Could not load file or assembly 'Microsoft.DotNet.MsBuildSdkResolver'

[Ralph-Sampson]

Issue Description

After upgrading to version 17.12.* and 17.14.* from 17.8.3, when attempting to programmatically build a project that references any SDK, i.e.

<Project Sdk="Microsoft.NET.Sdk"> <PropertyGroup> <TargetFramework>net9.0</TargetFramework> <RootNamespace>MsBuild.Test</RootNamespace> </PropertyGroup> <ItemGroup> <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" /> <PackageReference Include="MSTest.TestAdapter" Version="2.1.0" /> <PackageReference Include="MSTest.TestFramework" Version="2.1.0" /> <PackageReference Include="Microsoft.Build" Version ="17.14.8" ExcludeAssets="runtime"/> <PackageReference Include="Microsoft.Build.Locator" Version ="1.9.1"/> </ItemGroup> </Project>

I'm continually met with the following exception at runtime:

The SDK resolver assembly "C:\Program Files\Microsoft Visual Studio\2022\Professional\MSBuild\Current\Bin\SdkResolvers\Microsoft.DotNet.MSBuildSdkResolver\Microsoft.DotNet.MSBuildSdkResolver.dll" could not be loaded. Could not load file or assembly 'Microsoft.DotNet.MSBuildSdkResolver, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.

Inner Exception:

Message: Could not load file or assembly 'Microsoft.DotNet.MSBuildSdkResolver, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.

Source: System.Private.CoreLib

Stack Trace:    at System.Reflection.RuntimeAssembly.InternalLoad(AssemblyName assemblyName, StackCrawlMark& stackMark, AssemblyLoadContext assemblyLoadContext, RuntimeAssembly requestingAssembly, Boolean throwOnFileNotFound)
   at System.Reflection.Assembly.Load(AssemblyName assemblyRef)
   at Microsoft.Build.BackEnd.SdkResolution.SdkResolverLoader.LoadResolverAssembly(String resolverPath)
   at Microsoft.Build.BackEnd.SdkResolution.SdkResolverLoader.LoadResolvers(String resolverPath, ElementLocation location, List`1 resolvers)

Steps to Reproduce

I've got a dummy test here running the decompiled code for the resolver, which repros the failure I'm seeing. This code was pulled from https://github.com/dotnet/msbuild/blob/main/src/Build/BackEnd/Components/SdkResolution/SdkResolverLoader.cs :

    [TestMethod]
    public void ResolverTest()
    {
        // Default resolver location found from MSBuild using 
        // MSBuildLocator.RegisterMsBuildPath(C:\\Program Files\\Microsoft Visual Studio\\2022\\Professional\\MSBuild\\Current\\Bin);

        string resolverPath = "C:\\Program Files\\Microsoft Visual Studio\\2022\\Professional\\MSBuild\\Current\\Bin\\SdkResolvers\\Microsoft.DotNet.MSBuildSdkResolver\\Microsoft.DotNet.MSBuildSdkResolver.dll";
        string fileNameWithoutExtension = System.IO.Path.GetFileNameWithoutExtension(resolverPath);
        if (fileNameWithoutExtension.Equals("Microsoft.DotNet.MSBuildSdkResolver", StringComparison.OrdinalIgnoreCase))
        {
            Assert.ThrowsException<FileNotFoundException>(() =>
            {
                var i = Assembly.Load(new AssemblyName(fileNameWithoutExtension)
                {
                    CodeBase = resolverPath
                });
            }, "Expected Assembly.Load to fail");
        }
    }`

Expected Behavior

MsBuild can be used to programmatically build projects containing SDK references

Actual Behavior

Building projects with SDK references results in a failure related to the Microsoft.DotNet.MsBuildSdkResolver

Analysis

I believe the issue lies on line 241 here: https://github.com/dotnet/msbuild/blob/main/src/Build/BackEnd/Components/SdkResolution/SdkResolverLoader.cs

Versions & Configurations

Current Version:
MSBuild version 17.14.23+b0019275e for .NET Framework 17.14.23.42201

SDKS installed:
2.1.202 [C:\Program Files\dotnet\sdk] 2.1.526 [C:\Program Files\dotnet\sdk] 3.1.426 [C:\Program Files\dotnet\sdk] 5.0.214 [C:\Program Files\dotnet\sdk] 6.0.136 [C:\Program Files\dotnet\sdk] 6.0.321 [C:\Program Files\dotnet\sdk] 6.0.428 [C:\Program Files\dotnet\sdk] 8.0.120 [C:\Program Files\dotnet\sdk] 9.0.305 [C:\Program Files\dotnet\sdk]

[MichalPavlik]

@huulinhnguyen-dev, could you please repro the issue? Thanks.

[huulinhnguyen-dev]

Hi @Ralph-Sampson , I tried to reproduce the issue that you described. The unit test provided in the issue successfully reproduces the problem. However, the programmatic build scenario described in the issue cannot be reproduced the information you provided. To help us reproduce the exact scenario you're experiencing, could you please provide the exact .csproj file that triggers the issue and the exact code you're using to build programmatically.

Thank you so much

[Ralph-Sampson]

Absolutely, here's a zip file with a solution including a new test, executing code that throws the exception I'm seeing. The code provided attempts to load a project referencing an SDK with Project.FromFile() and that's where the exception is happening

MsBuild.Test.zip

[Ralph-Sampson]

Hi, @huulinhnguyen-dev / @MichalPavlik just checking in on this, any updates?

[huulinhnguyen-dev]

Hi @Ralph-Sampson , I apologize for the delay in providing you with a comprehensive analysis of this issue. The issue you're experiencing with MSBuild 17.12+ failing to load Microsoft.DotNet.MSBuildSdkResolver when used programmatically is caused by a change introduced in PR #10603 that was merged on September 11, 2024.
This pull request changed how the SDK resolver assembly is loaded from using Assembly.LoadFrom(path) to Assembly.Load(name) in order to enable NGEN (Native Image Generation) performance improvements, which saves approximately 50-100ms of MSBuild startup time. While this change works perfectly fine for command-line MSBuild usage and Visual Studio scenarios where the assembly is in the same directory and proper binding redirects are in place, it breaks programmatic usage such as unit tests, build tools, and CI/CD pipelines because the .NET runtime's assembly resolution context cannot find the SDK resolver assembly when the test runner or application is running from a different directory than where MSBuild is located. I need some additional time to implement and test the proper fix for this issue, and I will notify you as soon as possible once the solution is ready. You can use some methods to work around it as follows:

• Downgrade the MSBuild version to 17.8
• Create an assembly resolve handler. The handler will automatically load the SDK resolver from the correct path by using MSBuildLocator to find the MSBuild path when the .NET runtime cannot find the Microsoft.DotNet.MSBuildSdkResolver assembly, using Assembly.LoadFrom() instead of Assembly.Load().
• The pr that introduced the issue was included in the change wave 17.12, so you can set set MSBUILDDISABLEFEATURESFROMVERSION=17.12 then try again

Please try some workarounds and let me know the results.

[Ralph-Sampson]

Thanks for getting back to me, I'll look into the assembly resolve handler while you work on a fix for this issue. Do you have a potential ETA for when a fix can be expected?