added msbuild:UseBundledOnly option to force the usage of bundled MSBuild

src/OmniSharp.Host/MSBuild/Discovery/MSBuildLocator.cs

@@ -46,13 +46,23 @@ protected override void DisposeCore(bool disposing)
         }
 
         public static MSBuildLocator CreateDefault(ILoggerFactory loggerFactory, IAssemblyLoader assemblyLoader, IConfiguration msbuildConfiguration)
-            => new MSBuildLocator(loggerFactory, assemblyLoader,
+        {
+            var useBundledOnly = msbuildConfiguration.GetValue<bool>("UseBundledOnly");
+            if (useBundledOnly)
+            {
+                var logger = loggerFactory.CreateLogger<MSBuildLocator>();
+                logger.LogInformation("Because 'UseBundledOnly' is enabled in the configuration, OmniSharp will only use the bundled MSBuild.");
+                return CreateStandAlone(loggerFactory, assemblyLoader);
+            }
+
+            return new MSBuildLocator(loggerFactory, assemblyLoader,
                 ImmutableArray.Create<MSBuildInstanceProvider>(
                     new DevConsoleInstanceProvider(loggerFactory),
                     new VisualStudioInstanceProvider(loggerFactory),
                     new MonoInstanceProvider(loggerFactory),
                     new StandAloneInstanceProvider(loggerFactory),
                     new UserOverrideInstanceProvider(loggerFactory, msbuildConfiguration)));
+        }
 
         public static MSBuildLocator CreateStandAlone(ILoggerFactory loggerFactory, IAssemblyLoader assemblyLoader)
             => new MSBuildLocator(loggerFactory, assemblyLoader,

src/OmniSharp.MSBuild/Options/MSBuildOptions.cs

@@ -2,6 +2,7 @@ namespace OmniSharp.Options
 {
     public class MSBuildOptions
     {
+        public bool UseBundledOnly { get; set; } = false;
         public string ToolsVersion { get; set; }
         public string VisualStudioVersion { get; set; }
         public string Configuration { get; set; }

tests/OmniSharp.MSBuild.Tests/MSBuildSelectionTests.cs

@@ -1,5 +1,9 @@
 using System;
+using System.Collections.Generic;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
 using OmniSharp.MSBuild.Discovery;
+using OmniSharp.Services;
 using TestUtility;
 using Xunit;
 using Xunit.Abstractions;
@@ -255,6 +259,20 @@ public void StandAloneIsPreferredOverUnsupportedVS(string vsVersion)
             msbuildLocator.DeleteFakeInstancesFolders();
         }
 
+        [Fact]
+        public void CreateDefault_UseBundledOnly_True_LocatesOnlyStandAloneInstance()
+        {
+            var configBuilder = new Microsoft.Extensions.Configuration.ConfigurationBuilder().AddInMemoryCollection(new Dictionary<string, string>()
+            {
+                ["useBundledOnly"] = "true"
+            });
+            var loggerFactory = new LoggerFactory();
+            var locator = MSBuildLocator.CreateDefault(loggerFactory, new AssemblyLoader(loggerFactory), configBuilder.Build());
+            var instances = locator.GetInstances();
+            Assert.Single(instances);
+            Assert.Equal(DiscoveryType.StandAlone, instances[0].DiscoveryType);
+        }
+
         private static MSBuildInstance GetStandAloneMSBuildInstance()
         {
             return new MSBuildInstance(