Microsoft.Toolkit.Win32/Microsoft.Toolkit.Win32.UI.XamlHost/MetadataProviderDiscovery.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using WUX = Windows.UI.Xaml;

namespace Microsoft.Toolkit.Win32.UI.XamlHost
{
    /// <summary>
    /// MetadataProviderDiscovery is responsible for loading all metadata providers for custom UWP XAML
    /// types.  In this implementation, reflection is used at runtime to probe for metadata providers in
    /// the working directory, allowing any type that includes metadata (compiled in to a .NET framework
    /// assembly) to be used without explicit metadata handling by the application developer.  This
    /// internal class will be amended or removed when additional type loading support is available.
    /// </summary>
    internal static class MetadataProviderDiscovery
    {
        private static readonly List<Type> FilteredTypes = new List<Type>
        {
            typeof(XamlApplication),
            typeof(WUX.Markup.IXamlMetadataProvider)
        };

        /// <summary>
        /// Probes working directory for all available metadata providers
        /// </summary>
        /// <returns>List of UWP XAML metadata providers</returns>
        internal static IEnumerable<WUX.Markup.IXamlMetadataProvider> DiscoverMetadataProviders()
        {
            // Get all assemblies loaded in app domain and placed side-by-side from all DLL and EXE
            var loadedAssemblies = GetAssemblies();
#if NET462
            var uniqueAssemblies = new HashSet<Assembly>(loadedAssemblies, EqualityComparerFactory<Assembly>.CreateComparer(
                a => a.GetName().FullName.GetHashCode(),
                (a, b) => a.GetName().FullName.Equals(b.GetName().FullName, StringComparison.OrdinalIgnoreCase)));
#else
            var uniqueAssemblies = new HashSet<Assembly>(loadedAssemblies, EqualityComparerFactory<Assembly>.CreateComparer(
                a => a.GetName().FullName.GetHashCode(StringComparison.InvariantCulture),
                (a, b) => a.GetName().FullName.Equals(b.GetName().FullName, StringComparison.OrdinalIgnoreCase)));
#endif

            // Load all types loadable from the assembly, ignoring any types that could not be resolved due to an issue in the dependency chain
            foreach (var assembly in uniqueAssemblies)
            {
                foreach (var provider in LoadTypesFromAssembly(assembly))
                {
                    yield return provider;

                    if (typeof(WUX.Application).IsAssignableFrom(provider.GetType()))
                    {
                        System.Diagnostics.Debug.WriteLine("Xaml application has been created");
                        yield break;
                    }
                }
            }
        }

        private static IEnumerable<Assembly> GetAssemblies()
        {
            yield return Assembly.GetExecutingAssembly();

            // Get assemblies already loaded in the current app domain
            foreach (var a in AppDomain.CurrentDomain.GetAssemblies())
            {
                yield return a;
            }

            // Reflection-based runtime metadata probing
            var currentDirectory = new FileInfo(typeof(MetadataProviderDiscovery).Assembly.Location).Directory;

            foreach (var assembly in GetAssemblies(currentDirectory, "*.exe"))
            {
                yield return assembly;
            }

            foreach (var assembly in GetAssemblies(currentDirectory, "*.dll"))
            {
                yield return assembly;
            }
        }

        private static IEnumerable<Assembly> GetAssemblies(DirectoryInfo folder, string fileFilter)
        {
            foreach (var file in folder.EnumerateFiles(fileFilter))
            {
                Assembly a = null;

                try
                {
                    a = Assembly.LoadFrom(file.FullName);
                }
                catch (FileLoadException)
                {
                    // These exceptions are expected
                }
                catch (BadImageFormatException)
                {
                    // DLL is not loadable by CLR (e.g. Native)
                }

                if (a != null)
                {
                    yield return a;
                }
            }
        }

        /// <summary>
        /// Loads all types from the specified assembly and caches metadata providers
        /// </summary>
        /// <param name="assembly">Target assembly to load types from</param>
        /// <returns>The set of <seealso cref="WUX.Markup.IXamlMetadataProvider"/> found</returns>
        private static IEnumerable<WUX.Markup.IXamlMetadataProvider> LoadTypesFromAssembly(Assembly assembly)
        {
            // Load types inside the executing assembly
            foreach (var type in GetLoadableTypes(assembly))
            {
                // TODO: More type checking here
                // Not interface, not abstract, not generic, etc.
                if (typeof(WUX.Markup.IXamlMetadataProvider).IsAssignableFrom(type))
                {
                    var provider = (WUX.Markup.IXamlMetadataProvider)Activator.CreateInstance(type);
                    yield return provider;
                }
            }
        }

        // Algorithm from StackOverflow answer here:
        // http://stackoverflow.com/questions/7889228/how-to-prevent-reflectiontypeloadexception-when-calling-assembly-gettypes
        private static IEnumerable<Type> GetLoadableTypes(Assembly assembly)
        {
            if (assembly == null)
            {
                throw new ArgumentNullException(nameof(assembly));
            }

            try
            {
                var asmTypes = assembly.DefinedTypes
                    .Select(t => t.AsType());
                var filteredTypes = asmTypes.Where(t => !FilteredTypes.Contains(t));
                return filteredTypes;
            }
            catch (ReflectionTypeLoadException)
            {
                return Enumerable.Empty<Type>();
            }
            catch (FileLoadException)
            {
                return Enumerable.Empty<Type>();
            }
        }

        private static class EqualityComparerFactory<T>
        {
            private class MyComparer : IEqualityComparer<T>
            {
                private readonly Func<T, int> _getHashCodeFunc;
                private readonly Func<T, T, bool> _equalsFunc;

                public MyComparer(Func<T, int> getHashCodeFunc, Func<T, T, bool> equalsFunc)
                {
                    _getHashCodeFunc = getHashCodeFunc;
                    _equalsFunc = equalsFunc;
                }

                public bool Equals(T x, T y) => _equalsFunc(x, y);

                public int GetHashCode(T obj) => _getHashCodeFunc(obj);
            }

            public static IEqualityComparer<T> CreateComparer(Func<T, int> getHashCodeFunc, Func<T, T, bool> equalsFunc)
            {
                if (getHashCodeFunc == null)
                {
                    throw new ArgumentNullException(nameof(getHashCodeFunc));
                }

                if (equalsFunc == null)
                {
                    throw new ArgumentNullException(nameof(equalsFunc));
                }

                return new MyComparer(getHashCodeFunc, equalsFunc);
            }
        }
    }
}