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src/EditorFeatures/TestUtilities/Workspaces/TestWorkspace.cs

@@ -682,6 +682,6 @@ public void ChangeSolution(Solution solution)
         }
 
         public override bool CanApplyParseOptionChange(ParseOptions oldOptions, ParseOptions newOptions, Project project)
-        => true;
+            => true;
     }
 }

src/VisualStudio/CSharp/Impl/Utilities/CSharpParseOptionsChangingService.cs

@@ -3,6 +3,7 @@
 #nullable enable 
 
 using System.Composition;
+using System.Drawing.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Host.Mef;
@@ -25,20 +26,21 @@ public bool CanApplyChange(ParseOptions oldOptions, ParseOptions newOptions, str
             var oldCSharpOptions = (CSharpParseOptions)oldOptions;
             var newCSharpOptions = (CSharpParseOptions)newOptions;
 
-            LanguageVersion parsedMaxLanguageVersion;
+            // Currently, only changes to the LanguageVersion of parse options are supported.
+            if (oldCSharpOptions.WithLanguageVersion(newCSharpOptions.SpecifiedLanguageVersion) != newOptions)
+            {
+                return false;
+            }
 
             if (string.IsNullOrEmpty(maxLangVersion))
             {
-                parsedMaxLanguageVersion = LanguageVersion.Latest;
+                return true;
             }
             else
             {
-                Contract.ThrowIfFalse(LanguageVersionFacts.TryParse(maxLangVersion, out parsedMaxLanguageVersion));
+                Contract.ThrowIfFalse(LanguageVersionFacts.TryParse(maxLangVersion, out var parsedMaxLanguageVersion));
+                return newCSharpOptions.LanguageVersion <= parsedMaxLanguageVersion;
             }
-
-            // Currently, only changes to the LanguageVersion of parse options are supported.
-            return oldCSharpOptions.WithLanguageVersion(newCSharpOptions.SpecifiedLanguageVersion) == newOptions &&
-                newCSharpOptions.LanguageVersion <= parsedMaxLanguageVersion;
         }
 
         public void Apply(ParseOptions options, ProjectPropertyStorage storage)

src/VisualStudio/CSharp/Test/ProjectSystemShim/CPS/AdditionalPropertiesTests.cs

@@ -33,47 +33,37 @@ static string DefaultNamespaceOfSingleProject(TestEnvironment environment)
                 => environment.Workspace.CurrentSolution.Projects.Single().DefaultNamespace;
         }
 
-        [WpfFact]
+        [WpfTheory]
         [Trait(Traits.Feature, Traits.Features.ProjectSystemShims)]
-        public void SetProperty_MaxSupportedLangVersion_CPS()
+        [InlineData(LanguageVersion.CSharp7_3)]
+        [InlineData(LanguageVersion.CSharp8)]
+        [InlineData(LanguageVersion.Latest)]
+        [InlineData(LanguageVersion.LatestMajor)]
+        [InlineData(LanguageVersion.Preview)]
+        public void SetProperty_MaxSupportedLangVersion_CPS(LanguageVersion maxSupportedLangVersion)
         {
             var catalog = TestEnvironment.s_exportCatalog.Value
                 .WithParts(
                     typeof(CSharpParseOptionsChangingService));
 
+            const LanguageVersion attemptedVersion = LanguageVersion.CSharp8;
+
             var factory = ExportProviderCache.GetOrCreateExportProviderFactory(catalog);
 
             using (var environment = new TestEnvironment(exportProviderFactory: factory))
-            using (var project = CSharpHelpers.CreateCSharpCPSProject(environment, "Test"))
-            {
-                Assert.True(CanApplyVersionChange(environment, LanguageVersion.CSharp8));
-
-                // Test max version is less than attempted version
-                var maxSupportedLangVersion = LanguageVersion.CSharp7_3;
-                project.SetProperty(AdditionalPropertyNames.MaxSupportedLangVersion, maxSupportedLangVersion.ToDisplayString());
-                Assert.False(CanApplyVersionChange(environment, LanguageVersion.CSharp8));
-
-                // Test max version equals attempted version
-                maxSupportedLangVersion = LanguageVersion.CSharp8;
-                project.SetProperty(AdditionalPropertyNames.MaxSupportedLangVersion, maxSupportedLangVersion.ToDisplayString());
-                Assert.True(CanApplyVersionChange(environment, LanguageVersion.CSharp8));
-
-                // Test max version is greater than attempted version
-                maxSupportedLangVersion = LanguageVersion.Preview;
-                project.SetProperty(AdditionalPropertyNames.MaxSupportedLangVersion, maxSupportedLangVersion.ToDisplayString());
-                Assert.True(CanApplyVersionChange(environment, LanguageVersion.CSharp8));
-
-            }
-
-            static bool CanApplyVersionChange(TestEnvironment environment, LanguageVersion newVersion)
+            using (var cpsProject = CSharpHelpers.CreateCSharpCPSProject(environment, "Test"))
             {
                 var project = environment.Workspace.CurrentSolution.Projects.Single();
                 var oldParseOptions = (CSharpParseOptions)project.ParseOptions;
 
-                return environment.Workspace.CanApplyParseOptionChange(
+                cpsProject.SetProperty(AdditionalPropertyNames.MaxSupportedLangVersion, maxSupportedLangVersion.ToDisplayString());
+
+                var canApply = environment.Workspace.CanApplyParseOptionChange(
                     oldParseOptions,
-                    oldParseOptions.WithLanguageVersion(newVersion),
+                    oldParseOptions.WithLanguageVersion(attemptedVersion),
                     project);
+
+                Assert.Equal(attemptedVersion <= maxSupportedLangVersion, canApply);
             }
         }
     }

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioProject.cs

@@ -314,7 +314,14 @@ internal string DefaultNamespace
                        w => w.OnDefaultNamespaceChanged(Id, value));
         }
 
-        internal void SetMaxLangVersion(string version) => _workspace.SetMaxLanguageVersion(Id, version);
+        /// <summary>
+        /// The max language version supported for this project, if applicable. Useful to help indicate what 
+        /// language version features should be suggested to a user, as well as if they can be upgraded. 
+        /// </summary>
+        internal string MaxLangVersion
+        {
+            set => _workspace.SetMaxLanguageVersion(Id, value);
+        }
 
 
         #region Batching

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioWorkspaceImpl.cs

@@ -1889,11 +1889,11 @@ internal void EnsureDocumentOptionProvidersInitialized()
             }
         }
 
-        internal void SetMaxLanguageVersion(ProjectId projectId, string maxLangVer)
+        internal void SetMaxLanguageVersion(ProjectId projectId, string maxLanguageVersion)
         {
             lock (_gate)
             {
-                _projectToMaxSupportedLangVersionMap[projectId] = maxLangVer;
+                _projectToMaxSupportedLangVersionMap[projectId] = maxLanguageVersion;
             }
         }
     }