Allow provider-style initialization for section handlers.

Author: StephenMolloy

README.md

@@ -19,7 +19,7 @@ Version 2 is here with some new features:
   * Base Optional Tag - The [optional](#optional) tag that some of the builders in this project employed in V1 has been moved into the base class and is now available
 		on all key/value config builders.
   * Section Handlers - This feature allows users to develop extensions that will apply key/value config to sections other than `appSettings` and `connectionStrings`
-		if desired. Read more about this feature in the [Section Handlers](#sectionhandlers) segment below.
+		if desired. Read more about this feature in the [Section Handlers](#section-handlers) segment below.
 
 ## Key/Value Config Builders
 
@@ -339,7 +339,25 @@ collection. As an example, here is what their explicit declaration would look li
   </handlers>
 </Microsoft.Configuration.ConfigurationBuilders.SectionHandlers>
 ```
-When adding additional handlers, the name of the section must be 'Microsoft.Configuration.ConfigurationBuilders.SectionHandlers' so key/value config builders can find it.
+When adding additional handlers, the name of this section must be 'Microsoft.Configuration.ConfigurationBuilders.SectionHandlers' so key/value config builders can find it.
+Also note that a more qualified type will be required so the runtime can determine which assembly contains the new handler type. When working
+with ASP.Net applications, it is hit and miss regarding whether its able to define new section handlers in `App_Code` or not. Some configuration
+sections (such as `appSettings`) get loaded by ASP.Net before `App_Code` is compiled, so handlers for those sections will need to be
+compiled in a separate assembly in the 'bin' directory. For example:
+```xml
+<configSections>
+  <section name="Microsoft.Configuration.ConfigurationBuilders.SectionHandlers" type="Microsoft.Configuration.ConfigurationBuilders.SectionHandlersSection, Microsoft.Configuration.ConfigurationBuilders.Base" restartOnExternalChanges="false" requirePermission="false" />
+</configSections>
+
+<Microsoft.Configuration.ConfigurationBuilders.SectionHandlers>
+  <handlers>
+    <remove name="DefaultAppSettingsHandler" />
+    <add name="DefaultAppSettingsHandler" type="MyCustomAppSettingsSectionHandler, RefAssemblyInBin" />
+    <remove name="DefaultConnectionStringsHandler" />
+    <add name="DefaultConnectionStringsHandler" type="MyCustomConnectionStringsSectionHandler, App_Code" />
+  </handlers>
+</Microsoft.Configuration.ConfigurationBuilders.SectionHandlers>
+```
 
 ## Implementing More Key/Value Config Builders
 

src/Base/SectionHandler.cs

@@ -3,6 +3,9 @@
 
 using System.Configuration;
 using System.Collections.Generic;
+using System.Configuration.Provider;
+using System.Collections.Specialized;
+using System;
 
 namespace Microsoft.Configuration.ConfigurationBuilders
 {
@@ -17,7 +20,7 @@ internal interface ISectionHandler
     /// A class to be used by <see cref="KeyValueConfigBuilder"/>s to apply key/value config pairs to .Net configuration sections.
     /// </summary>
     /// <typeparam name="T">The type of <see cref="ConfigurationSection"/> that the implementing class can process.</typeparam>
-    public abstract class SectionHandler<T> : ISectionHandler where T : ConfigurationSection
+    public abstract class SectionHandler<T> : ProviderBase, ISectionHandler where T : ConfigurationSection
     {
         /// <summary>
         /// The <see cref="ConfigurationSection"/> instance being processed by this <see cref="SectionHandler{T}"/>.
@@ -41,6 +44,12 @@ public abstract class SectionHandler<T> : ISectionHandler where T : Configuratio
         /// <param name="oldKey">The old key name for the config item, or null.</param>
         /// <param name="oldItem">A reference to the old key/value pair obtained by <see cref="GetEnumerator"/>, or null.</param>
         public abstract void InsertOrUpdate(string newKey, string newValue, string oldKey = null, object oldItem = null);
+
+        private void Initialize(string name, T configSection, NameValueCollection config)
+        {
+            ConfigSection = configSection;
+            Initialize(name, config);
+        }
     }
 
     /// <summary>

src/Base/SectionHandlerSection.cs

@@ -2,7 +2,9 @@
 // Licensed under the MIT license. See the License.txt file in the project root for full license information.
 
 using System;
+using System.Collections.Specialized;
 using System.Configuration;
+using System.Reflection;
 
 namespace Microsoft.Configuration.ConfigurationBuilders
 {
@@ -62,13 +64,20 @@ static internal ISectionHandler GetSectionHandler<T>(T configSection) where T :
                     ISectionHandler handler = Activator.CreateInstance(handlerType) as ISectionHandler;
                     if (handler != null)
                     {
-                        sectionHandlerDesiredType.GetProperty("ConfigSection").SetValue(handler, configSection);
+                        ProviderSettings settings = handlerSection.Handlers[i];
+                        NameValueCollection clonedParams = new NameValueCollection(settings.Parameters.Count);
+                        foreach (string key in settings.Parameters)
+                            clonedParams[key] = settings.Parameters[key];
+
+                        MethodInfo init = sectionHandlerDesiredType.GetMethod("Initialize", BindingFlags.NonPublic | BindingFlags.Instance);
+                        init.Invoke(handler, new object[] { settings.Name, configSection, clonedParams });
+
+                        return handler;
                     }
-                    return handler;
                 }
             }
 
-            return null;
+            throw new Exception($"Error in Configuration: Cannot find ISectionHandler for '{configSection.SectionInformation.Name}' section.");
         }
     }
 }