Probe for .NET reference assemblies path

src/Microsoft.DotNet.ProjectModel/Resolution/FrameworkReferenceResolver.cs

@@ -8,6 +8,7 @@ using System.Linq;
 using System.Runtime.Versioning;
 using System.Xml.Linq;
 using Microsoft.DotNet.ProjectModel.Utilities;
+using Microsoft.Extensions.DependencyModel.Resolution;
 using Microsoft.Extensions.PlatformAbstractions;
 using NuGet.Frameworks;
 
@@ -53,7 +54,7 @@ namespace Microsoft.DotNet.ProjectModel.Resolution
         public static string GetDefaultReferenceAssembliesPath()
         {
             // Allow setting the reference assemblies path via an environment variable
-            var referenceAssembliesPath = Environment.GetEnvironmentVariable("DOTNET_REFERENCE_ASSEMBLIES_PATH");
+            var referenceAssembliesPath = DotNetReferenceAssembliesPathResolver.Resolve();
 
             if (!string.IsNullOrEmpty(referenceAssembliesPath))
             {

src/Microsoft.Extensions.DependencyModel/Resolution/DotNetReferenceAssembliesPathResolver.cs

@@ -0,0 +1,59 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.IO;
+using Microsoft.Extensions.EnvironmentAbstractions;
+using Microsoft.Extensions.PlatformAbstractions;
+
+namespace Microsoft.Extensions.DependencyModel.Resolution
+{
+    public class DotNetReferenceAssembliesPathResolver
+    {
+        public static readonly string DotNetReferenceAssembliesPathEnv = "DOTNET_REFERENCE_ASSEMBLIES_PATH";
+        
+        internal static string Resolve(IEnvironment envirnment)
+        {
+            var path = envirnment.GetEnvironmentVariable(DotNetReferenceAssembliesPathEnv);
+            if (!string.IsNullOrEmpty(path))
+            {
+                return path;
+            }
+            
+            return GetDefaultDotNetReferenceAssembliesPath();
+        }
+        
+        public static string Resolve()
+        {
+            return Resolve(EnvironmentWrapper.Default);
+        }
+                
+        private static string GetDefaultDotNetReferenceAssembliesPath()
+        {            
+            var os = PlatformServices.Default.Runtime.OperatingSystemPlatform;
+            
+            if (os == Platform.Windows)
+            {
+                return null;
+            }
+            
+            if (os == Platform.Darwin && 
+                Directory.Exists("/Library/Framework/Mono.Framework/Versions/Current/lib/mono/xbuild-frameworks"))
+            {
+                return "/Library/Framework/Mono.Framework/Versions/Current/lib/mono/xbuild-frameworks";
+            }
+            
+            if (Directory.Exists("/usr/local/lib/mono/xbuild-frameworks"))
+            {
+                return "/usr/local/lib/mono/xbuild-frameworks";
+            }
+            
+            if (Directory.Exists("/usr/lib/mono/xbuild-frameworks"))
+            {
+                return "/usr/lib/mono/xbuild-frameworks";
+            }
+            
+            return null;
+        }
+    }
+}

src/Microsoft.Extensions.DependencyModel/Resolution/ReferenceAssemblyPathResolver.cs

@@ -104,8 +104,7 @@ namespace Microsoft.Extensions.DependencyModel.Resolution
         internal static string GetDefaultReferenceAssembliesPath(IRuntimeEnvironment runtimeEnvironment, IEnvironment environment)
         {
             // Allow setting the reference assemblies path via an environment variable
-            var referenceAssembliesPath = environment.GetEnvironmentVariable("DOTNET_REFERENCE_ASSEMBLIES_PATH");
+            var referenceAssembliesPath = DotNetReferenceAssembliesPathResolver.Resolve(environment); 
-
             if (!string.IsNullOrEmpty(referenceAssembliesPath))
             {
                 return referenceAssembliesPath;
@@ -138,6 +137,5 @@ namespace Microsoft.Extensions.DependencyModel.Resolution
                 programFiles,
                 "Reference Assemblies", "Microsoft", "Framework");
         }
-
     }
 }