use an OmniSharp specific message for MSB3644

src/OmniSharp.MSBuild/Logging/ErrorMessages.cs

@@ -0,0 +1,7 @@
+namespace OmniSharp.MSBuild.Logging
+{
+    internal class ErrorMessages
+    {
+        internal static string ReferenceAssembliesNotFoundUnix = "This project targets .NET version that requires reference assemblies that do not ship with OmniSharp out of the box (e.g. .NET Framework). The most common solution is to make sure Mono is installed on your machine (https://mono-project.com/download/) and that OmniSharp is started with that Mono installation (e.g. 'omnisharp.useGlobalMono':'always' in C# Extension for VS Code).";
+    }
+}

src/OmniSharp.MSBuild/Logging/MSBuildDiagnostic.cs

@@ -1,5 +1,9 @@
-namespace OmniSharp.MSBuild.Logging
+using System;
+using OmniSharp.Utilities;
+
+namespace OmniSharp.MSBuild.Logging
 {
+
     public class MSBuildDiagnostic
     {
         public MSBuildDiagnosticSeverity Severity { get; }
@@ -31,9 +35,17 @@ private MSBuildDiagnostic(
         }
 
         public static MSBuildDiagnostic CreateFrom(Microsoft.Build.Framework.BuildErrorEventArgs args)
-            => new MSBuildDiagnostic(MSBuildDiagnosticSeverity.Error,
-                args.Message, args.File, args.ProjectFile, args.Subcategory, args.Code,
+        {
+            // https://github.com/dotnet/msbuild/blob/v16.8.3/src/Tasks/Resources/Strings.resx#L2155-L2158
+            // for MSB3644, we should print a different message on Unix because the default one is Windows-specific
+            var diagnosticText = args.Code.Equals("MSB3644", StringComparison.OrdinalIgnoreCase) 
+                && Platform.Current.OperatingSystem != Utilities.OperatingSystem.Windows 
+                ? ErrorMessages.ReferenceAssembliesNotFoundUnix : args.Message;
+
+            return new MSBuildDiagnostic(MSBuildDiagnosticSeverity.Error,
+                diagnosticText, args.File, args.ProjectFile, args.Subcategory, args.Code,
                 args.LineNumber, args.ColumnNumber, args.EndLineNumber, args.EndColumnNumber);
+        }
 
         public static MSBuildDiagnostic CreateFrom(Microsoft.Build.Framework.BuildWarningEventArgs args)
             => new MSBuildDiagnostic(MSBuildDiagnosticSeverity.Error,

src/OmniSharp.MSBuild/Logging/MSBuildLogger.cs

@@ -26,8 +26,9 @@ public ImmutableArray<MSBuildDiagnostic> GetDiagnostics() =>
 
         private void OnError(object sender, Microsoft.Build.Framework.BuildErrorEventArgs args)
         {
-            _logger.LogError(args.Message);
-            _diagnostics.Add(MSBuildDiagnostic.CreateFrom(args));
+            var msBuildDiagnostic = MSBuildDiagnostic.CreateFrom(args);
+            _logger.LogError(msBuildDiagnostic.Message);
+            _diagnostics.Add(msBuildDiagnostic);
         }
 
         private void OnWarning(object sender, Microsoft.Build.Framework.BuildWarningEventArgs args)

tests/OmniSharp.MSBuild.Tests/MSBuildDiagnosticTests.cs

@@ -0,0 +1,25 @@
+using Microsoft.Build.Framework;
+using OmniSharp.MSBuild.Logging;
+using OmniSharp.Utilities;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace OmniSharp.MSBuild.Tests
+{
+    public class MSBuildDiagnosticTests : AbstractMSBuildTestFixture
+    {
+        public MSBuildDiagnosticTests(ITestOutputHelper output) : base(output)
+        {
+        }
+
+        [Fact]
+        public void MSB3644_CustomMessage()
+        {
+            var sourceDiagnostic = new BuildErrorEventArgs("test-subcategory", "MSB3644", "foo.cs", 1, 1, 1, 1, "Reference assemblies not found!", "help-keyword", "dummy-sender");
+            var msbuildDiagnostic = MSBuildDiagnostic.CreateFrom(sourceDiagnostic);
+
+            Assert.Equal(Platform.Current.OperatingSystem != OperatingSystem.Windows
+                ? ErrorMessages.ReferenceAssembliesNotFoundUnix : sourceDiagnostic.Message, msbuildDiagnostic.Message);
+        }
+    }
+}

tests/OmniSharp.MSBuild.Tests/OmniSharp.MSBuild.Tests.csproj

@@ -12,6 +12,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\src\OmniSharp.MSBuild\OmniSharp.MSBuild.csproj" />
+    <ProjectReference Include="..\..\src\OmniSharp.Shared\OmniSharp.Shared.csproj" />
     <ProjectReference Include="..\TestUtility\TestUtility.csproj" />
   </ItemGroup>