Always use the Razor source generator in design time builds if cohosting is enabled

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/ProjectSystem/CohostingDesignTimeBuildPropertyProvider.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Immutable;
+using System.ComponentModel.Composition;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.Razor.Workspaces;
+using Microsoft.VisualStudio.ProjectSystem;
+using Microsoft.VisualStudio.ProjectSystem.Build;
+
+namespace Microsoft.VisualStudio.Razor.ProjectSystem;
+
+[ExportBuildGlobalPropertiesProvider(designTimeBuildProperties: true)]
+[AppliesTo("DotNetCoreRazor")]
+internal class CohostingDesignTimeBuildPropertyProvider : StaticGlobalPropertiesProviderBase
+{
+    private readonly Task<IImmutableDictionary<string, string>> _properties;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CohostingDesignTimeBuildPropertyProvider"/> class.
+    /// </summary>
+    [ImportingConstructor]
+    internal CohostingDesignTimeBuildPropertyProvider(IProjectService projectService, LanguageServerFeatureOptions featureOptions)
+        : base(projectService.Services)
+    {
+        var properties = Empty.PropertiesMap.Add(nameof(LanguageServerFeatureOptions.UseRazorCohostServer), featureOptions.UseRazorCohostServer ? "true" : "false");
+        if (featureOptions.UseRazorCohostServer)
+        {
+            // When cohosting is enabled we always want to use the source generator. Since this property provider only runs in design-time builds,
+            // this doesn't affect real build outputs.
+            properties = properties.Add("UseRazorSourceGenerator", "true");
+        }
+
+        _properties = Task.FromResult<IImmutableDictionary<string, string>>(properties);
+    }
+
+    [System.Diagnostics.CodeAnalysis.SuppressMessage("Usage", "VSTHRD003:Avoid awaiting foreign Tasks", Justification = "No awaiting involved")]
+    public override Task<IImmutableDictionary<string, string>> GetGlobalPropertiesAsync(CancellationToken cancellationToken)
+    {
+        return _properties;
+    }
+}