Enforce nullability for user code.

- Expanded the `ProjectWorkspaceStateGenerator` to extract the C# language version when building the `ProjectWorkspaceState`. This approach enables all platforms to get nullability support without any changes (as long as they support `ProjectWorkspaceState`, which they do). Also, Roslyn suggested that we avoid dealing with LangVersion directly because there are several factors that impact its "effective" value on a project when run in tooling.
- Updated the `LinePragma` code generation to include `#nullable restore` and `#nullable disable` lines to allow for project restored nullability state for user code.
- Added a new `RazorProjectEngineBuilderExtensions` class that adds Roslyn specific project engine modifications. In this case it allows us to set the C# language version for a project engine and configure underlying features accordingly.
- Added a `SuppressNullabilityEnforcement` flag that only turns on if C# < 8 is specified.
- Updated LiveShare, VS4Mac and RazorGenerate to understand CSharpLanguageVersion.
- Added a single test output to show the change.

dotnet/aspnetcore-tooling#5092
\n\nCommit migrated from 1df8128b87

src/Razor/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/src/InjectTargetExtension.cs

@@ -26,7 +26,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
 
             if (node.Source.HasValue)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     context.CodeWriter
                         .WriteLine(RazorInjectAttribute)

src/Razor/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version2_X/src/InjectTargetExtension.cs

@@ -26,7 +26,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version2_X
 
             if (node.Source.HasValue)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     context.CodeWriter
                         .WriteLine(RazorInjectAttribute)

src/Razor/Microsoft.AspNetCore.Mvc.Razor.Extensions/src/InjectTargetExtension.cs

@@ -26,7 +26,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             if (node.Source.HasValue)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     context.CodeWriter
                         .WriteLine(RazorInjectAttribute)

src/Razor/Microsoft.AspNetCore.Razor.Language/src/CodeGeneration/CodeWriterExtensions.cs

@@ -440,7 +440,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             return new CSharpCodeWritingScope(writer);
         }
 
-        public static IDisposable BuildLinePragma(this CodeWriter writer, SourceSpan? span)
+        public static IDisposable BuildLinePragma(this CodeWriter writer, SourceSpan? span, CodeRenderingContext context)
         {
             if (string.IsNullOrEmpty(span?.FilePath))
             {
@@ -448,7 +448,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
                 return NullDisposable.Default;
             }
 
-            return new LinePragmaWriter(writer, span.Value);
+            return new LinePragmaWriter(writer, span.Value, context.Options);
         }
 
         private static void WriteVerbatimStringLiteral(CodeWriter writer, string literal)
@@ -596,9 +596,13 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
         private class LinePragmaWriter : IDisposable
         {
             private readonly CodeWriter _writer;
+            private readonly RazorCodeGenerationOptions _codeGenerationOptions;
             private readonly int _startIndent;
 
-            public LinePragmaWriter(CodeWriter writer, SourceSpan span)
+            public LinePragmaWriter(
+                CodeWriter writer, 
+                SourceSpan span,
+                RazorCodeGenerationOptions codeGenerationOptions)
             {
                 if (writer == null)
                 {
@@ -606,8 +610,15 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
                 }
 
                 _writer = writer;
+                _codeGenerationOptions = codeGenerationOptions;
                 _startIndent = _writer.CurrentIndent;
                 _writer.CurrentIndent = 0;
+
+                if (!_codeGenerationOptions.SuppressNullabilityEnforcement)
+                {
+                    _writer.WriteLine("#nullable restore");
+                }
+
                 WriteLineNumberDirective(writer, span);
             }
 
@@ -630,6 +641,11 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
                     .WriteLine("#line default")
                     .WriteLine("#line hidden");
 
+                if (!_codeGenerationOptions.SuppressNullabilityEnforcement)
+                {
+                    _writer.WriteLine("#nullable disable");
+                }
+
                 _writer.CurrentIndent = _startIndent;
             }
         }

src/Razor/Microsoft.AspNetCore.Razor.Language/src/CodeGeneration/DesignTimeNodeWriter.cs

@@ -13,7 +13,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
         {
             if (node.Source.HasValue)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     context.AddSourceMappingFor(node);
                     context.CodeWriter.WriteUsing(node.Content);
@@ -44,7 +44,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
 
             if (node.Source != null)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     var offset = DesignTimeDirectivePass.DesignTimeVariable.Length + " = ".Length;
                     context.CodeWriter.WritePadding(offset, node.Source, context);
@@ -91,7 +91,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             IDisposable linePragmaScope = null;
             if (node.Source != null)
             {
-                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value);
+                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value, context);
 
                 context.CodeWriter.WritePadding(0, node.Source.Value, context);
             }
@@ -150,7 +150,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             var firstChild = node.Children[0];
             if (firstChild.Source != null)
             {
-                using (context.CodeWriter.BuildLinePragma(firstChild.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(firstChild.Source.Value, context))
                 {
                     var offset = DesignTimeDirectivePass.DesignTimeVariable.Length + " = ".Length;
                     context.CodeWriter.WritePadding(offset, firstChild.Source, context);
@@ -210,7 +210,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
                     {
                         if (!isWhitespaceStatement)
                         {
-                            linePragmaScope = context.CodeWriter.BuildLinePragma(token.Source.Value);
+                            linePragmaScope = context.CodeWriter.BuildLinePragma(token.Source.Value, context);
                         }
 
                         context.CodeWriter.WritePadding(0, token.Source.Value, context);

src/Razor/Microsoft.AspNetCore.Razor.Language/src/CodeGeneration/RuntimeNodeWriter.cs

@@ -32,7 +32,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
         {
             if (node.Source.HasValue)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     context.CodeWriter.WriteUsing(node.Content);
                 }
@@ -58,7 +58,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             IDisposable linePragmaScope = null;
             if (node.Source != null)
             {
-                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value);
+                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value, context);
                 context.CodeWriter.WritePadding(WriteCSharpExpressionMethod.Length + 1, node.Source, context);
             }
 
@@ -103,7 +103,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             IDisposable linePragmaScope = null;
             if (node.Source != null)
             {
-                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value);
+                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value, context);
                 context.CodeWriter.WritePadding(0, node.Source.Value, context);
             }
 
@@ -200,7 +200,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
 
         public override void WriteCSharpExpressionAttributeValue(CodeRenderingContext context, CSharpExpressionAttributeValueIntermediateNode node)
         {
-            using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+            using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
             {
                 var prefixLocation = node.Source.Value.AbsoluteIndex;
                 context.CodeWriter
@@ -266,7 +266,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
                         {
                             if (!isWhitespaceStatement)
                             {
-                                linePragmaScope = context.CodeWriter.BuildLinePragma(token.Source.Value);
+                                linePragmaScope = context.CodeWriter.BuildLinePragma(token.Source.Value, context);
                             }
 
                             context.CodeWriter.WritePadding(0, token.Source.Value, context);

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentDesignTimeNodeWriter.cs

@@ -61,7 +61,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
 
             if (node.Source.HasValue)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     context.AddSourceMappingFor(node);
                     context.CodeWriter.WriteUsing(node.Content);
@@ -92,7 +92,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
 
             if (node.Source != null)
             {
-                using (context.CodeWriter.BuildLinePragma(node.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(node.Source.Value, context))
                 {
                     var offset = DesignTimeVariable.Length + " = ".Length;
                     context.CodeWriter.WritePadding(offset, node.Source, context);
@@ -162,7 +162,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
             {
                 if (!isWhitespaceStatement)
                 {
-                    linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value);
+                    linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value, context);
                 }
 
                 context.CodeWriter.WritePadding(0, node.Source.Value, context);
@@ -247,7 +247,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
             var firstChild = node.Children[0];
             if (firstChild.Source != null)
             {
-                using (context.CodeWriter.BuildLinePragma(firstChild.Source.Value))
+                using (context.CodeWriter.BuildLinePragma(firstChild.Source.Value, context))
                 {
                     var offset = DesignTimeVariable.Length + " = ".Length;
                     context.CodeWriter.WritePadding(offset, firstChild.Source, context);
@@ -835,7 +835,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
                 return;
             }
 
-            using (context.CodeWriter.BuildLinePragma(token.Source))
+            using (context.CodeWriter.BuildLinePragma(token.Source, context))
             {
                 context.CodeWriter.WritePadding(0, token.Source.Value, context);
                 context.AddSourceMappingFor(token);

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentRuntimeNodeWriter.cs

@@ -55,7 +55,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
             IDisposable linePragmaScope = null;
             if (node.Source != null)
             {
-                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value);
+                linePragmaScope = context.CodeWriter.BuildLinePragma(node.Source.Value, context);
                 context.CodeWriter.WritePadding(0, node.Source.Value, context);
             }
 

src/Razor/Microsoft.AspNetCore.Razor.Language/src/DefaultRazorCodeGenerationOptions.cs

@@ -12,7 +12,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             string rootNamespace,
             bool suppressChecksum,
             bool supressMetadataAttributes,
-            bool suppressPrimaryMethodBody)
+            bool suppressPrimaryMethodBody,
+            bool suppressNullabilityEnforcement)
         {
             IndentWithTabs = indentWithTabs;
             IndentSize = indentSize;
@@ -21,6 +22,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             SuppressChecksum = suppressChecksum;
             SuppressMetadataAttributes = supressMetadataAttributes;
             SuppressPrimaryMethodBody = suppressPrimaryMethodBody;
+            SuppressNullabilityEnforcement = suppressNullabilityEnforcement;
         }
 
         public override bool DesignTime { get; }
@@ -32,5 +34,7 @@ namespace Microsoft.AspNetCore.Razor.Language
         public override string RootNamespace { get; }
 
         public override bool SuppressChecksum { get; }
+
+        public override bool SuppressNullabilityEnforcement { get; }
     }
 }

src/Razor/Microsoft.AspNetCore.Razor.Language/src/DefaultRazorCodeGenerationOptionsBuilder.cs

@@ -37,6 +37,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 
         public override bool SuppressChecksum { get; set; }
 
+        public override bool SuppressNullabilityEnforcement { get; set; }
+
         public override RazorCodeGenerationOptions Build()
         {
             return new DefaultRazorCodeGenerationOptions(
@@ -46,7 +48,8 @@ namespace Microsoft.AspNetCore.Razor.Language
                 RootNamespace,
                 SuppressChecksum,
                 SuppressMetadataAttributes,
-                SuppressPrimaryMethodBody);
+                SuppressPrimaryMethodBody,
+                SuppressNullabilityEnforcement);
         }
 
         public override void SetDesignTime(bool designTime)

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Extensions/DefaultTagHelperTargetExtension.cs

@@ -373,7 +373,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                     var firstMappedChild = node.Children.FirstOrDefault(child => child.Source != null) as IntermediateNode;
                     var valueStart = firstMappedChild?.Source;
 
-                    using (context.CodeWriter.BuildLinePragma(node.Source))
+                    using (context.CodeWriter.BuildLinePragma(node.Source, context))
                     {
                         var accessor = GetPropertyAccessor(node);
                         var assignmentPrefixLength = accessor.Length + " = ".Length;
@@ -422,7 +422,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 }
                 else
                 {
-                    using (context.CodeWriter.BuildLinePragma(node.Source))
+                    using (context.CodeWriter.BuildLinePragma(node.Source, context))
                     {
                         context.CodeWriter.WriteStartAssignment(GetPropertyAccessor(node));
 

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Extensions/DesignTimeDirectiveTargetExtension.cs

@@ -65,7 +65,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                         }
 
                         // {node.Content} __typeHelper = default({node.Content});
-                        using (context.CodeWriter.BuildLinePragma(node.Source))
+                        using (context.CodeWriter.BuildLinePragma(node.Source, context))
                         {
                             context.AddSourceMappingFor(node);
                             context.CodeWriter
@@ -88,7 +88,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                         }
 
                         // global::System.Object {node.content} = null;
-                        using (context.CodeWriter.BuildLinePragma(node.Source))
+                        using (context.CodeWriter.BuildLinePragma(node.Source, context))
                         {
                             context.CodeWriter
                             .Write("global::")
@@ -112,7 +112,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                         }
 
                         // global::System.Object __typeHelper = nameof({node.Content});
-                        using (context.CodeWriter.BuildLinePragma(node.Source))
+                        using (context.CodeWriter.BuildLinePragma(node.Source, context))
                         {
                             context.CodeWriter
                             .Write("global::")
@@ -132,7 +132,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                     case DirectiveTokenKind.String:
 
                         // global::System.Object __typeHelper = "{node.Content}";
-                        using (context.CodeWriter.BuildLinePragma(node.Source))
+                        using (context.CodeWriter.BuildLinePragma(node.Source, context))
                         {
                             context.CodeWriter
                             .Write("global::")
@@ -173,7 +173,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             // for consistency so when a C# completion session starts, filling user code doesn't result in
             // a previously non-existent line pragma from being added and destroying the context in which
             // the completion session was started.
-            using (context.CodeWriter.BuildLinePragma(node.Source))
+            using (context.CodeWriter.BuildLinePragma(node.Source, context))
             {
                 context.AddSourceMappingFor(node);
                 context.CodeWriter.Write(" ");

src/Razor/Microsoft.AspNetCore.Razor.Language/src/RazorCodeGenerationOptions.cs

@@ -16,7 +16,8 @@ namespace Microsoft.AspNetCore.Razor.Language
                 suppressChecksum: false,
                 rootNamespace: null,
                 supressMetadataAttributes: false,
-                suppressPrimaryMethodBody: false);
+                suppressPrimaryMethodBody: false,
+                suppressNullabilityEnforcement: false);
         }
 
         public static RazorCodeGenerationOptions CreateDesignTimeDefault()
@@ -28,7 +29,8 @@ namespace Microsoft.AspNetCore.Razor.Language
                 rootNamespace: null,
                 suppressChecksum: false,
                 supressMetadataAttributes: true,
-                suppressPrimaryMethodBody: false);
+                suppressPrimaryMethodBody: false,
+                suppressNullabilityEnforcement: false);
         }
 
         public static RazorCodeGenerationOptions Create(Action<RazorCodeGenerationOptionsBuilder> configure)
@@ -107,5 +109,10 @@ namespace Microsoft.AspNetCore.Razor.Language
         /// Gets or sets a value that determines if an empty body is generated for the primary method.
         /// </summary>
         public virtual bool SuppressPrimaryMethodBody { get; protected set; }
+
+        /// <summary>
+        /// Gets or sets a value that determines if nullability type enforcement is restored to project settings for user code.
+        /// </summary>
+        public virtual bool SuppressNullabilityEnforcement { get; }
     }
 }

src/Razor/Microsoft.AspNetCore.Razor.Language/src/RazorCodeGenerationOptionsBuilder.cs

@@ -54,6 +54,11 @@ namespace Microsoft.AspNetCore.Razor.Language
         /// </summary>
         public virtual bool SuppressPrimaryMethodBody { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value that determines if nullability type enforcement is restored to project settings for user code.
+        /// </summary>
+        public virtual bool SuppressNullabilityEnforcement { get; set; }
+
         public abstract RazorCodeGenerationOptions Build();
 
         public virtual void SetDesignTime(bool designTime)

src/Razor/Microsoft.AspNetCore.Razor.Tools/src/GenerateCommand.cs

@@ -7,6 +7,7 @@ using System.Diagnostics;
 using System.IO;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Razor.Language;
+using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Razor;
 using Microsoft.Extensions.CommandLineUtils;
 using Microsoft.VisualStudio.LanguageServices.Razor.Serialization;
@@ -30,6 +31,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
             ExtensionNames = Option("-n", "extension name", CommandOptionType.MultipleValue);
             ExtensionFilePaths = Option("-e", "extension file path", CommandOptionType.MultipleValue);
             RootNamespace = Option("--root-namespace", "root namespace for generated code", CommandOptionType.SingleValue);
+            CSharpLanguageVersion = Option("--csharp-language-version", "csharp language version generated code", CommandOptionType.SingleValue);
             GenerateDeclaration = Option("--generate-declaration", "Generate declaration", CommandOptionType.NoValue);
         }
 
@@ -55,6 +57,8 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
         public CommandOption RootNamespace { get; }
 
+        public CommandOption CSharpLanguageVersion { get; }
+
         public CommandOption GenerateDeclaration { get; }
 
         protected override Task<int> ExecuteCoreAsync()
@@ -170,6 +174,22 @@ namespace Microsoft.AspNetCore.Razor.Tools
                 RazorProjectFileSystem.Create(projectDirectory),
             });
 
+            var success = true;
+            var csharpLanguageVersion = LanguageVersion.Default;
+            if (CSharpLanguageVersion.HasValue())
+            {
+                var rawLanguageVersion = CSharpLanguageVersion.Value();
+                if (!LanguageVersionFacts.TryParse(CSharpLanguageVersion.Value(), out var parsedLanguageVersion))
+                {
+                    success = false;
+                    Error.WriteLine($"Unknown C# language version {rawLanguageVersion}.");
+                }
+                else
+                {
+                    csharpLanguageVersion = parsedLanguageVersion;
+                }
+            }
+
             var engine = RazorProjectEngine.Create(configuration, compositeFileSystem, b =>
             {
                 b.Features.Add(new StaticTagHelperFeature() { TagHelpers = tagHelpers, });
@@ -184,12 +204,12 @@ namespace Microsoft.AspNetCore.Razor.Tools
                 {
                     b.SetRootNamespace(RootNamespace.Value());
                 }
+
+                b.SetCSharpLanguageVersion(csharpLanguageVersion);
             });
 
             var results = GenerateCode(engine, sourceItems);
 
-            var success = true;
-
             foreach (var result in results)
             {
                 var errorCount = result.CSharpDocument.Diagnostics.Count;

src/Razor/Microsoft.CodeAnalysis.Razor/src/RazorProjectEngineBuilderExtensions.cs

@@ -0,0 +1,85 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Linq;
+using Microsoft.AspNetCore.Razor.Language;
+using Microsoft.CodeAnalysis.CSharp;
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    /// <summary>
+    /// Roslyn specific <see cref="RazorProjectEngineBuilder"/> extensions.
+    /// </summary>
+    public static class RazorProjectEngineBuilderExtensions
+    {
+        /// <summary>
+        /// Sets the C# language version to respect when generating code. 
+        /// </summary>
+        /// <param name="builder">The <see cref="RazorProjectEngineBuilder"/>.</param>
+        /// <param name="csharpLanguageVersion">The C# <see cref="LanguageVersion"/>.</param>
+        /// <returns>The <see cref="RazorProjectEngineBuilder"/>.</returns>
+        public static RazorProjectEngineBuilder SetCSharpLanguageVersion(this RazorProjectEngineBuilder builder, LanguageVersion csharpLanguageVersion)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            if (builder.Configuration.LanguageVersion.Major < 3)
+            {
+                // Prior to 3.0 there were no C# version specific controlled features so there's no value in setting a CSharp language version, noop.
+                return builder;
+            }
+
+            var existingFeature = builder.Features.OfType<ConfigureParserForCSharpVersionFeature>().FirstOrDefault();
+            if (existingFeature != null)
+            {
+                builder.Features.Remove(existingFeature);
+            }
+
+            builder.Features.Add(new ConfigureParserForCSharpVersionFeature(csharpLanguageVersion));
+
+            return builder;
+        }
+
+        private class ConfigureParserForCSharpVersionFeature : IConfigureRazorCodeGenerationOptionsFeature
+        {
+            public ConfigureParserForCSharpVersionFeature(LanguageVersion csharpLanguageVersion)
+            {
+                CSharpLanguageVersion = csharpLanguageVersion;
+            }
+
+            public LanguageVersion CSharpLanguageVersion { get; }
+
+            public int Order { get; set; }
+
+            public RazorEngine Engine { get; set; }
+
+            public void Configure(RazorCodeGenerationOptionsBuilder options)
+            {
+                if (options == null)
+                {
+                    throw new ArgumentNullException(nameof(options));
+                }
+
+                if (CSharpLanguageVersion < LanguageVersion.CSharp8)
+                {
+                    options.SuppressNullabilityEnforcement = true;
+                }
+                else
+                {
+                    // Given that nullability enforcement can be a compile error we only turn it on for C# >= 8.0. There are
+                    // cases in tooling when the project isn't fully configured yet at which point the CSharpLanguageVersion
+                    // may be Default (value 0). In those cases that C# version is equivalently "unspecified" and is up to the consumer
+                    // to act in a safe manner to not cause unneeded errors for older compilers. Therefore if the version isn't
+                    // >= 8.0 (or Latest) then nullability enforcement is suppressed.
+                    //
+                    // Once the project finishes configuration the C# language version will be updated to reflect the effective 
+                    // language version for the project.
+                    options.SuppressNullabilityEnforcement = false;
+                }
+            }
+        }
+    }
+}

src/Razor/Microsoft.NET.Sdk.Razor/src/RazorGenerate.cs

@@ -26,6 +26,8 @@ namespace Microsoft.AspNetCore.Razor.Tasks
 
         public string RootNamespace { get; set; }
 
+        public string CSharpLanguageVersion { get; set; }
+
         [Required]
         public string Version { get; set; }
 
@@ -138,16 +140,24 @@ namespace Microsoft.AspNetCore.Razor.Tasks
             builder.AppendLine(Configuration[0].GetMetadata(Identity));
 
             // Added in 3.0
-            if (parsedVersion.Major >= 3 && !string.IsNullOrEmpty(RootNamespace))
+            if (parsedVersion.Major >= 3)
             {
-                builder.AppendLine("--root-namespace");
+                if (!string.IsNullOrEmpty(RootNamespace))
-                builder.AppendLine(RootNamespace);
+                {
-            }
+                    builder.AppendLine("--root-namespace");
+                    builder.AppendLine(RootNamespace);
+                }
 
-            // Added in 3.0
+                if (!string.IsNullOrEmpty(CSharpLanguageVersion))
-            if (parsedVersion.Major >= 3 && GenerateDeclaration)
+                {
-            {
+                    builder.AppendLine("--csharp-language-version");
-                builder.AppendLine("--generate-declaration");
+                    builder.AppendLine(CSharpLanguageVersion);
+                }
+
+                if (GenerateDeclaration)
+                {
+                    builder.AppendLine("--generate-declaration");
+                }
             }
 
             for (var i = 0; i < Extensions.Length; i++)

src/Razor/Microsoft.NET.Sdk.Razor/src/build/netstandard2.0/Microsoft.NET.Sdk.Razor.CodeGeneration.targets

@@ -156,6 +156,7 @@ Copyright (c) .NET Foundation. All rights reserved.
       PipeName="$(_RazorBuildServerPipeName)"
       Version="$(RazorLangVersion)"
       RootNamespace="$(RootNamespace)"
+      CSharpLanguageVersion="$(LangVersion)"
       Configuration="@(ResolvedRazorConfiguration)"
       Extensions="@(ResolvedRazorExtension)"
       Sources="@(RazorGenerateWithTargetPath)"

src/Razor/Microsoft.NET.Sdk.Razor/src/build/netstandard2.0/Microsoft.NET.Sdk.Razor.Component.targets

@@ -107,6 +107,7 @@ Copyright (c) .NET Foundation. All rights reserved.
       PipeName="$(_RazorBuildServerPipeName)"
       Version="$(RazorLangVersion)"
       RootNamespace="$(RootNamespace)"
+      CSharpLanguageVersion="$(LangVersion)"
       Configuration="@(ResolvedRazorConfiguration)"
       Extensions="@(ResolvedRazorExtension)"
       Sources="@(_RazorComponentDeclarationSources)"