Fixing some issues with the AggregateSettingsProvider

The settings provider didn't work if one of the providers returned null for a setting, but the return type wasn't nullable. To sort this I've added a new method to the `ISettingsProvider` interface to return the unconverted string value, which allows the aggregate provider to get the value and do the conversion separately.

AppConfigFacility.Azure/AzureSettingsProvider.cs

@@ -7,12 +7,8 @@
     /// </summary>
     public class AzureSettingsProvider : SettingsProviderBase
     {
-        /// <summary>
-        /// Gets the setting as a string from some config source.
-        /// </summary>
-        /// <param name="key">The key of the setting.</param>
-        /// <returns>The string value of the setting.</returns>
-        protected override string GetSetting(string key)
+        /// <inheritdoc />
+        public override string GetSetting(string key)
         {
             return CloudConfigurationManager.GetSetting(key);
         }

AppConfigFacility/AggregateSettingsProvider.cs

@@ -8,7 +8,7 @@ namespace AppConfigFacility
     /// A wrapper around several <see cref="ISettingsProvider"/> objects that will attempt to get
     /// settings from each one in turn.
     /// </summary>
-    public class AggregateSettingsProvider : ISettingsProvider
+    public class AggregateSettingsProvider : SettingsProviderBase
     {
         private readonly IReadOnlyCollection<ISettingsProvider> _settingsProviders;
 
@@ -34,11 +34,16 @@ public AggregateSettingsProvider(IReadOnlyCollection<ISettingsProvider> settings
         /// </summary>
         public IReadOnlyCollection<ISettingsProvider> SettingsProviders => _settingsProviders;
 
-        public object GetSetting(string key, Type returnType)
+        public override object GetSetting(string key, Type returnType)
+        {
+            return ConvertSetting(GetSetting(key), returnType);
+        }
+
+        public override string GetSetting(string key)
         {
             foreach (var provider in _settingsProviders)
             {
-                var value = provider.GetSetting(key, returnType);
+                var value = provider.GetSetting(key);
                 if (value != null)
                 {
                     return value;

AppConfigFacility/AppSettingsProvider.cs

@@ -7,12 +7,8 @@
     /// </summary>
     public class AppSettingsProvider : SettingsProviderBase
     {
-        /// <summary>
-        /// Gets the setting as a string from some config source.
-        /// </summary>
-        /// <param name="key">The key of the setting.</param>
-        /// <returns>The string value of the setting.</returns>
-        protected override string GetSetting(string key)
+        /// <inheritdoc />
+        public override string GetSetting(string key)
         {
             return ConfigurationManager.AppSettings[key];
         }

AppConfigFacility/EnvironmentSettingsProvider.cs

@@ -8,7 +8,8 @@ namespace AppConfigFacility
     /// </summary>
     public class EnvironmentSettingsProvider : SettingsProviderBase
     {
-        protected override string GetSetting(string key)
+        /// <inheritdoc />
+        public override string GetSetting(string key)
         {
             return Environment.GetEnvironmentVariable(key);
         }

AppConfigFacility/ISettingsProvider.cs

@@ -16,5 +16,12 @@ public interface ISettingsProvider
         /// The setting.
         /// </returns>
         object GetSetting(string key, Type returnType);
+
+        /// <summary>
+        /// Gets the setting as a string from some config source.
+        /// </summary>
+        /// <param name="key">The key of the setting.</param>
+        /// <returns>The string value of the setting.</returns>
+        string GetSetting(string key);
     }
 }

AppConfigFacility/SettingsProviderBase.cs

@@ -8,35 +8,42 @@ namespace AppConfigFacility
     public abstract class SettingsProviderBase : ISettingsProvider
     {
         /// <summary>
-        /// Gets the specified setting.
+        /// Gets the specified setting and converts it to the specified type.
         /// </summary>
         /// <param name="key">The setting key.</param>
         /// <param name="returnType">The type of the setting.</param>
         /// <returns>
         /// The setting.
         /// </returns>
-        public object GetSetting(string key, Type returnType)
+        public virtual object GetSetting(string key, Type returnType)
         {
             var value = GetSetting(key);
 
+            return ConvertSetting(value, returnType);
+        }
+
+        /// <inheritdoc />
+        public abstract string GetSetting(string key);
+
+        /// <summary>
+        /// Converts the specified value into the specified type.
+        /// </summary>
+        /// <param name="value">The string representation.</param>
+        /// <param name="returnType">The return type.</param>
+        /// <returns>The value converted to the specified type.</returns>
+        protected object ConvertSetting(string value, Type returnType)
+        {
             if (returnType.IsEnum)
             {
                 return Enum.Parse(returnType, value);
             }
-            
+
             if (returnType == typeof(Uri))
             {
                 return new Uri(value);
             }
 
             return Convert.ChangeType(value, returnType);
         }
-
-        /// <summary>
-        /// Gets the setting as a string from some config source.
-        /// </summary>
-        /// <param name="key">The key of the setting.</param>
-        /// <returns>The string value of the setting.</returns>
-        protected abstract string GetSetting(string key);
     }
 }