Calcium: A modular application toolset leveraging PRISM – Part 1

//===================================================================================
// Microsoft patterns & practices
// Composite Application Guidance for Windows Presentation Foundation and Silverlight
//===================================================================================
// Copyright (c) Microsoft Corporation.  All rights reserved.
// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY
// OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT
// LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
// FITNESS FOR A PARTICULAR PURPOSE.
//===================================================================================
// The example companies, organizations, products, domain names,
// e-mail addresses, logos, people, places, and events depicted
// herein are fictitious.  No association with any real company,
// organization, product, domain name, email address, logo, person,
// places, or events is intended or should be inferred.
//===================================================================================
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Security.Permissions;
using Microsoft.Practices.Composite.Properties;

namespace Microsoft.Practices.Composite.Modularity
{
    /// <summary>
    /// Handles AppDomain's AssemblyResolve event to be able to load assemblies dynamically in 
    /// the LoadFrom context, but be able to reference the type from assemblies loaded in the Load context.
    /// </summary>
    [SecurityPermission(SecurityAction.LinkDemand)]
    [SecurityPermission(SecurityAction.InheritanceDemand)]
    public class AssemblyResolver : IAssemblyResolver, IDisposable
    {
        private readonly List<AssemblyInfo> registeredAssemblies = new List<AssemblyInfo>();

        private bool handlesAssemblyResolve;

        /// <summary>
        /// Registers the specified assembly and resolves the types in it when the AppDomain requests for it.
        /// </summary>
        /// <param name="assemblyFilePath">The path to the assemly to load in the LoadFrom context.</param>
        /// <remarks>This method does not load the assembly immediately, but lazily until someone requests a <see cref="Type"/>
        /// declared in the assembly.</remarks>
        public void LoadAssemblyFrom(string assemblyFilePath)
        {
            if (!this.handlesAssemblyResolve)
            {
                AppDomain.CurrentDomain.AssemblyResolve += this.CurrentDomain_AssemblyResolve;
                this.handlesAssemblyResolve = true;
            }

            Uri assemblyUri = GetFileUri(assemblyFilePath);

            if (assemblyUri == null)
            {
                throw new ArgumentException(Resources.InvalidArgumentAssemblyUri, "assemblyFilePath");
            }

            if (!File.Exists(assemblyUri.LocalPath))
            {
                throw new FileNotFoundException();
            }

            AssemblyName assemblyName = AssemblyName.GetAssemblyName(assemblyUri.LocalPath);
            AssemblyInfo assemblyInfo = this.registeredAssemblies.FirstOrDefault(a => assemblyName == a.AssemblyName);

            if (assemblyInfo != null)
            {
                return;
            }

            assemblyInfo = new AssemblyInfo() { AssemblyName = assemblyName, AssemblyUri = assemblyUri };
            this.registeredAssemblies.Add(assemblyInfo);
        }

        private static Uri GetFileUri(string filePath)
        {
            if (String.IsNullOrEmpty(filePath))
            {
                return null;
            }

            Uri uri;
            if (!Uri.TryCreate(filePath, UriKind.Absolute, out uri))
            {
                return null;
            }

            if (!uri.IsFile)
            {
                return null;
            }

            return uri;
        }

        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Reliability", "CA2001:AvoidCallingProblematicMethods", MessageId = "System.Reflection.Assembly.LoadFrom")]
        private Assembly CurrentDomain_AssemblyResolve(object sender, ResolveEventArgs args)
        {
            AssemblyName assemblyName = new AssemblyName(args.Name);

            AssemblyInfo assemblyInfo = this.registeredAssemblies.FirstOrDefault(a => AssemblyName.ReferenceMatchesDefinition(assemblyName, a.AssemblyName));

            if (assemblyInfo != null)
            {
                if (assemblyInfo.Assembly == null)
                {
                    assemblyInfo.Assembly = Assembly.LoadFrom(assemblyInfo.AssemblyUri.LocalPath);
                }

                return assemblyInfo.Assembly;
            }

            return null;
        }

        private class AssemblyInfo
        {
            public AssemblyName AssemblyName { get; set; }

            public Uri AssemblyUri { get; set; }

            public Assembly Assembly { get; set; }
        }

        #region Implementation of IDisposable

        /// <summary>
        /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
        /// </summary>
        /// <remarks>Calls <see cref="Dispose(bool)"/></remarks>.
        /// <filterpriority>2</filterpriority>
        public void Dispose()
        {
            this.Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Disposes the associated <see cref="AssemblyResolver"/>.
        /// </summary>
        /// <param name="disposing">When <see langword="true"/>, it is being called from the Dispose method.</param>
        protected virtual void Dispose(bool disposing)
        {
            if (this.handlesAssemblyResolve)
            {
                AppDomain.CurrentDomain.AssemblyResolve -= this.CurrentDomain_AssemblyResolve;
                this.handlesAssemblyResolve = false;
            }
        }

        #endregion
    }
}