Changed default toolchain for .Net Framework to CsProjClassicNetToolchain.

Removed check in test.

Author: timcassell

src/BenchmarkDotNet/Toolchains/ToolchainExtensions.cs

@@ -36,7 +36,12 @@ internal static IToolchain GetToolchain(this Runtime runtime, Descriptor? descri
             {
                 case ClrRuntime clrRuntime:
                     if (!preferMsBuildToolchains && RuntimeInformation.IsFullFramework
-                        && RuntimeInformation.GetCurrentRuntime().MsBuildMoniker == runtime.MsBuildMoniker)
+                        && RuntimeInformation.GetCurrentRuntime().MsBuildMoniker == runtime.MsBuildMoniker
+                        // If dotnet SDK is installed, we use CsProjClassicNetToolchain.
+                        && (!HostEnvironmentInfo.GetCurrent().IsDotNetCliInstalled()
+                            // Integration tests take too much time, because each benchmark run rebuilds the test suite and BenchmarkDotNet itself.
+                            // To reduce the total duration of the CI workflows, we just use RoslynToolchain.
+                            || XUnitHelper.IsIntegrationTest.Value))
                     {
                         return RoslynToolchain.Instance;
                     }

tests/BenchmarkDotNet.IntegrationTests.ManualRunning.MultipleFrameworks/MultipleFrameworksTest.cs

@@ -22,20 +22,6 @@ public class MultipleFrameworksTest : BenchmarkTestExecutor
         [InlineData(RuntimeMoniker.Net80)]
         public void EachFrameworkIsRebuilt(RuntimeMoniker runtime)
         {
-#if NET461
-            // We cannot detect what target framework version the host was compiled for on full Framework,
-            // which causes the RoslynToolchain to be used instead of CsProjClassicNetToolchain when the host is full Framework
-            // (because full Framework always uses the version that's installed on the machine, unlike Core),
-            // which means if the machine has net48 installed (not net481), the net461 host with net48 runtime moniker
-            // will not be recompiled, causing the test to fail.
-
-            // If we ever change the default toolchain to CsProjClassicNetToolchain instead of RoslynToolchain, we can remove this check.
-            if (runtime == RuntimeMoniker.Net48)
-            {
-                // XUnit doesn't provide Assert.Skip API yet.
-                return;
-            }
-#endif
             var config = ManualConfig.CreateEmpty().AddJob(Job.Dry.WithRuntime(runtime.GetRuntime()).WithEnvironmentVariable(TfmEnvVarName, runtime.ToString()));
             CanExecute<ValuePerTfm>(config);
         }