LoadProviderConfiguration()方法出错ＢＵＧ修改方法

错误信息：

[NullReferenceException: 未将对象引用设置到对象的实例。]
   AspNetForums.Components.ForumsDataProvider.LoadProviderConfiguration() +37
   AspNetForums.Components.ForumsDataProvider.Instance(HttpContext context, String providerTypeName, String databaseOwner, String connectionString) +567
   AspNetForums.Components.ForumException.Log() +20
   AspNetForums.ForumsHttpModule.Application_BeginRequest(Object source, EventArgs e) +396
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute() +60
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +87

这个问题主要是由于Cache引起的。

修改方法：

Instance#region Instance
        /**//// <summary>
        /// Returns an instance of the user-specified data provider class.
        /// </summary>
        /// <returns>An instance of the user-specified data provider class.  This class must inherit the
        /// ForumsDataProvider interface.</returns>
        public static ForumsDataProvider Instance() 
        {
            return Instance(HttpContext.Current, null, null, null);
        }

        public static ForumsDataProvider Instance (HttpContext context) 
        {
            return Instance(context, null, null, null);
        }

        public static ForumsDataProvider Instance (HttpContext context, string providerTypeName, string databaseOwner, string connectionString) 
        {

            // Use the cache because the reflection used later is expensive
            //
            Cache cache = HttpRuntime.Cache;
            Type type = null;

            // Get the names of the providers
            //
            ForumConfiguration config = ForumConfiguration.GetConfig();

            // Read the configuration specific information
            // for this provider
            //
            Provider sqlForumsProvider = (Provider) config.Providers[config.DefaultProvider];

            // Read the connection string for this provider
            //
            if (connectionString == null)
                connectionString = sqlForumsProvider.Attributes["connectionString"];

            // Read the database owner name for this provider
            //
            if (databaseOwner == null)
                databaseOwner = sqlForumsProvider.Attributes["databaseOwner"];

            if (providerTypeName == null)
                providerTypeName = ((Provider) config.Providers[config.DefaultProvider]).Type;

            // In the provider instance in the cache?
            //
            if ( cache["DataProvider"] == null ) 
            {

                // The assembly should be in in or GAC, so we simply need
                // to get an instance of the type
                //
                try 
                {

                    type = Type.GetType( providerTypeName );

                    // Insert the type into the cache
                    //
                    Type[] paramTypes = new Type[2];
                    paramTypes[0] = typeof(string);
                    paramTypes[1] = typeof(string);
                    cache.Insert( "DataProvider", type.GetConstructor(paramTypes) );


                } 
                catch 
                {

                    if (context != null) 
                    {
                        // We can't load the dataprovider
                        //
                        StreamReader reader = new StreamReader( context.Server.MapPath("~/Languages/" + config.DefaultLanguage + "/errors/DataProvider.htm") );
                        string html = reader.ReadToEnd();
                        reader.Close();

                        html = html.Replace("[DATAPROVIDERCLASS]", config.DefaultProvider);
                        html = html.Replace("[DATAPROVIDERASSEMBLY]", config.DefaultProvider);
                        context.Response.Write(html);
                        context.Response.End();
                    } 
                    else 
                    {
                        throw new ForumException(ForumExceptionType.DataProvider, "Unable to load " + config.DefaultProvider);
                    }

                }

            }

            // Load the configuration settings
            //
            object[] paramArray = new object[2];
            paramArray[0] = databaseOwner;
            paramArray[1] = connectionString;

            return (ForumsDataProvider)(  ((ConstructorInfo)cache["DataProvider"]).Invoke(paramArray) );
        }
        #endregion