diff --git a/src/QsCompiler/CompilationManager/ProjectManager.cs b/src/QsCompiler/CompilationManager/ProjectManager.cs
index 16bc290cc6..0ad42a3005 100644
--- a/src/QsCompiler/CompilationManager/ProjectManager.cs
+++ b/src/QsCompiler/CompilationManager/ProjectManager.cs
@@ -738,7 +738,7 @@ public bool ManagerTask(Uri file, Action<CompilationUnitManager> executeTask, ID
         /// <summary>
         /// used to log exceptions raised during processing -> may be null!
         /// </summary>
-        private readonly Action<Exception> logException;
+        private readonly Action<Exception>? logException;
 
         /// <summary>
         /// general purpose logging routine used for major loading events -> may be null!
@@ -750,7 +750,7 @@ public bool ManagerTask(Uri file, Action<CompilationUnitManager> executeTask, ID
         /// that action is called whenever diagnostics for the project have changed and are ready for publishing.
         /// Any exceptions caught during processing are logged using the given exception logger.
         /// </summary>
-        public ProjectManager(Action<Exception> exceptionLogger, Action<string, MessageType>? log = null, Action<PublishDiagnosticParams>? publishDiagnostics = null)
+        public ProjectManager(Action<Exception>? exceptionLogger, Action<string, MessageType>? log = null, Action<PublishDiagnosticParams>? publishDiagnostics = null)
         {
             this.load = new ProcessingQueue(exceptionLogger);
             this.projects = new ConcurrentDictionary<Uri, Project>();
diff --git a/src/QsCompiler/LanguageServer/EditorState.cs b/src/QsCompiler/LanguageServer/EditorState.cs
index f9583d0d31..35cbe022d4 100644
--- a/src/QsCompiler/LanguageServer/EditorState.cs
+++ b/src/QsCompiler/LanguageServer/EditorState.cs
@@ -30,7 +30,7 @@ internal class EditorState : IDisposable
 
         private readonly Action<PublishDiagnosticParams> publish;
         private readonly Action<string, Dictionary<string, string?>, Dictionary<string, int>> sendTelemetry;
-        private readonly Action<Uri> onTemporaryProjectLoaded;
+        private readonly Action<Uri>? onTemporaryProjectLoaded;
 
         /// <summary>
         /// needed to determine if the reality of a source file that has changed on disk is indeed given by the content on disk,
@@ -58,11 +58,11 @@ internal class EditorState : IDisposable
         /// </summary>
         internal EditorState(
             ProjectLoader projectLoader,
-            Action<PublishDiagnosticParams> publishDiagnostics,
-            Action<string, Dictionary<string, string?>, Dictionary<string, int>> sendTelemetry,
-            Action<string, MessageType> log,
-            Action<Exception> onException,
-            Action<Uri> onTemporaryProjectLoaded)
+            Action<PublishDiagnosticParams>? publishDiagnostics,
+            Action<string, Dictionary<string, string?>, Dictionary<string, int>>? sendTelemetry,
+            Action<string, MessageType>? log,
+            Action<Exception>? onException,
+            Action<Uri>? onTemporaryProjectLoaded)
         {
             this.ignoreEditorUpdatesForFiles = new ConcurrentDictionary<Uri, byte>();
             this.sendTelemetry = sendTelemetry ?? ((eventName, properties, measurements) => { });
@@ -258,7 +258,7 @@ internal Task OpenFileAsync(
                     {
                         var projectUri = this.QsTemporaryProjectLoader(textDocument.Uri, sdkVersion: null); // null means the Sdk version will be set to the extension version
                         this.projects.ProjectChangedOnDiskAsync(projectUri, this.QsProjectLoader, this.GetOpenFile);
-                        this.onTemporaryProjectLoaded(projectUri);
+                        this.onTemporaryProjectLoaded?.Invoke(projectUri);
                         createdTemporaryProject = true;
                     }
                     catch (Exception ex)
diff --git a/src/QsCompiler/TestTargets/Simulation/Target/Driver.cs b/src/QsCompiler/TestTargets/Simulation/Target/Driver.cs
index 65422a3c9b..1fec88445b 100644
--- a/src/QsCompiler/TestTargets/Simulation/Target/Driver.cs
+++ b/src/QsCompiler/TestTargets/Simulation/Target/Driver.cs
@@ -32,9 +32,9 @@ static void Main(string[] args)
 
             using (var qsim = new QuantumSimulator())
             {
-                Task task = entryPoint.Invoke(null, new[] { qsim }) as Task<QVoid>;
+                var task = entryPoint.Invoke(null, new[] { qsim }) as Task<QVoid>;
                 task?.Wait();
             }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/QsCompiler/TestTargets/Simulation/Target/Program.cs b/src/QsCompiler/TestTargets/Simulation/Target/Program.cs
index 2902715bd8..3c13ec56cc 100644
--- a/src/QsCompiler/TestTargets/Simulation/Target/Program.cs
+++ b/src/QsCompiler/TestTargets/Simulation/Target/Program.cs
@@ -31,7 +31,8 @@ public CsharpGeneration() =>
         public IDictionary<string, string> AssemblyConstants { get; }
 
         /// <inheritdoc/>
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics { get; private set; }
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics { get; private set; } =
+            Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         /// <inheritdoc/>
         public bool ImplementsTransformation => true;
diff --git a/src/QsCompiler/TestTargets/Simulation/Target/Simulation.csproj b/src/QsCompiler/TestTargets/Simulation/Target/Simulation.csproj
index 179585d9cf..60927301c2 100644
--- a/src/QsCompiler/TestTargets/Simulation/Target/Simulation.csproj
+++ b/src/QsCompiler/TestTargets/Simulation/Target/Simulation.csproj
@@ -6,6 +6,8 @@
     <TargetFramework>netstandard2.1</TargetFramework>
     <PlatformTarget>x64</PlatformTarget>
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/QsCompiler/Tests.DocGenerator/Tests.DocGenerator.csproj b/src/QsCompiler/Tests.DocGenerator/Tests.DocGenerator.csproj
index a0328017db..963b1ecbd5 100644
--- a/src/QsCompiler/Tests.DocGenerator/Tests.DocGenerator.csproj
+++ b/src/QsCompiler/Tests.DocGenerator/Tests.DocGenerator.csproj
@@ -6,6 +6,8 @@
     <IsPackable>false</IsPackable>
     <RootNamespace>Microsoft.Quantum.QsCompiler.Documentation.Testing</RootNamespace>
     <AssemblyName>Tests.Microsoft.Quantum.QsDocumentationParser</AssemblyName>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/QsCompiler/Tests.LanguageServer/ProjectLoaderTests.cs b/src/QsCompiler/Tests.LanguageServer/ProjectLoaderTests.cs
index 54b9cab3de..562030ff8e 100644
--- a/src/QsCompiler/Tests.LanguageServer/ProjectLoaderTests.cs
+++ b/src/QsCompiler/Tests.LanguageServer/ProjectLoaderTests.cs
@@ -38,7 +38,7 @@ private static string ProjectFileName(string project) =>
         private static string SourceFileName(string project, string fileName) =>
             Path.Combine("TestProjects", project, fileName);
 
-        private (string, ProjectInformation) Context(string project)
+        private (string, ProjectInformation?) Context(string project)
         {
             var relativePath = ProjectFileName(project);
             var uri = new Uri(Path.GetFullPath(relativePath));
@@ -73,18 +73,18 @@ public void SupportedTargetFrameworks()
         [TestMethod]
         public void FindProjectTargetFramework()
         {
-            void CompareFramework(string project, string expected)
+            void CompareFramework(string project, string? expected)
             {
                 var projectFileName = ProjectFileName(project);
                 var props = new ProjectLoader().DesignTimeBuildProperties(projectFileName, out var _, (x, y) => (y.Contains('.') ? 1 : 0) - (x.Contains('.') ? 1 : 0));
-                if (!props.TryGetValue("TargetFramework", out string actual))
+                if (!props.TryGetValue("TargetFramework", out var actual))
                 {
                     actual = null;
                 }
                 Assert.AreEqual(expected, actual);
             }
 
-            var testProjects = new (string, string)[]
+            var testProjects = new (string, string?)[]
             {
                 ("test1", "netcoreapp2.1"),
                 ("test2", "netstandard2.0"),
@@ -129,10 +129,10 @@ public void LoadNonQsharpProjects()
         public void LoadOutdatedQsharpProject()
         {
             var (projectFile, context) = this.Context("test9");
-            var projDir = Path.GetDirectoryName(projectFile);
+            var projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test9.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test9.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             var qsFiles = new string[]
             {
@@ -149,10 +149,10 @@ public void LoadOutdatedQsharpProject()
         public void LoadQsharpCoreLibraries()
         {
             var (projectFile, context) = this.Context("test3");
-            var projDir = Path.GetDirectoryName(projectFile);
+            var projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test3.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test3.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             var qsFiles = new string[]
             {
@@ -168,10 +168,10 @@ public void LoadQsharpCoreLibraries()
             CollectionAssert.AreEquivalent(qsFiles, context.SourceFiles.ToArray());
 
             (projectFile, context) = this.Context("test12");
-            projDir = Path.GetDirectoryName(projectFile);
+            projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test12.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test12.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             qsFiles = new string[]
             {
@@ -191,10 +191,10 @@ public void LoadQsharpCoreLibraries()
         public void LoadQsharpFrameworkLibrary()
         {
             var (projectFile, context) = this.Context("test7");
-            var projDir = Path.GetDirectoryName(projectFile);
+            var projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test7.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test7.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             var qsFiles = new string[]
             {
@@ -211,10 +211,10 @@ public void LoadQsharpFrameworkLibrary()
         public void LoadQsharpConsoleApps()
         {
             var (projectFile, context) = this.Context("test4");
-            var projDir = Path.GetDirectoryName(projectFile);
+            var projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test4.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test4.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             var qsFiles = new string[]
             {
@@ -228,10 +228,10 @@ public void LoadQsharpConsoleApps()
             CollectionAssert.AreEquivalent(qsFiles, context.SourceFiles.ToArray());
 
             (projectFile, context) = this.Context("test10");
-            projDir = Path.GetDirectoryName(projectFile);
+            projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test10.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test10.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             qsFiles = new string[]
             {
@@ -243,10 +243,10 @@ public void LoadQsharpConsoleApps()
             CollectionAssert.AreEquivalent(qsFiles, context.SourceFiles.ToArray());
 
             (projectFile, context) = this.Context("test11");
-            projDir = Path.GetDirectoryName(projectFile);
+            projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test11.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test11.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             qsFiles = new string[]
             {
@@ -262,10 +262,10 @@ public void LoadQsharpConsoleApps()
         public void LoadQsharpUnitTest()
         {
             var (projectFile, context) = this.Context("test5");
-            var projDir = Path.GetDirectoryName(projectFile);
+            var projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test5.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test5.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             var qsFiles = new string[]
             {
@@ -286,10 +286,10 @@ public void LoadQsharpUnitTest()
         public void LoadQsharpMultiFrameworkLibrary()
         {
             var (projectFile, context) = this.Context("test6");
-            var projDir = Path.GetDirectoryName(projectFile);
+            var projDir = Path.GetDirectoryName(projectFile) ?? "";
             Assert.IsNotNull(context);
-            Assert.AreEqual("test6.dll", Path.GetFileName(context.Properties.OutputPath));
-            Assert.IsTrue(Path.GetDirectoryName(context.Properties.OutputPath).StartsWith(projDir));
+            Assert.AreEqual("test6.dll", Path.GetFileName(context!.Properties.OutputPath));
+            Assert.IsTrue((Path.GetDirectoryName(context.Properties.OutputPath) ?? "").StartsWith(projDir));
 
             var qsFiles = new string[]
             {
@@ -314,8 +314,9 @@ public void LoadQsharpTemporaryProject()
 
             var qsFiles = new string[] { sourceFile };
             var projectInformation = CompilationContext.Load(projectUri);
-            Assert.IsTrue(projectInformation.UsesCanon());
-            CollectionAssert.AreEquivalent(qsFiles, projectInformation.SourceFiles.ToArray());
+            Assert.IsNotNull(projectInformation);
+            Assert.IsTrue(projectInformation!.UsesCanon());
+            CollectionAssert.AreEquivalent(qsFiles, projectInformation!.SourceFiles.ToArray());
         }
     }
 
@@ -324,7 +325,7 @@ internal static class CompilationContext
         private static void LogOutput(string msg, MessageType level) =>
             Console.WriteLine($"[{level}]: {msg}");
 
-        internal static ProjectInformation Load(Uri projectFile) =>
+        internal static ProjectInformation? Load(Uri projectFile) =>
             new EditorState(new ProjectLoader(LogOutput), null, null, null, null, null)
                 .QsProjectLoader(projectFile, out var loaded) ? loaded : null;
 
diff --git a/src/QsCompiler/Tests.LanguageServer/TestSetup.cs b/src/QsCompiler/Tests.LanguageServer/TestSetup.cs
index 82fb6912d6..4ee4cbd245 100644
--- a/src/QsCompiler/Tests.LanguageServer/TestSetup.cs
+++ b/src/QsCompiler/Tests.LanguageServer/TestSetup.cs
@@ -18,8 +18,8 @@ public sealed partial class BasicFunctionality : IDisposable
     {
         // basic setup
 
-        private Connection connection;
-        private JsonRpc rpc;
+        private Connection? connection;
+        private JsonRpc rpc = null!; // Initialized in SetupServerConnectionAsync.
         private readonly RandomInput inputGenerator = new RandomInput();
         private readonly Stack<PublishDiagnosticParams> receivedDiagnostics = new Stack<PublishDiagnosticParams>();
 
@@ -28,7 +28,7 @@ public Task<string[]> GetFileContentInMemoryAsync(string filename) =>
                 Methods.WorkspaceExecuteCommand.Name,
                 TestUtils.ServerCommand(CommandIds.FileContentInMemory, TestUtils.GetTextDocumentIdentifier(filename)));
 
-        public Task<Diagnostic[]> GetFileDiagnosticsAsync(string filename = null) =>
+        public Task<Diagnostic[]> GetFileDiagnosticsAsync(string? filename = null) =>
             this.rpc.InvokeWithParameterObjectAsync<Diagnostic[]>(
                 Methods.WorkspaceExecuteCommand.Name,
                 TestUtils.ServerCommand(CommandIds.FileDiagnostics, filename == null ? new TextDocumentIdentifier { Uri = null } : TestUtils.GetTextDocumentIdentifier(filename)));
diff --git a/src/QsCompiler/Tests.LanguageServer/TestUtils.cs b/src/QsCompiler/Tests.LanguageServer/TestUtils.cs
index 9f0443f69f..3c8163e905 100644
--- a/src/QsCompiler/Tests.LanguageServer/TestUtils.cs
+++ b/src/QsCompiler/Tests.LanguageServer/TestUtils.cs
@@ -107,16 +107,12 @@ internal static int GetRangeLength(VisualStudio.LanguageServer.Protocol.Range ra
 
         internal static void ApplyEdit(TextDocumentContentChangeEvent change, ref List<string> content)
         {
-            if (content == null)
-            {
-                throw new ArgumentNullException(nameof(content));
-            }
             if (!content.Any())
             {
                 throw new ArgumentException("the given content has to have at least on line");
             }
 
-            Assert.IsTrue(IsValidRange(change?.Range) && change.Text != null);
+            Assert.IsTrue(IsValidRange(change.Range) && change.Text != null);
             Assert.IsTrue(change.Range.End.Line < content.Count());
             Assert.IsTrue(change.Range.Start.Character <= content[change.Range.Start.Line].Length);
             Assert.IsTrue(change.Range.End.Character <= content[change.Range.End.Line].Length);
@@ -143,7 +139,7 @@ internal static void ApplyEdit(TextDocumentContentChangeEvent change, ref List<s
             content.InsertRange(startLine, lineChanges);
         }
 
-        internal static bool IsValidRange(Range range)
+        internal static bool IsValidRange(Range? range)
         {
             static bool IsValidPosition(Position position) => position.Line >= 0 && position.Character >= 0;
 
diff --git a/src/QsCompiler/Tests.LanguageServer/Tests.LanguageServer.csproj b/src/QsCompiler/Tests.LanguageServer/Tests.LanguageServer.csproj
index b0c6a44f06..16cfc61b0f 100644
--- a/src/QsCompiler/Tests.LanguageServer/Tests.LanguageServer.csproj
+++ b/src/QsCompiler/Tests.LanguageServer/Tests.LanguageServer.csproj
@@ -8,6 +8,8 @@
     <PlatformTarget>x64</PlatformTarget>
     <OutputType>Library</OutputType>
     <NoWarn>NU1701</NoWarn>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
diff --git a/src/QsCompiler/Tests.LanguageServer/Tests.cs b/src/QsCompiler/Tests.LanguageServer/Tests.cs
index 7cfe8f2577..b5644cd652 100644
--- a/src/QsCompiler/Tests.LanguageServer/Tests.cs
+++ b/src/QsCompiler/Tests.LanguageServer/Tests.cs
@@ -81,9 +81,9 @@ public async Task ServerCapabilitiesAsync()
             Assert.IsNotNull(initReply.Capabilities.CompletionProvider.TriggerCharacters);
             Assert.IsTrue(initReply.Capabilities.CompletionProvider.TriggerCharacters.SequenceEqual(new[] { ".", "(" }));
             Assert.IsNotNull(initReply.Capabilities.SignatureHelpProvider?.TriggerCharacters);
-            Assert.IsTrue(initReply.Capabilities.SignatureHelpProvider.TriggerCharacters.Any());
+            Assert.IsTrue(initReply.Capabilities.SignatureHelpProvider!.TriggerCharacters.Any());
             Assert.IsNotNull(initReply.Capabilities.ExecuteCommandProvider?.Commands);
-            Assert.IsNotNull(initReply.Capabilities.ExecuteCommandProvider.Commands.Contains(CommandIds.ApplyEdit));
+            Assert.IsTrue(initReply.Capabilities.ExecuteCommandProvider!.Commands.Contains(CommandIds.ApplyEdit));
             Assert.IsTrue(initReply.Capabilities.TextDocumentSync.OpenClose);
             Assert.IsTrue(initReply.Capabilities.TextDocumentSync.Save.IncludeText);
             Assert.AreEqual(TextDocumentSyncKind.Incremental, initReply.Capabilities.TextDocumentSync.Change);