diff --git a/src/Compilers/Core/CommandLine/CompilerServerLogger.cs b/src/Compilers/Core/CommandLine/CompilerServerLogger.cs
index 0efc3761e0516d048c9fb3a2c4208883a663e4bc..74352c0d31c3b46f9b7f66ab418e20041d41d22a 100644
--- a/src/Compilers/Core/CommandLine/CompilerServerLogger.cs
+++ b/src/Compilers/Core/CommandLine/CompilerServerLogger.cs
@@ -41,7 +41,7 @@ static CompilerServerLogger()
             try
             {
                 // Check if the environment
-                string loggingFileName = Environment.GetEnvironmentVariable(environmentVariable);
+                string? loggingFileName = Environment.GetEnvironmentVariable(environmentVariable);
 
                 if (loggingFileName != null)
                 {
@@ -75,15 +75,15 @@ public static void Initialize(string outputPrefix)
         /// <summary>
         /// Log an exception. Also logs information about inner exceptions.
         /// </summary>
-        public static void LogException(Exception e, string reason)
+        public static void LogException(Exception exception, string reason)
         {
             if (s_loggingStream != null)
             {
-                Log("Exception '{0}' occurred during '{1}'. Stack trace:\r\n{2}", e.Message, reason, e.StackTrace);
+                Log("Exception '{0}' occurred during '{1}'. Stack trace:\r\n{2}", exception.Message, reason, exception.StackTrace);
 
                 int innerExceptionLevel = 0;
 
-                e = e.InnerException;
+                Exception? e = exception.InnerException;
                 while (e != null)
                 {
                     Log("Inner exception[{0}] '{1}'. Stack trace: \r\n{1}", innerExceptionLevel, e.Message, e.StackTrace);
diff --git a/src/Compilers/Core/MSBuildTask/ManagedCompiler.cs b/src/Compilers/Core/MSBuildTask/ManagedCompiler.cs
index 4470b8ec286b98729014f76125cd86fe72b2d2a4..617b695dd32bdc8a43519e42bcdc07a4f5e78b83 100644
--- a/src/Compilers/Core/MSBuildTask/ManagedCompiler.cs
+++ b/src/Compilers/Core/MSBuildTask/ManagedCompiler.cs
@@ -343,7 +343,12 @@ public bool SkipCompilerExecution
 
         public string? TargetType
         {
-            set { _store[nameof(TargetType)] = CultureInfo.InvariantCulture.TextInfo.ToLower(value); }
+            set
+            {
+                _store[nameof(TargetType)] = value != null
+                    ? CultureInfo.InvariantCulture.TextInfo.ToLower(value)
+                    : null;
+            }
             get { return (string?)_store[nameof(TargetType)]; }
         }
 
@@ -467,11 +472,11 @@ protected override int ExecuteTool(string pathToTool, string responseFileCommand
             try
             {
                 var workingDir = CurrentDirectoryToUse();
-                string tempDir = BuildServerConnection.GetTempPath(workingDir);
+                string? tempDir = BuildServerConnection.GetTempPath(workingDir);
 
                 if (!UseSharedCompilation ||
                     HasToolBeenOverridden ||
-                    !BuildServerConnection.IsCompilerServerSupported(tempDir))
+                    !BuildServerConnection.IsCompilerServerSupported)
                 {
                     return base.ExecuteTool(pathToTool, responseFileCommands, commandLineCommands);
                 }
@@ -483,6 +488,10 @@ protected override int ExecuteTool(string pathToTool, string responseFileCommand
                     CompilerServerLogger.Log($"BuildResponseFile = '{responseFileCommands}'");
 
                     var clientDir = Path.GetDirectoryName(PathToManagedTool);
+                    if (clientDir is null || tempDir is null)
+                    {
+                        return base.ExecuteTool(pathToTool, responseFileCommands, commandLineCommands);
+                    }
 
                     // Note: we can't change the "tool path" printed to the console when we run
                     // the Csc/Vbc task since MSBuild logs it for us before we get here. Instead,
@@ -567,10 +576,10 @@ private string CurrentDirectoryToUse()
         /// <summary>
         /// Get the "LIB" environment variable, or NULL if none.
         /// </summary>
-        private string LibDirectoryToUse()
+        private string? LibDirectoryToUse()
         {
             // First check the real environment.
-            string libDirectory = Environment.GetEnvironmentVariable("LIB");
+            string? libDirectory = Environment.GetEnvironmentVariable("LIB");
 
             // Now go through additional environment variables.
             string[] additionalVariables = EnvironmentVariables;
diff --git a/src/Compilers/Core/MSBuildTask/MapSourceRoots.cs b/src/Compilers/Core/MSBuildTask/MapSourceRoots.cs
index 5611691b8048037511aa374772ca30984ca5fef2..6d83946e4825dc430e0e15dc46ee3c7645a6750f 100644
--- a/src/Compilers/Core/MSBuildTask/MapSourceRoots.cs
+++ b/src/Compilers/Core/MSBuildTask/MapSourceRoots.cs
@@ -89,7 +89,7 @@ private static bool EndsWithDirectorySeparator(string path)
 
         private void MergeSourceRootMetadata(ITaskItem left, ITaskItem right)
         {
-            foreach (string metadataName in right.MetadataNames)
+            foreach (string? metadataName in right.MetadataNames)
             {
                 var leftValue = left.GetMetadata(metadataName);
                 var rightValue = right.GetMetadata(metadataName);
diff --git a/src/Compilers/Core/MSBuildTask/Microsoft.Build.Tasks.CodeAnalysis.csproj b/src/Compilers/Core/MSBuildTask/Microsoft.Build.Tasks.CodeAnalysis.csproj
index ed371546c2a24087b522457df19b948ab246c70b..1892d067053fed467409e4a78587294b18d9d118 100644
--- a/src/Compilers/Core/MSBuildTask/Microsoft.Build.Tasks.CodeAnalysis.csproj
+++ b/src/Compilers/Core/MSBuildTask/Microsoft.Build.Tasks.CodeAnalysis.csproj
@@ -12,7 +12,7 @@
     <AssemblyVersion/>
     <!-- CA1819 (Properties should not return arrays) disabled as it is very common across this project. -->
     <NoWarn>$(NoWarn);CA1819</NoWarn>
-
+    
     <!-- NuGet -->
     <IsPackable>true</IsPackable>
     <PackageId>Microsoft.CodeAnalysis.Build.Tasks</PackageId>
@@ -23,6 +23,12 @@
     </PackageDescription>
     <GenerateMicrosoftCodeAnalysisCommitHashAttribute>true</GenerateMicrosoftCodeAnalysisCommitHashAttribute>
   </PropertyGroup>
+
+  <!-- Disable the nullable warnings when not compiling for netcoreapp3.1 -->
+  <PropertyGroup Condition="'$(TargetFramework)' != 'netcoreapp3.1'">
+    <NoWarn>$(NoWarn);8600;8601;8602;8603;8604</NoWarn>
+  </PropertyGroup>
+
   <ItemGroup>
     <Content Include="*.targets">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
diff --git a/src/Compilers/Core/MSBuildTask/MvidReader.cs b/src/Compilers/Core/MSBuildTask/MvidReader.cs
index e79fbd28d3ca4b2e457a183e3de67aa2acbdfe0b..42135ab83a81675e88dea8f6bed4b1003eb683c9 100755
--- a/src/Compilers/Core/MSBuildTask/MvidReader.cs
+++ b/src/Compilers/Core/MSBuildTask/MvidReader.cs
@@ -99,7 +99,7 @@ private static Guid FindMvidInSections(ushort count, BinaryReader reader)
                     return s_empty;
                 }
 
-                if (name.Length == 8 && name[0] == '.' &&
+                if (name!.Length == 8 && name[0] == '.' &&
                     name[1] == 'm' && name[2] == 'v' && name[3] == 'i' && name[4] == 'd' && name[5] == '\0')
                 {
                     // Section: VirtualSize (4)
@@ -150,7 +150,7 @@ private static Guid ReadMvidSection(BinaryReader reader, uint pointerToMvidSecti
                 return s_empty;
             }
 
-            return new Guid(guidBytes);
+            return new Guid(guidBytes!);
         }
 
         private static bool ReadUInt16(BinaryReader reader, out ushort output)
@@ -177,7 +177,7 @@ private static bool ReadUInt32(BinaryReader reader, out uint output)
             return true;
         }
 
-        private static bool ReadBytes(BinaryReader reader, int count, [NotNullWhen(true)] out byte[]? output)
+        private static bool ReadBytes(BinaryReader reader, int count, out byte[]? output)
         {
             if (reader.BaseStream.Position + count >= reader.BaseStream.Length)
             {
diff --git a/src/Compilers/Core/MSBuildTask/Utilities.cs b/src/Compilers/Core/MSBuildTask/Utilities.cs
index 01ae73908aa1cd1dbaf8593d0d09c42f4042375d..74da0c2bdd328dd2d3b8b95988d46ad78d447188 100644
--- a/src/Compilers/Core/MSBuildTask/Utilities.cs
+++ b/src/Compilers/Core/MSBuildTask/Utilities.cs
@@ -178,8 +178,8 @@ internal static string GenerateFullPathToTool(string toolName)
             var assemblyDirectory = Path.GetDirectoryName(assemblyPath);
 
             return RuntimeHostInfo.IsDesktopRuntime
-                ? Path.Combine(assemblyDirectory, toolName)
-                : Path.Combine(assemblyDirectory, "bincore", toolName);
+                ? Path.Combine(assemblyDirectory!, toolName)
+                : Path.Combine(assemblyDirectory!, "bincore", toolName);
         }
     }
 }
diff --git a/src/Compilers/Core/MSBuildTask/ValidateBootstrap.cs b/src/Compilers/Core/MSBuildTask/ValidateBootstrap.cs
index 33a867385079a054f0b8d3a33285cb5472f16bb7..2b0a90fd9a9fc74e3725cb233b5250222f25ad4a 100644
--- a/src/Compilers/Core/MSBuildTask/ValidateBootstrap.cs
+++ b/src/Compilers/Core/MSBuildTask/ValidateBootstrap.cs
@@ -36,7 +36,7 @@ public sealed partial class ValidateBootstrap : Task
         public string? TasksAssemblyFullPath
         {
             get { return _tasksAssemblyFullPath; }
-            set { _tasksAssemblyFullPath = NormalizePath(Path.GetFullPath(value)); }
+            set { _tasksAssemblyFullPath = NormalizePath(Path.GetFullPath(value!)); }
         }
 
         public ValidateBootstrap()
@@ -103,7 +103,7 @@ public override bool Execute()
             return path;
         }
 
-        private string GetDirectory(Assembly assembly) => Path.GetDirectoryName(Utilities.TryGetAssemblyPath(assembly));
+        private string? GetDirectory(Assembly assembly) => Path.GetDirectoryName(Utilities.TryGetAssemblyPath(assembly));
 
         internal static void AddFailedLoad(AssemblyName name)
         {
diff --git a/src/Compilers/Core/Portable/InternalUtilities/NullableAttributes.cs b/src/Compilers/Core/Portable/InternalUtilities/NullableAttributes.cs
index 0943bd89d3ebbc8736fd064b6e8a95ac7f6c4bec..fde55dce4fac9f1b91e6f1ca4a3e45186d0fb295 100644
--- a/src/Compilers/Core/Portable/InternalUtilities/NullableAttributes.cs
+++ b/src/Compilers/Core/Portable/InternalUtilities/NullableAttributes.cs
@@ -5,7 +5,7 @@
 // This was copied from https://github.com/dotnet/coreclr/blob/60f1e6265bd1039f023a82e0643b524d6aaf7845/src/System.Private.CoreLib/shared/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
 // and updated to have the scope of the attributes be internal.
 
-#if NETSTANDARD2_0 || NET472 || NET20 || NETSTANDARD1_3 || NET45
+#if !NETCOREAPP
 #nullable enable
 
 namespace System.Diagnostics.CodeAnalysis
@@ -88,7 +88,4 @@ internal sealed class DoesNotReturnIfAttribute : Attribute
     }
 }
 
-#elif NETCOREAPP3_1
-#else
-#error "Unsupported configuration"
 #endif
\ No newline at end of file
diff --git a/src/Compilers/Shared/BuildServerConnection.cs b/src/Compilers/Shared/BuildServerConnection.cs
index fcb138d88584fd3a77e2a62c06d716d58c6dd9d1..cfd9c9bf1c4f6070fba49f599c8e002f49309a22 100644
--- a/src/Compilers/Shared/BuildServerConnection.cs
+++ b/src/Compilers/Shared/BuildServerConnection.cs
@@ -31,7 +31,7 @@ namespace Microsoft.CodeAnalysis.CommandLine
     /// communication layer which is shared between MSBuild and non-MSBuild components. This is the problem that 
     /// BuildPathsAlt fixes as the type lives with the build server communication code.
     /// </summary>
-    internal readonly struct BuildPathsAlt
+    internal sealed class BuildPathsAlt
     {
         /// <summary>
         /// The path which contains the compiler binaries and response files.
@@ -77,7 +77,7 @@ internal sealed class BuildServerConnection
         /// <summary>
         /// Determines if the compiler server is supported in this environment.
         /// </summary>
-        internal static bool IsCompilerServerSupported(string tempPath) => GetPipeNameForPathOpt("") is object;
+        internal static bool IsCompilerServerSupported => GetPipeNameForPathOpt("") is object;
 
         public static Task<BuildResponse> RunServerCompilation(
             RequestLanguage language,
@@ -85,7 +85,7 @@ internal sealed class BuildServerConnection
             List<string> arguments,
             BuildPathsAlt buildPaths,
             string? keepAlive,
-            string libEnvVariable,
+            string? libEnvVariable,
             CancellationToken cancellationToken)
         {
             var pipeNameOpt = sharedCompilationId ?? GetPipeNameForPathOpt(buildPaths.ClientDirectory);
@@ -108,7 +108,7 @@ internal sealed class BuildServerConnection
             BuildPathsAlt buildPaths,
             string? pipeName,
             string? keepAlive,
-            string libEnvVariable,
+            string? libEnvVariable,
             int? timeoutOverride,
             CreateServerFunc createServerFunc,
             CancellationToken cancellationToken)
@@ -588,7 +588,7 @@ internal static string GetClientMutexName(string pipeName)
         /// is <paramref name="workingDir"/>.  This function must emulate <see cref="Path.GetTempPath"/> as 
         /// closely as possible.
         /// </summary>
-        public static string GetTempPath(string? workingDir)
+        public static string? GetTempPath(string? workingDir)
         {
             if (PlatformInformation.IsUnix)
             {
@@ -656,7 +656,7 @@ internal sealed class FileMutex : IDisposable
         internal static string GetMutexDirectory()
         {
             var tempPath = BuildServerConnection.GetTempPath(null);
-            var result = Path.Combine(tempPath, ".roslyn");
+            var result = Path.Combine(tempPath!, ".roslyn");
             Directory.CreateDirectory(result);
             return result;
         }
diff --git a/src/Compilers/Shared/RuntimeHostInfo.cs b/src/Compilers/Shared/RuntimeHostInfo.cs
index d04b6ac8ce4ddc0f3ba4a50a766cc67e3830a9ff..ce4296e5651ca61aa5b4aab0c2704fe7a321be3b 100644
--- a/src/Compilers/Shared/RuntimeHostInfo.cs
+++ b/src/Compilers/Shared/RuntimeHostInfo.cs
@@ -33,7 +33,7 @@ internal static (string processFilePath, string commandLineArguments, string too
             if (IsDotNetHost(out string? pathToDotNet))
             {
                 commandLineArguments = $@"exec ""{toolFilePath}"" {commandLineArguments}";
-                return (pathToDotNet, commandLineArguments, toolFilePath);
+                return (pathToDotNet!, commandLineArguments, toolFilePath);
             }
             else
             {
@@ -58,7 +58,7 @@ internal static bool IsDotNetHost([NotNullWhen(true)] out string? pathToDotNet)
 
         private static string DotNetHostPathEnvironmentName = "DOTNET_HOST_PATH";
 
-        private static bool IsDotNetHost([NotNullWhen(true)] out string? pathToDotNet)
+        private static bool IsDotNetHost(out string? pathToDotNet)
         {
             pathToDotNet = GetDotNetPathOrDefault();
             return true;