ITestResult r)
 		{
-			TestResult result = r as TestResult;
+			if (r is not TestResult result)
+				return;
+
+			var writer = Writer ?? Console.Out;
 
 			if (result.Test is TestSuite) {
 				if (!result.IsFailure () && !result.IsSuccess () && !result.IsInconclusive () && !result.IsIgnored ())
-					Writer.WriteLine ("\t[INFO] {0}", result.Message);
+					writer.WriteLine ("\t[INFO] {0}", result.Message);
 				var skip = result.Test.Properties ["_SKIPREASON"];
 				if (skip.Count > 0)
-					Writer.WriteLine ("\t[SKIPREASON] {0}", skip [0]);
+					writer.WriteLine ("\t[SKIPREASON] {0}", skip [0]);
 
 #if NUNITLITE_NUGET
 				string name = result.Test.FullName;
@@ -514,46 +532,46 @@ public virtual void TestFinished (ITestResult r)
 				string name = result.Test.Name;
 #endif
 				if (!String.IsNullOrEmpty (name))
-					Writer.WriteLine ("{0} : {1} ms", name, result.GetDuration ().TotalMilliseconds);
+					writer.WriteLine ("{0} : {1} ms", name, result.GetDuration ().TotalMilliseconds);
 			} else {
 				if (result.IsSuccess ()) {
-					Writer.Write ("\t[PASS] ");
+					writer.Write ("\t[PASS] ");
 					PassedCount++;
 				} else if (result.IsIgnored ()) {
-					Writer.Write ("\t[IGNORED] ");
+					writer.Write ("\t[IGNORED] ");
 					IgnoredCount++;
 				} else if (result.IsFailure ()) {
-					Writer.Write ("\t[FAIL] ");
+					writer.Write ("\t[FAIL] ");
 					FailedCount++;
 				} else if (result.IsInconclusive ()) {
-					Writer.Write ("\t[INCONCLUSIVE] ");
+					writer.Write ("\t[INCONCLUSIVE] ");
 					InconclusiveCount++;
 				} else {
-					Writer.Write ("\t[INFO] ");
+					writer.Write ("\t[INFO] ");
 				}
 #if NUNITLITE_NUGET
-				Writer.Write (result.Test.Name);
+				writer.Write (result.Test.Name);
 #else
-				Writer.Write (result.Test.FixtureType.Name);
-				Writer.Write (".");
-				Writer.Write (result.Test.Name);
+				writer.Write (result.Test.FixtureType.Name);
+				writer.Write (".");
+				writer.Write (result.Test.Name);
 #endif
 
-				string message = result.Message;
+				string? message = result.Message;
 				if (!String.IsNullOrEmpty (message)) {
-					Writer.Write (" : {0}", message.Replace ("\r\n", "\\r\\n"));
+					writer.Write (" : {0}", message.Replace ("\r\n", "\\r\\n"));
 				}
-				Writer.WriteLine ();
+				writer.WriteLine ();
 #if NUNITLITE_NUGET
 				if (!string.IsNullOrEmpty (result.Output))
-					Writer.WriteLine (result.Output);
+					writer.WriteLine (result.Output);
 #endif
 
-				string stacktrace = result.StackTrace;
+				string? stacktrace = result.StackTrace;
 				if (!String.IsNullOrEmpty (result.StackTrace)) {
 					string [] lines = stacktrace.Split (new char [] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
 					foreach (string line in lines)
-						Writer.WriteLine ("\t\t{0}", line);
+						writer.WriteLine ("\t\t{0}", line);
 				}
 			}
 		}
@@ -564,7 +582,7 @@ public virtual void TestFinished (ITestResult r)
 #endif
 		};
 
-		SettingsDictionary CreateSettings (SettingsDictionary settings)
+		SettingsDictionary CreateSettings (SettingsDictionary? settings)
 		{
 			if (fixtures is null && (settings is null || settings.Count == 0))
 				return default_settings;
@@ -572,8 +590,13 @@ SettingsDictionary CreateSettings (SettingsDictionary settings)
 			var dict = new Dictionary<string, object> (default_settings);
 
 			if (settings is not null) {
-				foreach (var key in settings.Keys)
-					dict [key?.ToString ()] = settings [key];
+				foreach (var key in settings.Keys) {
+					if (key is not null) {
+						var name = key.ToString ();
+						if (name is not null)
+							dict [name] = settings [key];
+					}
+				}
 			}
 
 			if (fixtures is not null)
@@ -587,27 +610,36 @@ SettingsDictionary CreateSettings (SettingsDictionary settings)
 		DefaultTestAssemblyBuilder builder = new DefaultTestAssemblyBuilder ();
 		List<NUnitTestAssemblyRunner> runners = new List<NUnitTestAssemblyRunner> ();
 
-		public bool Load (string assemblyName, IDictionary<string, object> settings = null)
+		public bool Load (string assemblyName, IDictionary<string, object>? settings = null)
 		{
+			if (string.IsNullOrEmpty (assemblyName))
+				return false;
 			var runner = new NUnitTestAssemblyRunner (builder);
 			runners.Add (runner);
 			return AddSuite ((TestSuite) runner.Load (assemblyName, CreateSettings (settings)));
 		}
 
-		public bool Load (Assembly assembly, IDictionary<string, object> settings = null)
+		public bool Load (Assembly assembly, IDictionary<string, object>? settings = null)
 		{
-			return Load (assembly.GetName ().Name, settings);
+			var assemblyName = assembly.GetName ().Name;
+			if (string.IsNullOrEmpty (assemblyName))
+				return false;
+			return Load (assemblyName, settings);
 		}
 #else
 		NUnitLiteTestAssemblyBuilder builder = new NUnitLiteTestAssemblyBuilder ();
 
-		public bool Load (string assemblyName, SettingsDictionary settings = null)
+		public bool Load (string assemblyName, SettingsDictionary? settings = null)
 		{
+			if (string.IsNullOrEmpty (assemblyName))
+				return false;
 			return AddSuite (builder.Build (assemblyName, CreateSettings (settings)));
 		}
 
-		public bool Load (Assembly assembly, SettingsDictionary settings = null)
+		public bool Load (Assembly assembly, SettingsDictionary? settings = null)
 		{
+			if (assembly.GetName ().Name is null)
+				return false;
 			return AddSuite (builder.Build (assembly, CreateSettings (settings)));
 		}
 #endif
@@ -643,7 +675,7 @@ public void Run (Test test)
 			// The TestResult we get back from the runner is for the top-most test suite,
 			// which isn't necessarily the test that we ran. So look for the TestResult
 			// for the test we ran.
-			ITestResult find_result (ITestResult tr)
+			ITestResult? find_result (ITestResult tr)
 			{
 				if (tr.Test == test)
 					return tr;
@@ -690,7 +722,7 @@ public void TestOutput (TestOutput testOutput)
 #if NUNITLITE_NUGET
 		public void SendMessage (TestMessage message)
 		{
-			Writer.WriteLine (message.ToString ());
+			(Writer ?? Console.Out).WriteLine (message.ToString ());
 		}
 #endif
 	}
@@ -718,7 +750,11 @@ public TouchRunner (UIWindow window)
 		}
 
 		public UINavigationController NavigationController {
-			get { return (UINavigationController) window.RootViewController; }
+			get {
+				if (window.RootViewController is UINavigationController navigationController)
+					return navigationController;
+				throw new InvalidOperationException ("The root view controller must be a navigation controller.");
+			}
 		}
 
 		protected override void TerminateWithSuccess ()
@@ -826,16 +862,12 @@ TestSuiteElement Setup (TestSuite suite)
 
 			Section section = new Section (suite.Name);
 			foreach (ITest test in suite.Tests) {
-				TestSuite ts = (test as TestSuite);
-				if (ts is not null) {
+				if (test is TestSuite ts) {
 					section.Add (Setup (ts));
+				} else if (test is TestMethod tc) {
+					section.Add (Setup (tc));
 				} else {
-					TestMethod tc = (test as TestMethod);
-					if (tc is not null) {
-						section.Add (Setup (tc));
-					} else {
-						throw new NotImplementedException (test.GetType ().ToString ());
-					}
+					throw new NotImplementedException (test.GetType ().ToString ());
 				}
 			}
 
@@ -853,7 +885,7 @@ TestSuiteElement Setup (TestSuite suite)
 			root.Add (options);
 
 			var tvc = new TouchViewController (root);
-			tvc.ViewAppearing += (object sender, EventArgs ea) => {
+			tvc.ViewAppearing += (object? sender, EventArgs ea) => {
 				NotifySelectedTest (suite);
 			};
 			suites_dvc.Add (suite, tvc);
@@ -870,18 +902,16 @@ TestCaseElement Setup (TestMethod test)
 		public override void TestFinished (ITestResult r)
 		{
 			base.TestFinished (r);
+			if (r is not TestResult result)
+				return;
 
 			ExecuteOnMainThread (() => {
-				TestResult result = r as TestResult;
-				TestSuite ts = result.Test as TestSuite;
-				if (ts is not null) {
-					TestSuiteElement tse;
-					if (suite_elements.TryGetValue (ts, out tse))
+				if (result.Test is TestSuite ts) {
+					TestSuiteElement? tse;
+					if (suite_elements.TryGetValue (ts, out tse) && tse is not null)
 						tse.TestFinished (result);
-				} else {
-					TestMethod tc = result.Test as TestMethod;
-					if (tc is not null)
-						case_elements [tc].TestFinished (result);
+				} else if (result.Test is TestMethod tc) {
+					case_elements [tc].TestFinished (result);
 				}
 			});
 		}
@@ -903,7 +933,7 @@ static string UniqueIdentifier {
 			get {
 				IntPtr handle = UIDevice.CurrentDevice.Handle;
 				if (UIDevice.CurrentDevice.RespondsToSelector (new Selector ("uniqueIdentifier")))
-					return CFString.FromHandle (objc_msgSend (handle, Selector.GetHandle ("uniqueIdentifier")));
+					return CFString.FromHandle (objc_msgSend (handle, Selector.GetHandle ("uniqueIdentifier"))) ?? "unknown";
 				return "unknown";
 			}
 		}
@@ -917,7 +947,7 @@ protected override void ExecuteOnMainThread (Action action)
 
 	// A filter that matches a specific test
 	class MatchTestFilter : TestFilter {
-		public ITest MatchTest;
+		public ITest? MatchTest;
 		public List<TestFilter> AndFilters = new List<TestFilter> ();
 
 #if NUNITLITE_NUGET
@@ -929,6 +959,9 @@ public override TNode AddToXml (TNode parentNode, bool recursive)
 
 		public override bool Match (ITest test)
 		{
+			if (MatchTest is null)
+				return false;
+
 			if (AndFilters is not null) {
 				// If any of the And filters returns false, then return false too.
 				if (AndFilters.Any ((v) => !v.Match (test)))
@@ -986,13 +1019,13 @@ public override void Write (char value)
 				writer.Write (value);
 		}
 
-		public override void Write (char [] buffer)
+		public override void Write (char []? buffer)
 		{
 			foreach (var writer in writers)
 				writer.Write (buffer);
 		}
 
-		public override void WriteLine (string value)
+		public override void WriteLine (string? value)
 		{
 			foreach (var writer in writers)
 				writer.WriteLine (value);
diff --git a/tests/common/Touch.Unit/Touch.Client/Runner/TouchViewController.cs b/tests/common/Touch.Unit/Touch.Client/Runner/TouchViewController.cs
index a314bdfe0572..a9192e18a04b 100644
--- a/tests/common/Touch.Unit/Touch.Client/Runner/TouchViewController.cs
+++ b/tests/common/Touch.Unit/Touch.Client/Runner/TouchViewController.cs
@@ -61,7 +61,10 @@ public TouchViewController (RootElement root) : base (root, true)
 
 		UIBarButtonItem FilterButton {
 			get {
-				return NavigationItem.RightBarButtonItems [1];
+				var items = NavigationItem.RightBarButtonItems;
+				if (items is null || items.Length < 2)
+					throw new InvalidOperationException ("The filter button has not been initialized.");
+				return items [1];
 			}
 		}
 
@@ -72,12 +75,12 @@ enum ResultFilter {
 			Success,
 		}
 
-		string OriginalCaption { get; set; }
-		List<Element> Unfiltered { get; set; }
+		string? OriginalCaption { get; set; }
+		List<Element> Unfiltered { get; set; } = new List<Element> ();
 
 		ResultFilter CurrentFilter { get; set; }
 
-		void ChangeFilter (object sender, EventArgs e)
+		void ChangeFilter (object? sender, EventArgs e)
 		{
 			switch (CurrentFilter) {
 			case ResultFilter.All:
@@ -137,7 +140,10 @@ public void Filter ()
 
 		UIBarButtonItem SortButton {
 			get {
-				return NavigationItem.RightBarButtonItems [0];
+				var items = NavigationItem.RightBarButtonItems;
+				if (items is null || items.Length == 0)
+					throw new InvalidOperationException ("The sort button has not been initialized.");
+				return items [0];
 			}
 		}
 
@@ -152,7 +158,7 @@ enum SortOrder {
 		static ElementComparer Ascending = new ElementComparer (SortOrder.Ascending);
 		static ElementComparer Descending = new ElementComparer (SortOrder.Descending);
 
-		void ChangeSort (object sender, EventArgs e)
+		void ChangeSort (object? sender, EventArgs e)
 		{
 			List<Element> list = Root [0].Elements;
 			switch (CurrentSortOrder) {
@@ -178,20 +184,20 @@ public ElementComparer (SortOrder sortOrder)
 				order = sortOrder == SortOrder.Descending ? -1 : 1;
 			}
 
-			public int Compare (Element x, Element y)
+			public int Compare (Element? x, Element? y)
 			{
-				return order * x.Caption.CompareTo (y.Caption);
+				return order * string.Compare (x?.Caption, y?.Caption, StringComparison.Ordinal);
 			}
 		}
 
 		// UI
 
-		static UIImage arrow_up;
-		static UIImage arrow_down;
-		static UIImage ok_sign;
-		static UIImage remove_sign;
-		static UIImage question_sign;
-		static UIImage asterisk;
+		static UIImage? arrow_up;
+		static UIImage? arrow_down;
+		static UIImage? ok_sign;
+		static UIImage? remove_sign;
+		static UIImage? question_sign;
+		static UIImage? asterisk;
 
 		static UIImage ArrowUp {
 			get {
@@ -250,15 +256,17 @@ static UIImage GetAwesomeIcon (Action<CGContext> render)
 			float size = 20f;
 			UIGraphics.BeginImageContextWithOptions (new CGSize (size, size), false, 0);
 			using (var c = UIGraphics.GetCurrentContext ()) {
-				c.SetFillColor (1.0f, 1.0f, 1.0f, 1.0f);
-				c.SetStrokeColor (1.0f, 1.0f, 1.0f, 1.0f);
-				c.TranslateCTM (3f, size - 3f);
-				c.ScaleCTM (size / 1000, -size / 1000);
-				render (c);
+				if (c is not null) {
+					c.SetFillColor (1.0f, 1.0f, 1.0f, 1.0f);
+					c.SetStrokeColor (1.0f, 1.0f, 1.0f, 1.0f);
+					c.TranslateCTM (3f, size - 3f);
+					c.ScaleCTM (size / 1000, -size / 1000);
+					render (c);
+				}
 			}
-			UIImage img = UIGraphics.GetImageFromCurrentImageContext ();
+			var img = UIGraphics.GetImageFromCurrentImageContext ();
 			UIGraphics.EndImageContext ();
-			return img;
+			return img ?? new UIImage ();
 		}
 
 		#region generated code
diff --git a/tests/common/Touch.Unit/Touch.Client/dotnet/shared.csproj b/tests/common/Touch.Unit/Touch.Client/dotnet/shared.csproj
index c473f7ddd166..d2d40080acff 100644
--- a/tests/common/Touch.Unit/Touch.Client/dotnet/shared.csproj
+++ b/tests/common/Touch.Unit/Touch.Client/dotnet/shared.csproj
@@ -8,6 +8,9 @@
     <NoWarn>$(NoWarn);CA1416</NoWarn>
     <!-- warning CA1422: This call site is reachable on: '...'. '...' is obsoleted on: '...'. -->
     <NoWarn>$(NoWarn);CA1422</NoWarn>
+
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>Nullable</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\Runner\ExcludedCategoryFilter.cs">
diff --git a/tests/introspection/ApiBaseTest.cs b/tests/introspection/ApiBaseTest.cs
index af67f58c996e..02cd37df96cd 100644
--- a/tests/introspection/ApiBaseTest.cs
+++ b/tests/introspection/ApiBaseTest.cs
@@ -101,7 +101,7 @@ protected TextWriter Writer {
 #if MONOMAC
 			get { return Console.Out; }
 #else
-			get { return AppDelegate.Runner.Writer; }
+			get { return AppDelegate.Runner.Writer!; }
 #endif
 		}