Fix reload behaviors + Add OnLoadError event for file based config providers

src/Microsoft.Extensions.Configuration.FileExtensions/FileConfigurationExtensions.cs

@@ -12,6 +12,7 @@ namespace Microsoft.Extensions.Configuration
     public static class FileConfigurationExtensions
     {
         private static string FileProviderKey = "FileProvider";
+        private static string FileLoadExceptionHandlerKey = "FileLoadExceptionHandler";
 
         /// <summary>
         /// Sets the default <see cref="IFileProvider"/> to be used for file-based providers.
@@ -54,8 +55,8 @@ public static IFileProvider GetFileProvider(this IConfigurationBuilder builder)
             }
 
 #if NET451
-            var stringBasePath = AppDomain.CurrentDomain.GetData("APP_CONTEXT_BASE_DIRECTORY") as string 
-                ?? AppDomain.CurrentDomain.BaseDirectory 
+            var stringBasePath = AppDomain.CurrentDomain.GetData("APP_CONTEXT_BASE_DIRECTORY") as string
+                ?? AppDomain.CurrentDomain.BaseDirectory
                 ?? string.Empty;
             return new PhysicalFileProvider(stringBasePath);
 #else
@@ -83,5 +84,42 @@ public static IConfigurationBuilder SetBasePath(this IConfigurationBuilder build
 
             return builder.SetFileProvider(new PhysicalFileProvider(basePath));
         }
+
+        /// <summary>
+        /// Sets a default action to be invoked for file-based providers when an error occurs.
+        /// </summary>
+        /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param>
+        /// <param name="handler">The Action to be invoked on a file load exception.</param>
+        /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>
+        public static IConfigurationBuilder SetFileLoadExceptionHandler(this IConfigurationBuilder builder, Action<FileLoadExceptionContext> handler)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            builder.Properties[FileLoadExceptionHandlerKey] = handler;
+            return builder;
+        }
+
+        /// <summary>
+        /// Gets the default <see cref="IFileProvider"/> to be used for file-based providers.
+        /// </summary>
+        /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param>
+        /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>
+        public static Action<FileLoadExceptionContext> GetFileLoadExceptionHandler(this IConfigurationBuilder builder)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            object handler;
+            if (builder.Properties.TryGetValue(FileLoadExceptionHandlerKey, out handler))
+            {
+                return builder.Properties[FileLoadExceptionHandlerKey] as Action<FileLoadExceptionContext>;
+            }
+            return null;
+        }
     }
-}
+}

src/Microsoft.Extensions.Configuration.FileExtensions/FileConfigurationProvider.cs

@@ -5,7 +5,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Text;
-using Microsoft.Extensions.FileProviders;
+using System.Threading;
 using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.Extensions.Configuration
@@ -31,7 +31,10 @@ public FileConfigurationProvider(FileConfigurationSource source)
             {
                 ChangeToken.OnChange(
                     () => Source.FileProvider.Watch(Source.Path),
-                    () => Load(reload: true));
+                    () => {
+                        Thread.Sleep(Source.ReloadDelay);
+                        Load(reload: true);
+                    });
             }
         }
 
@@ -61,9 +64,35 @@ private void Load(bool reload)
             }
             else
             {
+                // Always create new Data on reload to drop old keys
+                if (reload)
+                {
+                    Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                }
                 using (var stream = file.CreateReadStream())
                 {
-                    Load(stream);
+                    try
+                    {
+                        Load(stream);
+                    }
+                    catch (Exception e)
+                    {
+                        bool ignoreException = false;
+                        if (Source.OnLoadException != null)
+                        {
+                            var exceptionContext = new FileLoadExceptionContext
+                            {
+                                Provider = this,
+                                Exception = e
+                            };
+                            Source.OnLoadException.Invoke(exceptionContext);
+                            ignoreException = exceptionContext.Ignore;
+                        }
+                        if (!ignoreException)
+                        {
+                            throw e;
+                        }
+                    }
                 }
             }
             // REVIEW: Should we raise this in the base as well / instead?

src/Microsoft.Extensions.Configuration.FileExtensions/FileConfigurationSource.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.IO;
 using Microsoft.Extensions.FileProviders;
 
@@ -31,13 +32,34 @@ public abstract class FileConfigurationSource : IConfigurationSource
         /// </summary>
         public bool ReloadOnChange { get; set; }
 
+        /// <summary>
+        /// Number of milliseconds that reload will wait before calling Load.  This helps
+        /// avoid triggering reload before a file is completely written. Default is 250.
+        /// </summary>
+        public int ReloadDelay { get; set; } = 250;
+
+        /// <summary>
+        /// Will be called if an uncaught exception occurs in FileConfigurationProvider.Load.
+        /// </summary>
+        public Action<FileLoadExceptionContext> OnLoadException { get; set; }
+
         /// <summary>
         /// Builds the <see cref="IConfigurationProvider"/> for this source.
         /// </summary>
         /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param>
         /// <returns>A <see cref="IConfigurationProvider"/></returns>
         public abstract IConfigurationProvider Build(IConfigurationBuilder builder);
 
+        /// <summary>
+        /// Called to use any default settings on the builder like the FileProvider or FileLoadExceptionHandler.
+        /// </summary>
+        /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param>
+        public void EnsureDefaults(IConfigurationBuilder builder)
+        {
+            FileProvider = FileProvider ?? builder.GetFileProvider();
+            OnLoadException = OnLoadException ?? builder.GetFileLoadExceptionHandler();
+        }
+
         /// <summary>
         /// If no file provider has been set, for absolute Path, this will creates a physical file provider 
         /// for the nearest existing directory.

src/Microsoft.Extensions.Configuration.FileExtensions/FileLoadExceptionContext.cs

@@ -0,0 +1,28 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+
+namespace Microsoft.Extensions.Configuration
+{
+    /// <summary>
+    /// Contains information about a file load exception.
+    /// </summary>
+    public class FileLoadExceptionContext
+    {
+        /// <summary>
+        /// The <see cref="FileConfigurationProvider"/> that caused the exception.
+        /// </summary>
+        public FileConfigurationProvider Provider { get; set; }
+
+        /// <summary>
+        /// The exception that occured in Load.
+        /// </summary>
+        public Exception Exception { get; set; }
+
+        /// <summary>
+        /// If true, the exception will not be rethrown.
+        /// </summary>
+        public bool Ignore { get; set; }
+    }
+}

src/Microsoft.Extensions.Configuration.FileExtensions/project.json

@@ -29,7 +29,8 @@
     },
     "netstandard1.3": {
       "dependencies": {
-        "System.AppContext": "4.1.0-*"
+        "System.AppContext": "4.1.0-*",
+        "System.Threading.Thread": "4.0.0-*"
       }
     }
   }

src/Microsoft.Extensions.Configuration.Ini/IniConfigurationSource.cs

@@ -24,7 +24,7 @@ public class IniConfigurationSource : FileConfigurationSource
         /// <returns>An <see cref="IniConfigurationProvider"/></returns>
         public override IConfigurationProvider Build(IConfigurationBuilder builder)
         {
-            FileProvider = FileProvider ?? builder.GetFileProvider();
+            EnsureDefaults(builder);
             return new IniConfigurationProvider(this);
         }
     }

src/Microsoft.Extensions.Configuration.Json/JsonConfigurationSource.cs

@@ -17,7 +17,7 @@ public class JsonConfigurationSource : FileConfigurationSource
         /// <returns>A <see cref="JsonConfigurationProvider"/></returns>
         public override IConfigurationProvider Build(IConfigurationBuilder builder)
         {
-            FileProvider = FileProvider ?? builder.GetFileProvider();
+            EnsureDefaults(builder);
             return new JsonConfigurationProvider(this);
         }
     }

src/Microsoft.Extensions.Configuration.Xml/XmlConfigurationProvider.cs

@@ -33,12 +33,12 @@ public override void Load(Stream stream)
             var data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
             var readerSettings = new XmlReaderSettings()
-                {
-                    CloseInput = false, // caller will close the stream
-                    DtdProcessing = DtdProcessing.Prohibit,
-                    IgnoreComments = true,
-                    IgnoreWhitespace = true
-                };
+            {
+                CloseInput = false, // caller will close the stream
+                DtdProcessing = DtdProcessing.Prohibit,
+                IgnoreComments = true,
+                IgnoreWhitespace = true
+            };
 
             using (var reader = Decryptor.CreateDecryptingXmlReader(stream, readerSettings))
             {

src/Microsoft.Extensions.Configuration.Xml/XmlConfigurationSource.cs

@@ -15,7 +15,7 @@ public class XmlConfigurationSource : FileConfigurationSource
         /// <returns>A <see cref="XmlConfigurationProvider"/></returns>
         public override IConfigurationProvider Build(IConfigurationBuilder builder)
         {
-            FileProvider = FileProvider ?? builder.GetFileProvider();
+            EnsureDefaults(builder);
             return new XmlConfigurationProvider(this);
         }
     }