Moving version validation to only when a SDK is not overriden. Also adding a validation for the minimum MSBuild version.

src/Microsoft.DotNet.MSBuildSdkResolver/MSBuildSdkResolver.cs

@@ -50,20 +50,34 @@ namespace Microsoft.DotNet.MSBuildSdkResolver
                 }
 
                 msbuildSdksDir = Path.Combine(netcoreSdkDir, "Sdks");
-                netcoreSdkVersion = new DirectoryInfo(netcoreSdkDir).Name;;
-            }
+                netcoreSdkVersion = new DirectoryInfo(netcoreSdkDir).Name;
 
-            if (!IsNetCoreSDKOveridden(netcoreSdkVersion) && 
-                IsNetCoreSDKSmallerThanTheMinimumVersion(netcoreSdkVersion, sdkReference.MinimumVersion))
-            {
-                return factory.IndicateFailure(
-                    new[]
-                    {
-                        $"Version {netcoreSdkVersion} of the SDK is smaller than the minimum version"
-                        + $" {sdkReference.MinimumVersion} requested. Check that a recent enough .NET Core SDK is"
-                        + " installed, increase the minimum version specified in the project, or increase"
-                        + " the version specified in global.json."
-                    });
+                if (IsNetCoreSDKSmallerThanTheMinimumVersion(netcoreSdkVersion, sdkReference.MinimumVersion))
+                {
+                    return factory.IndicateFailure(
+                        new[]
+                        {
+                            $"Version {netcoreSdkVersion} of the SDK is smaller than the minimum version"
+                            + $" {sdkReference.MinimumVersion} requested. Check that a recent enough .NET Core SDK is"
+                            + " installed, increase the minimum version specified in the project, or increase"
+                            + " the version specified in global.json."
+                        });
+                }
+
+                var minimumMSBuildVersionString =
+                    File.ReadAllLines(Path.Combine(netcoreSdkDir, "minimumMSBuildVersion"))[0];
+                var minimumMSBuildVersion = Version.Parse(minimumMSBuildVersionString);
+                if (context.MSBuildVersion.CompareTo(minimumMSBuildVersion) == -1)
+                {
+                    return factory.IndicateFailure(
+                        new[]
+                        {
+                            $"Version {netcoreSdkVersion} of the SDK requires at least version {minimumMSBuildVersionString}"
+                            + $" of msbuild. The current available version of msbuild is {context.MSBuildVersion.ToString()}."
+                            + " Change the SDK specified in global.json to an older version that requires the msbuild"
+                            + " version currently available."
+                        });
+                }
             }
 
             string msbuildSdkDir = Path.Combine(msbuildSdksDir, sdkReference.Name, "Sdk");
@@ -80,11 +94,6 @@ namespace Microsoft.DotNet.MSBuildSdkResolver
             return factory.IndicateSuccess(msbuildSdkDir, netcoreSdkVersion);
         }
 
-        private bool IsNetCoreSDKOveridden(string netcoreSdkVersion)
-        {
-            return netcoreSdkVersion == null;
-        }
-
         private bool IsNetCoreSDKSmallerThanTheMinimumVersion(string netcoreSdkVersion, string minimumVersion)
         {
             FXVersion netCoreSdkFXVersion;

src/redist/minimumMSBuildVersion

@@ -0,0 +1 @@
+15.3.246

src/redist/redist.csproj

@@ -36,6 +36,12 @@
     <ProjectReference Include="..\tool_nuget\tool_nuget.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <Content Include="minimumMSBuildVersion">
+      <CopyToPublishDirectory>PreserveNewest</CopyToPublishDirectory>
+    </Content>
+  </ItemGroup>
+
   <Target Name="PublishAspNetRuntimePackageStore"
           AfterTargets="CrossgenPublishDir">
     <ItemGroup>

test/Microsoft.DotNet.MSBuildSdkResolver.Tests/GivenAnMSBuildSdkResolver.cs

@@ -78,6 +78,32 @@ namespace Microsoft.DotNet.Cli.Utils.Tests
                 + " specified in the project, or increase the version specified in global.json.");
         }
 
+        [Fact]
+        public void ItReturnsNullWhenTheSDKRequiresAHigherVersionOfMSBuildThanTheOneAvailable()
+        {
+            var environment = new TestEnvironment();
+            var expected =
+                environment.CreateSdkDirectory(ProgramFiles.X64, "Some.Test.Sdk", "99.99.99", new Version(2, 0));
+            environment.CreateMuxerAndAddToPath(ProgramFiles.X64);
+
+            var resolver = environment.CreateResolver();
+            var result = (MockResult)resolver.Resolve(
+                new SdkReference("Some.Test.Sdk", null, "99.99.99"),
+                new MockContext
+                {
+                    ProjectFilePath = environment.TestDirectory.FullName
+                },
+                new MockFactory());
+
+            result.Success.Should().BeFalse();
+            result.Path.Should().BeNull();
+            result.Version.Should().BeNull();
+            result.Warnings.Should().BeNullOrEmpty();
+            result.Errors.Should().Contain("Version 99.99.99 of the SDK requires at least version 2.0 of msbuild."
+                + " The current available version of msbuild is 1.0. Change the SDK specified in global.json to an older"
+                + " version that requires the msbuild version currently available.");
+        }
+
         [Fact]
         public void ItReturnsTheVersionIfItIsEqualToTheMinVersion()
         {
@@ -147,27 +173,63 @@ namespace Microsoft.DotNet.Cli.Utils.Tests
                 => new DotNetMSBuildSdkResolver(GetEnvironmentVariable);
 
             public DirectoryInfo GetSdkDirectory(ProgramFiles programFiles, string sdkName, string sdkVersion)
-                => TestDirectory.GetDirectory(GetProgramFilesDirectory(programFiles).FullName, "dotnet", "sdk", sdkVersion, "Sdks", sdkName, "Sdk");
+                => TestDirectory.GetDirectory(
+                    GetProgramFilesDirectory(programFiles).FullName,
+                    "dotnet",
+                    "sdk",
+                    sdkVersion,
+                    "Sdks",
+                    sdkName,
+                    "Sdk");
 
             public DirectoryInfo GetProgramFilesDirectory(ProgramFiles programFiles)
                 => TestDirectory.GetDirectory($"ProgramFiles{programFiles}");
             
-            public DirectoryInfo CreateSdkDirectory(ProgramFiles programFiles, string sdkVersion, string sdkName)
+            public DirectoryInfo CreateSdkDirectory(
+                ProgramFiles programFiles,
+                string sdkName,
+                string sdkVersion,
+                Version minimumMSBuildVersion = null)
             {
-                var dir = GetSdkDirectory(programFiles, sdkVersion, sdkName);
+                var dir = GetSdkDirectory(programFiles, sdkName, sdkVersion);
                 dir.Create();
+
+                CreateMSBuildRequiredVersionFile(programFiles, sdkVersion, minimumMSBuildVersion);
+
                 return dir;
             }
 
             public void CreateMuxerAndAddToPath(ProgramFiles programFiles)
             {
-                var muxerDirectory = TestDirectory.GetDirectory(GetProgramFilesDirectory(programFiles).FullName, "dotnet");
+                var muxerDirectory =
+                    TestDirectory.GetDirectory(GetProgramFilesDirectory(programFiles).FullName, "dotnet");
 
                 new FileInfo(Path.Combine(muxerDirectory.FullName, Muxer)).Create();
 
                 PathEnvironmentVariable = $"{muxerDirectory}{Path.PathSeparator}{PathEnvironmentVariable}";
             }
 
+            private void CreateMSBuildRequiredVersionFile(
+                ProgramFiles programFiles,
+                string sdkVersion,
+                Version minimumMSBuildVersion)
+            {
+                if (minimumMSBuildVersion == null)
+                {
+                    minimumMSBuildVersion = new Version(1, 0);
+                }
+
+                var cliDirectory = TestDirectory.GetDirectory(
+                    GetProgramFilesDirectory(programFiles).FullName,
+                    "dotnet",
+                    "sdk",
+                    sdkVersion);
+
+                File.WriteAllText(
+                    Path.Combine(cliDirectory.FullName, "minimumMSBuildVersion"),
+                    minimumMSBuildVersion.ToString());
+            }
+
             public void CreateGlobalJson(DirectoryInfo directory, string version)
                 => File.WriteAllText(directory.GetFile("global.json").FullName, 
                     $@"{{ ""sdk"": {{ ""version"":  ""{version}"" }} }}");
@@ -188,6 +250,12 @@ namespace Microsoft.DotNet.Cli.Utils.Tests
         {
             public new string ProjectFilePath { get => base.ProjectFilePath; set => base.ProjectFilePath = value; }
             public new string SolutionFilePath { get => base.SolutionFilePath; set => base.SolutionFilePath = value; }
+            public new Version MSBuildVersion { get => base.MSBuildVersion; set => base.MSBuildVersion = value; }
+
+            public MockContext()
+            {
+                MSBuildVersion = new Version(1, 0);
+            }
         }
 
         private sealed class MockFactory : SdkResultFactory