add Windows check to our CsProjClassicNetToolchain

src/BenchmarkDotNet.Core/Toolchains/CsProj/CsProjClassicNetToolchain.cs

@@ -4,9 +4,9 @@
 using BenchmarkDotNet.Extensions;
 using BenchmarkDotNet.Jobs;
 using BenchmarkDotNet.Loggers;
+using BenchmarkDotNet.Portability;
 using BenchmarkDotNet.Running;
 using JetBrains.Annotations;
-using Microsoft.Win32;
 
 namespace BenchmarkDotNet.Toolchains.CsProj
 {
@@ -21,6 +21,7 @@ public class CsProjClassicNetToolchain : Toolchain
         [PublicAPI] public static readonly IToolchain Net461 = new CsProjClassicNetToolchain("net461");
         [PublicAPI] public static readonly IToolchain Net462 = new CsProjClassicNetToolchain("net462");
         [PublicAPI] public static readonly IToolchain Net47 = new CsProjClassicNetToolchain("net47");
+        private static readonly IToolchain Default = Net46; // the lowest version we support
 
         [PublicAPI]
         public static readonly Lazy<IToolchain> Current = new Lazy<IToolchain>(GetCurrentVersion);
@@ -41,6 +42,12 @@ public override bool IsSupported(Benchmark benchmark, ILogger logger, IResolver
                 return false;
             }
 
+            if (!RuntimeInformation.IsWindows())
+            {
+                logger.WriteLineError($"Classic .NET toolchain is supported only for Windows, benchmark '{benchmark.DisplayInfo}' will not be executed");
+                return false;
+            }
+
             if (!HostEnvironmentInfo.GetCurrent().IsDotNetCliInstalled())
             {
                 logger.WriteLineError($"BenchmarkDotNet requires dotnet cli toolchain to be installed, benchmark '{benchmark.DisplayInfo}' will not be executed");
@@ -58,26 +65,32 @@ public override bool IsSupported(Benchmark benchmark, ILogger logger, IResolver
 
         private static IToolchain GetCurrentVersion()
         {
-            try
+            if (!RuntimeInformation.IsWindows())
+                return Net46; // we return .NET 4.6 which during validaiton will tell the user about lack of support
+
+            return GetCurrentVersionBasedOnWindowsRegistry();
+        }
+
+        // this logic is put to a separate method to avoid any assembly loading issues on non Windows systems
+        private static IToolchain GetCurrentVersionBasedOnWindowsRegistry()
+        {   
+            using (var ndpKey = Microsoft.Win32.RegistryKey
+                .OpenBaseKey(Microsoft.Win32.RegistryHive.LocalMachine, Microsoft.Win32.RegistryView.Registry32)
+                .OpenSubKey("SOFTWARE\\Microsoft\\NET Framework Setup\\NDP\\v4\\Full\\"))
             {
-                using (RegistryKey ndpKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry32)
-                    .OpenSubKey("SOFTWARE\\Microsoft\\NET Framework Setup\\NDP\\v4\\Full\\"))
-                {
-                    int releaseKey = Convert.ToInt32(ndpKey.GetValue("Release"));
-                    // magic! https://msdn.microsoft.com/en-us/library/hh925568(v=vs.110).aspx
-                    if (releaseKey >= 460798)
-                        return Net47;
-                    if (releaseKey >= 394802)
-                        return Net462;
-                    if (releaseKey >= 394254)
-                        return Net461;
+                if (ndpKey == null)
+                    return Default;
 
+                int releaseKey = Convert.ToInt32(ndpKey.GetValue("Release"));
+                // magic numbers come from https://msdn.microsoft.com/en-us/library/hh925568(v=vs.110).aspx
+                if (releaseKey >= 460798)
+                    return Net47;
+                if (releaseKey >= 394802)
+                    return Net462;
+                if (releaseKey >= 394254)
                     return Net461;
-                }
-            }
-            catch (Exception e)
-            {
-                throw new Exception("You need to run on Windows with .NET 4.6+ installed", e);
+
+                return Default;
             }
         }
     }