IT:AD:NET:IoC:Unity:HttpContextLifetimeManager [Notes]

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# IT:AD:NET:IoC:Unity:HttpContextLifetimeManager #



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## Situation ##

Incredibly, Unity only offers a Thread based lifetime manager -- which is not appropriate for Web Scenarios.


## Solution ##
Either use Ninject (as it has one baked in), or use the following when registering Services.



Code:


    using System;
    using System.Collections;
    using System.Collections.Generic;
    using System.Reflection;
    using Microsoft.Practices.Unity;

    namespace XAct.Services.IoC
    {
        /// <summary>
        /// An implementation of the Unity Lifetime Manager specifically for 
        /// Web scenarios.
        /// </summary>
        /// <remarks>
        /// <para>
        /// Used by <see cref="XAct.Services.ServiceLocatorService"/> when 
        /// trying to register services by WebRequest.
        /// </para>
        /// </remarks>
        /// <internal>
        /// See: http://bit.ly/yyguHs
        /// </internal>
        public class HttpContextLifetimeManager :  PerThreadLifetimeManager, IDisposable
        {

            private readonly string _key = Guid.NewGuid().ToString();




            #region Constants

            /// <summary>
            ///   Class of constants used within the
            ///   <see cref = "Environment" />
            ///   class.
            /// </summary>
            public static class Constants
            {
                /// <summary>
                ///   Constant used to save the ApplicationName in the Request's hashtable.
                /// </summary>
                public const string ApplicationNameKey = "ApplicationName";

                /// <summary>
                ///   FQN of System.Web
                /// </summary>
                public const string SystemWebAssemblyString =
                    "System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a";
            }

            #endregion

            /// <summary>
            /// Retrieve a value from the backing store associated with this Lifetime policy.
            /// </summary>
            /// <returns>
            /// the object desired, or null if no such object is currently stored.
            /// </returns>
            public override object GetValue()
            {
                if (HttpContext == null)
                {
                    return base.GetValue();
                }
                return InnerDict[_key];
            }

            /// <summary>
            /// Remove the given object from backing store.
            /// </summary>
            public override void RemoveValue()
            {
                if (HttpContext==null)
                {
                    base.RemoveValue();
                    return;
                }
                InnerDict.Remove(_key);
            }
            
            /// <summary>
            /// Stores the given value into backing store for retrieval later.
            /// </summary>
            /// <param name="newValue">The object being stored.</param>
            public override void SetValue(object newValue)
            {
                if (HttpContext == null)
                {
                    base.SetValue(newValue);
                }

                InnerDict[_key] = newValue;
            }
            /// <summary>
            /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
            /// </summary>
            public void Dispose()
            {
                RemoveValue();
            }

            private IDictionary InnerDict
            {
                get
                {
                    const string key = "Unity.HttpContextLifetimeManager.Cache";
                    IDictionary results = HttpContextItems[key] as IDictionary;

                    if (results == null)
                    {
                        results = new Dictionary<object, object>();

                        HttpContextItems.Add(key, results);
                    }
                    return results;
                }
            }

            ///<summary>
            ///  The HttpContext by Reflection 
            ///  (so that this assembly does not end up having a reference
            ///  to System.Web)
            ///</summary>
            public static object HttpContext
            {
                get
                {
                    if (_cachedHttpContextPropertyInfo == null)
                    {
                        const string path = "System.Web.HttpContext";
                        const string assemblyName = Constants.SystemWebAssemblyString;

                        const string contextPath = "Current";

                        Type type = Type.GetType(path + ", " + assemblyName, true, true);

                        if (type == null)
                        {
                            return null;
                        }


                        //get the HTTP context property info
                        _cachedHttpContextPropertyInfo = type.GetProperty(contextPath,
                                                                          (BindingFlags.Public | BindingFlags.Static));
                    }
                    //get a reference to the current HTTP context
                    return _cachedHttpContextPropertyInfo.GetValue(null, null);
                }
            }
            private static PropertyInfo _cachedHttpContextPropertyInfo;


            /// <summary>
            /// Gets the items.
            /// </summary>
            public IDictionary HttpContextItems
            {
                get
                {
                    if (HttpContext != null)
                    {
                        IDictionary items = HttpContext.GetValue("Items") as IDictionary;
                        return items;
                    }
                    throw new Exception("No HttpContext to back HttpContextLifetimeManager");
                }
            }

        }
    }