Allow multiple web applications under IIS to use the same SQL server session state database without modifying the database stored procedures.

gistfile1.cs

/*
 * This code is a workaround for the fact that session state in ASP.NET SqlServer mode is isolated by application. The AppDomain's appId is used to
 * segregate applications, and there is no official way exposed to modify this behaviour.
 * 
 * Some workarounds tackle the problem by modifying the ASP.NET state database. This workaround approaches the problem from the application code
 * and will be appropriate for those who do not want to alter the database. We are using it during a migration process from old to new technology stacks, 
 * where we want the transition between the two sites to be seamless.
 * 
 * As always, when relying on implementation details, the reflection based approach used here may break in future / past versions of the .NET framework.
 * Test thoroughly.
 * 
 * Usage: add this to your Global.asax:
 *       protected void Application_BeginRequest()
 *       {
 *           SessionStateCrossApplicationHacker.SetSessionStateApplicationName("an application");
 *       }
 */

using System;
using System.Data.SqlClient;
using System.Globalization;
using System.Reflection;
using System.Web.SessionState;

public static class SessionStateCrossApplicationHacker
{
    static string _appName;
    static readonly object _appNameLock = new object();

    public static void SetSessionStateApplicationName(string appName)
    {
        if (_appName != appName)
        {
            lock (_appNameLock)
            {
                if (_appName != appName)
                {
                    SetSessionStateApplicationNameOnceOnly(appName);

                    _appName = appName;
                }
            }
        }
    }

    static void SetSessionStateApplicationNameOnceOnly(string appName)
    {
        //get the static instance of SqlSessionStateStore.SqlPartitionInfo from System.Web.SessionState.SqlSessionStateStore.s_singlePartitionInfo
        var sqlSessionStateStoreType = typeof (SessionStateMode).Assembly.GetType("System.Web.SessionState.SqlSessionStateStore");
        var sqlSessionStatePartitionInfoInstance = GetStaticFieldValue(sqlSessionStateStoreType, "s_singlePartitionInfo");
        if (sqlSessionStatePartitionInfoInstance == null)
            throw new InvalidOperationException("You'll need to call this method later in the pipeline - the session state mechanism (SessionStateModule, which is an IHttpModule) has not been initialised yet. Try calling this method in Global.asax's Application_BeginRequest. Also make sure that you do not specify a partitionResolverType in your web.config.");

        //ensure that the session has not been used prior to this with an incorrect app ID
        var isStaticSqlPartitionInfoInitialised = GetFieldValue<bool>(sqlSessionStatePartitionInfoInstance, "_sqlInfoInited");
        if (isStaticSqlPartitionInfoInitialised)
            throw new InvalidOperationException("You'll need to call this method earlier in the pipeline - before any sessions have been loaded.");

        //force initialisation of the static SqlSessionStateStore.SqlPartitionInfo instance - otherwise this will happen later and overwrite our change
        var connectionString = GetFieldValue<string>(sqlSessionStatePartitionInfoInstance, "_sqlConnectionString");
        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();
            CallInstanceMethod(sqlSessionStatePartitionInfoInstance, "InitSqlInfo", connection);
        }

        //calculate and set the application hash code
        string applicationNameHashCode = GetHashCode(appName).ToString("x8", CultureInfo.InvariantCulture);
        GetField(sqlSessionStatePartitionInfoInstance, "_appSuffix").SetValue(sqlSessionStatePartitionInfoInstance, applicationNameHashCode);
    }

    static int GetHashCode(string appName)
    {
        string s = appName.ToLower();
        int hash = 5381;
        int len = s.Length;

        for (int i = 0; i < len; i++)
        {
            int c = Convert.ToInt32(s[i]);
            hash = ((hash << 5) + hash) ^ c;
        }

        return hash;
    }

    static void CallInstanceMethod(object instance, string methodName, params object[] parameters)
    {
        var methodInfo = instance.GetType().GetMethod(methodName, BindingFlags.NonPublic | BindingFlags.Instance);
        methodInfo.Invoke(instance, parameters);
    }

    static object GetStaticFieldValue(Type typeWithStaticField, string staticFieldName)
    {
        return typeWithStaticField.GetField(staticFieldName, BindingFlags.NonPublic | BindingFlags.Static).GetValue(null);
    }

    static FieldInfo GetField(object instance, string name)
    {
        return instance.GetType().GetField(name, BindingFlags.NonPublic | BindingFlags.Instance);
    }

    static T GetFieldValue<T>(object instance, string name)
    {
        return (T)GetField(instance, name).GetValue(instance);
    }
}