Added configurable Python.Runtime.dll loader.

src/config/FodyWeavers.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<Weavers>
+  <ModuleInit/>
+</Weavers>

src/config/ModuleInitializer.cs

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+using System;
+
+using Python.Config;
+
+/// <summary>
+/// Used by the ModuleInit. All code inside the Initialize method is ran as soon as the assembly is loaded.
+/// </summary>
+public static class ModuleInitializer
+{
+    /// <summary>
+    /// Initializes the module.
+    /// </summary>
+    public static void Initialize()
+    {
+        PythonConfig.EnsureInitialized();
+    }
+}

src/config/Properties/AssemblyInfo.cs

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+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+// General Information about an assembly is controlled through the following 
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyTitle("config")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("")]
+[assembly: AssemblyProduct("config")]
+[assembly: AssemblyCopyright("Copyright ©  2016")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+
+// Setting ComVisible to false makes the types in this assembly not visible 
+// to COM components.  If you need to access a type in this assembly from 
+// COM, set the ComVisible attribute to true on that type.
+[assembly: ComVisible(false)]
+
+// The following GUID is for the ID of the typelib if this project is exposed to COM
+[assembly: Guid("c4779672-f707-4778-b901-7c63548a3d6a")]
+
+// PythonVersion information for an assembly consists of the following four values:
+//
+//      Major PythonVersion
+//      Minor PythonVersion 
+//      Build Number
+//      Revision
+//
+// You can specify all the values or you can default the Build and Revision Numbers 
+// by using the '*' as shown below:
+// [assembly: AssemblyVersion("1.0.*")]
+[assembly: AssemblyVersion("1.0.0.0")]
+[assembly: AssemblyFileVersion("1.0.0.0")]

src/config/Python.Config.csproj

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+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{C4779672-F707-4778-B901-7C63548A3D6A}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>Python.Config</RootNamespace>
+    <AssemblyName>Python.Config</AssemblyName>
+    <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <NuGetPackageImportStamp>
+    </NuGetPackageImportStamp>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="ModuleInitializer.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="PythonConfig.cs" />
+    <Compile Include="PythonConfigSection.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Configuration" />
+  </ItemGroup>
+  <ItemGroup>
+    <Content Include="FodyWeavers.xml" />
+    <EmbeddedResource Include="Python.Runtime-Py27-Win64.dll">
+      <LogicalName>Python.Runtime-Py27-Win64.dll</LogicalName>
+    </EmbeddedResource>
+    <EmbeddedResource Include="Python.Runtime-Py35-Win64.dll">
+      <LogicalName>Python.Runtime-Py35-Win64.dll</LogicalName>
+    </EmbeddedResource>
+    <EmbeddedResource Include="Python.Runtime-Py27-Linux64.dll">
+      <LogicalName>Python.Runtime-Py27-Linux64.dll</LogicalName>
+    </EmbeddedResource>
+    <EmbeddedResource Include="Python.Runtime-Py35-Linux64.dll">
+      <LogicalName>Python.Runtime-Py35-Linux64.dll</LogicalName>
+    </EmbeddedResource>
+  </ItemGroup>
+  <ItemGroup>
+    <None Include="packages.config" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <Import Project="..\..\packages\Fody.1.29.4\build\portable-net+sl+win+wpa+wp\Fody.targets" Condition="Exists('..\..\packages\Fody.1.29.4\build\portable-net+sl+win+wpa+wp\Fody.targets')" />
+  <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
+    <PropertyGroup>
+      <ErrorText>This project references NuGet package(s) that are missing on this computer. Use NuGet Package Restore to download them.  For more information, see http://go.microsoft.com/fwlink/?LinkID=322105. The missing file is {0}.</ErrorText>
+    </PropertyGroup>
+    <Error Condition="!Exists('..\..\packages\Fody.1.29.4\build\portable-net+sl+win+wpa+wp\Fody.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\..\packages\Fody.1.29.4\build\portable-net+sl+win+wpa+wp\Fody.targets'))" />
+  </Target>
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>

src/config/PythonConfig.cs

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+namespace Python.Config
+{
+    using System;
+    using System.Configuration;
+    using System.IO;
+    using System.Reflection;
+    using System.Text.RegularExpressions;
+
+    /// <summary>
+    /// Python.Net runtime configuration.
+    /// </summary>
+    public static class PythonConfig
+    {
+        private static object _assemblyLoadLock = new object();
+
+        private static Exception _configException;
+
+        private static Assembly _pythonRuntimeAssembly;
+
+        private static string _pythonVersion;
+
+        private static Regex _versionNumberRegex = new Regex("^(?<major>\\d)\\.(?<minor>\\d)$");
+
+        /// <summary>
+        /// Initializes static members of the <see cref="PythonConfig"/> class. 
+        /// </summary>
+        static PythonConfig()
+        {
+            if (IntPtr.Size == 4)
+            {
+                throw new NotSupportedException("32 bit platform not supported by Python.Config library.");
+            }
+            AppDomain.CurrentDomain.AssemblyResolve += AssemblyResolveHandler;
+        }
+
+        /// <summary>
+        /// Loaded assembly name.
+        /// </summary>
+        public static string LoadedRuntimeAssembly { get; private set; }
+
+        /// <summary>
+        /// Pyhon runtime version. Can be changed only 
+        /// </summary>
+        public static string PythonVersion
+        {
+            get
+            {
+                if (_pythonVersion == null)
+                {
+                    try
+                    {
+                        var pythonConfigSection = (PythonConfigSection)ConfigurationManager.GetSection("pythonConfig");
+                        _pythonVersion = pythonConfigSection.PythonVersion;
+
+                        ValidatePythonVersion(_pythonVersion);
+                    }
+                    catch (Exception ex)
+                    {
+                        _pythonVersion = "2.7";
+                        _configException = ex;
+                        AppDomain.CurrentDomain.SetData("PythonConfigException", _configException);
+                    }
+                }
+
+                return _pythonVersion;
+            }
+
+            private set
+            {
+                // Disabling this feature. Dynamic version select should be implemented through callback in configuration file.
+                if (IsRuntimeAssemblyLoaded)
+                {
+                    throw new InvalidOperationException(
+                              "Python version can be changed only before Python.Runtime assembly loaded by CLR.");
+                }
+
+                ValidatePythonVersion(value);
+                _pythonVersion = value;
+            }
+        }
+
+        /// <summary>
+        /// Used internally to determine that Python.Runtime assembly already loaded.
+        /// </summary>
+        internal static bool IsRuntimeAssemblyLoaded { get; set; }
+
+        /// <summary>
+        /// Forces Python Config library to be initialized.
+        /// </summary>
+        public static void EnsureInitialized()
+        {
+            // Do nothing, but forces CLR to load PythonConfig type.
+        }
+
+        private static Assembly AssemblyResolveHandler(object sender, ResolveEventArgs args)
+        {
+            if (args.Name.StartsWith("Python.Runtime"))
+            {
+                lock (_assemblyLoadLock)
+                {
+                    if (_pythonRuntimeAssembly != null)
+                    {
+                        return _pythonRuntimeAssembly;
+                    }
+
+                    string platform = Path.DirectorySeparatorChar == '\\' ? "Win64" : "Linux64";
+
+                    // We will load assembly here.
+                    string resourceName = $"Python.Runtime-Py{PythonVersion.Replace(".", string.Empty)}-{platform}.dll";
+
+                    // looks for the assembly from the resources and load it
+                    using (var stream = typeof(PythonConfig).Assembly.GetManifestResourceStream(resourceName))
+                    {
+                        if (stream != null)
+                        {
+                            byte[] assemblyData = new byte[stream.Length];
+                            stream.Read(assemblyData, 0, assemblyData.Length);
+                            try
+                            {
+                                _pythonRuntimeAssembly = Assembly.Load(assemblyData);
+                                LoadedRuntimeAssembly = resourceName;
+                                return _pythonRuntimeAssembly;
+                            }
+                            catch (Exception ex)
+                            {
+                                AppDomain.CurrentDomain.SetData("PythonConfigException", ex);
+                            }
+                        }
+                    }
+                }
+            }
+
+            return null;
+        }
+
+        private static void ValidatePythonVersion(string version)
+        {
+            if (!_versionNumberRegex.Match(version).Success)
+            {
+                throw new ArgumentException("Python version should be specified in x.y, for example \"2.7\".");
+            }
+
+            if (version != "2.7" && version != "3.5")
+            {
+                throw new ArgumentException("Unsupported python version. Only 2.7 and 3.5 are supported.");
+            }
+        }
+    }
+}

src/config/PythonConfigSection.cs

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+namespace Python.Config
+{
+    using System;
+    using System.Configuration;
+
+    public class PythonConfigSection : ConfigurationSection
+    {
+        [ConfigurationProperty("pythonVersion", DefaultValue = "2.7", IsRequired = true)]
+        public string PythonVersion
+        {
+            get
+            {
+                return (string)this["pythonVersion"];
+            }
+
+            set
+            {
+                this["pythonVersion"] = value;
+            }
+        }
+    }
+}

src/config/packages.config

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+<?xml version="1.0" encoding="utf-8"?>
+<packages>
+  <package id="Fody" version="1.29.4" targetFramework="net40" developmentDependency="true" />
+  <package id="ModuleInit.Fody" version="1.5.9.0" targetFramework="net40" developmentDependency="true" />
+</packages>

src/configtest/App.config

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+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+  <configSections>
+    <section name="pythonConfig" type="Python.Config.PythonConfigSection, Python.Config, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
+  </configSections>
+  <pythonConfig pythonVersion="3.5"/>
+</configuration>

src/configtest/Program.cs

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+namespace Python.Config.Test
+{
+    using System;
+
+    using Python.Runtime;
+
+    internal class Program
+    {
+        static Program()
+        {
+            Console.WriteLine("Starting application...");
+            // Required to be placed in the static constructor for Mono.
+            PythonConfig.EnsureInitialized();
+        }
+
+        [STAThread]
+        private static int Main(string[] args)
+        {
+            // Mono workaround required to fix AssemblyResolve + EntryPoint class bug.
+            // Classes that was referenced from EntryPoint class cannot use assemblies resolved through "AssemblyResolve"
+            Action monoWorkaround = () =>
+                {
+                    try
+                    {
+                        if (PythonConfig.LoadedRuntimeAssembly != null)
+                        {
+                            Console.WriteLine(
+                                $"Python.runtime.dll substituted by {PythonConfig.LoadedRuntimeAssembly}.");
+                        }
+                        else
+                        {
+                            Console.WriteLine(
+                                $"Python.runtime.dll was loaded from application directory.");
+                        }
+
+                        // You should put this initialized only if some component starting to use it before first application configuration file read attempt.
+                        // So in rare cases.
+                        PythonEngine.Initialize();
+
+                        // Like that.
+                        try
+                        {
+                            using (Py.GIL())
+                            {
+                                dynamic sysModule = Py.Import("sys");
+                                Console.WriteLine("Python engine version:");
+                                Console.WriteLine(sysModule.version);
+                            }
+                        }
+                        finally
+                        {
+                            PythonEngine.Shutdown();
+                        }
+                    }
+                    catch (Exception ex)
+                    {
+                        Console.WriteLine(ex);
+                    }
+                };
+            monoWorkaround();
+
+            return 0;
+        }
+    }
+}