Fix MultipleFrameworksTest

Author: timcassell

src/BenchmarkDotNet/Helpers/FrameworkVersionHelper.cs

@@ -24,6 +24,11 @@ private static readonly (int minReleaseNumber, string version)[] FrameworkVersio
 
         internal static string? GetTargetFrameworkVersion(Assembly? assembly)
         {
+            if (assembly is null)
+            {
+                return null;
+            }
+
             // Look for a TargetFrameworkAttribute with a supported Framework version.
             foreach (var attribute in assembly.GetCustomAttributes<TargetFrameworkAttribute>())
             {

src/BenchmarkDotNet/Toolchains/ToolchainExtensions.cs

@@ -25,7 +25,8 @@ internal static IToolchain GetToolchain(this BenchmarkCase benchmarkCase)
                 : GetToolchain(
                     benchmarkCase.GetRuntime(),
                     benchmarkCase.Descriptor,
-                    benchmarkCase.Job.HasDynamicBuildCharacteristic()
+                    benchmarkCase.Job.HasDynamicBuildCharacteristic(),
+                    benchmarkCase.Job.Environment.HasValue(EnvironmentMode.RuntimeCharacteristic)
                 );
 
         internal static IToolchain GetToolchain(this Job job)
@@ -34,20 +35,26 @@ internal static IToolchain GetToolchain(this Job job)
                 : GetToolchain(
                     job.ResolveValue(EnvironmentMode.RuntimeCharacteristic, EnvironmentResolver.Instance),
                     null,
-                    job.HasDynamicBuildCharacteristic()
+                    job.HasDynamicBuildCharacteristic(),
+                    job.Environment.HasValue(EnvironmentMode.RuntimeCharacteristic)
                 );
 
-        internal static IToolchain GetToolchain(this Runtime runtime, Descriptor? descriptor = null, bool preferMsBuildToolchains = false)
+        internal static IToolchain GetToolchain(this Runtime runtime, Descriptor? descriptor = null, bool preferMsBuildToolchains = false, bool isRuntimeExplicit = false)
         {
             switch (runtime)
             {
                 case ClrRuntime clrRuntime:
-                    if (!preferMsBuildToolchains && RuntimeInformation.IsFullFramework
-                        && RuntimeInformation.GetCurrentRuntime().MsBuildMoniker == runtime.MsBuildMoniker)
+                    bool ShouldUseCurrentRuntime()
                     {
-                        return RoslynToolchain.Instance;
+                        var assembly = isRuntimeExplicit
+                            ? descriptor?.WorkloadMethod.DeclaringType.Assembly
+                            : null;
+                        return runtime.MsBuildMoniker == ClrRuntime.GetTargetOrCurrentVersion(assembly).MsBuildMoniker;
                     }
 
+                    if (!preferMsBuildToolchains && RuntimeInformation.IsFullFramework && ShouldUseCurrentRuntime())
+                        return RoslynToolchain.Instance;
+
                     return clrRuntime.RuntimeMoniker != RuntimeMoniker.NotRecognized
                         ? GetToolchain(clrRuntime.RuntimeMoniker)
                         : CsProjClassicNetToolchain.From(clrRuntime.MsBuildMoniker);