Replace generic dictionary with specialized collection type.

src/ImageSharp.Web/Commands/CommandCollection.cs

@@ -0,0 +1,89 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+
+namespace SixLabors.ImageSharp.Web.Commands
+{
+    /// <summary>
+    /// Represents an ordered collection of processing commands.
+    /// </summary>
+    public sealed class CommandCollection : KeyedCollection<string, KeyValuePair<string, string>>
+    {
+        private readonly List<string> keys = new();
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CommandCollection"/> class.
+        /// </summary>
+        public CommandCollection()
+            : this(StringComparer.OrdinalIgnoreCase)
+        {
+        }
+
+        private CommandCollection(IEqualityComparer<string> comparer)
+            : base(comparer)
+        {
+        }
+
+        /// <summary>
+        /// Gets an <see cref="ICollection{T}"/> representing the keys of the collection.
+        /// </summary>
+        public ICollection<string> Keys => this.keys;
+
+        /// <summary>
+        /// Gets the command value with the specified key.
+        /// </summary>
+        /// <param name="key">The key of the element to get.</param>
+        /// <returns>
+        /// The command value with the specified key. If a value with the specified key is not
+        /// found, an exception is thrown.
+        /// </returns>
+        /// <exception cref="ArgumentNullException"><paramref name="key"/> is null.</exception>
+        /// <exception cref="KeyNotFoundException">An element with the specified key does not exist in the collection.</exception>
+        public new string this[string key]
+        {
+            get
+            {
+                if (this.TryGetValue(key, out KeyValuePair<string, string> item))
+                {
+                    return item.Key;
+                }
+
+                throw new KeyNotFoundException();
+            }
+        }
+
+        /// <inheritdoc/>
+        protected override void InsertItem(int index, KeyValuePair<string, string> item)
+        {
+            base.InsertItem(index, item);
+            this.keys.Insert(index, item.Key);
+        }
+
+        /// <inheritdoc/>
+        protected override void RemoveItem(int index)
+        {
+            base.RemoveItem(index);
+            this.keys.RemoveAt(index);
+        }
+
+        /// <inheritdoc/>
+        protected override void SetItem(int index, KeyValuePair<string, string> item)
+        {
+            base.SetItem(index, item);
+            this.keys[index] = item.Key;
+        }
+
+        /// <inheritdoc/>
+        protected override void ClearItems()
+        {
+            base.ClearItems();
+            this.keys.Clear();
+        }
+
+        /// <inheritdoc/>
+        protected override string GetKeyForItem(KeyValuePair<string, string> item) => item.Key;
+    }
+}

src/ImageSharp.Web/Commands/CommandCollectionExtensions.cs

@@ -0,0 +1,25 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Collections.Generic;
+
+namespace SixLabors.ImageSharp.Web.Commands
+{
+    /// <summary>
+    /// Extension methods for <see cref="CommandCollectionExtensions"/>.
+    /// </summary>
+    public static class CommandCollectionExtensions
+    {
+        /// <summary>
+        /// Gets the value associated with the specified key or the default value.
+        /// </summary>
+        /// <param name="collection">The collection instance.</param>
+        /// <param name="key">The key of the value to get.</param>
+        /// <returns>The value associated with the specified key or the default value.</returns>
+        public static string GetValueOrDefault(this CommandCollection collection, string key)
+        {
+            collection.TryGetValue(key, out KeyValuePair<string, string> result);
+            return result.Value;
+        }
+    }
+}

src/ImageSharp.Web/Commands/IRequestParser.cs

@@ -16,6 +16,6 @@ public interface IRequestParser
         /// </summary>
         /// <param name="context">Encapsulates all HTTP-specific information about an individual HTTP request.</param>
         /// <returns>The <see cref="IDictionary{TKey,TValue}"/>.</returns>
-        IDictionary<string, string> ParseRequestCommands(HttpContext context);
+        CommandCollection ParseRequestCommands(HttpContext context);
     }
 }

src/ImageSharp.Web/Commands/PresetOnlyQueryCollectionRequestParser.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -16,7 +16,7 @@ namespace SixLabors.ImageSharp.Web.Commands
     /// </summary>
     public class PresetOnlyQueryCollectionRequestParser : IRequestParser
     {
-        private readonly IDictionary<string, IDictionary<string, string>> presets;
+        private readonly IDictionary<string, CommandCollection> presets;
 
         /// <summary>
         /// The command constant for the preset query parameter.
@@ -31,31 +31,33 @@ public PresetOnlyQueryCollectionRequestParser(IOptions<PresetOnlyQueryCollection
             this.presets = ParsePresets(presetOptions.Value.Presets);
 
         /// <inheritdoc/>
-        public IDictionary<string, string> ParseRequestCommands(HttpContext context)
+        public CommandCollection ParseRequestCommands(HttpContext context)
         {
             if (context.Request.Query.Count == 0 || !context.Request.Query.ContainsKey(QueryKey))
             {
-                return new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                return new();
             }
 
-            var requestedPreset = context.Request.Query["preset"][0];
-            return this.presets.GetValueOrDefault(requestedPreset) ?? new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+            string requestedPreset = context.Request.Query["preset"][0];
+            return this.presets.GetValueOrDefault(requestedPreset) ?? new();
         }
 
-        private static IDictionary<string, IDictionary<string, string>> ParsePresets(
+        private static IDictionary<string, CommandCollection> ParsePresets(
             IDictionary<string, string> unparsedPresets) =>
             unparsedPresets
                 .Select(keyValue =>
-                    new KeyValuePair<string, IDictionary<string, string>>(keyValue.Key, ParsePreset(keyValue.Value)))
+                    new KeyValuePair<string, CommandCollection>(keyValue.Key, ParsePreset(keyValue.Value)))
                 .ToDictionary(keyValue => keyValue.Key, keyValue => keyValue.Value, StringComparer.OrdinalIgnoreCase);
 
-        private static IDictionary<string, string> ParsePreset(string unparsedPresetValue)
+        private static CommandCollection ParsePreset(string unparsedPresetValue)
         {
+            // TODO: Investigate skipping the double allocation here.
+            // In .NET 6 we can directly use the QueryStringEnumerable type and enumerate stright to our command collection
             Dictionary<string, StringValues> parsed = QueryHelpers.ParseQuery(unparsedPresetValue);
-            var transformed = new Dictionary<string, string>(parsed.Count, StringComparer.OrdinalIgnoreCase);
-            foreach (KeyValuePair<string, StringValues> keyValue in parsed)
+            CommandCollection transformed = new();
+            foreach (KeyValuePair<string, StringValues> pair in parsed)
             {
-                transformed[keyValue.Key] = keyValue.Value.ToString();
+                transformed.Add(new(pair.Key, pair.Value.ToString()));
             }
 
             return transformed;

src/ImageSharp.Web/Commands/QueryCollectionRequestParser.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using System.Collections.Generic;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.WebUtilities;
@@ -15,18 +14,20 @@ namespace SixLabors.ImageSharp.Web.Commands
     public sealed class QueryCollectionRequestParser : IRequestParser
     {
         /// <inheritdoc/>
-        public IDictionary<string, string> ParseRequestCommands(HttpContext context)
+        public CommandCollection ParseRequestCommands(HttpContext context)
         {
             if (context.Request.Query.Count == 0)
             {
-                return new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                return new();
             }
 
+            // TODO: Investigate skipping the double allocation here.
+            // In .NET 6 we can directly use the QueryStringEnumerable type and enumerate stright to our command collection
             Dictionary<string, StringValues> parsed = QueryHelpers.ParseQuery(context.Request.QueryString.ToUriComponent());
-            var transformed = new Dictionary<string, string>(parsed.Count, StringComparer.OrdinalIgnoreCase);
+            CommandCollection transformed = new();
             foreach (KeyValuePair<string, StringValues> pair in parsed)
             {
-                transformed[pair.Key] = pair.Value.ToString();
+                transformed.Add(new(pair.Key, pair.Value.ToString()));
             }
 
             return transformed;

src/ImageSharp.Web/Middleware/ImageCommandContext.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
-using System.Collections.Generic;
 using System.Globalization;
 using Microsoft.AspNetCore.Http;
 using SixLabors.ImageSharp.Web.Commands;
@@ -22,7 +21,7 @@ public class ImageCommandContext
         /// <param name="culture">The culture used to parse commands.</param>
         public ImageCommandContext(
             HttpContext context,
-            IDictionary<string, string> commands,
+            CommandCollection commands,
             CommandParser parser,
             CultureInfo culture)
         {
@@ -38,9 +37,9 @@ public ImageCommandContext(
         public HttpContext Context { get; }
 
         /// <summary>
-        /// Gets the dictionary containing the collection of URI derived processing commands.
+        /// Gets the collection of URI derived processing commands.
         /// </summary>
-        public IDictionary<string, string> Commands { get; }
+        public CommandCollection Commands { get; }
 
         /// <summary>
         /// Gets the command parser for parsing URI derived processing commands.

src/ImageSharp.Web/Middleware/ImageProcessingContext.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.IO;
 using Microsoft.AspNetCore.Http;
+using SixLabors.ImageSharp.Web.Commands;
 
 namespace SixLabors.ImageSharp.Web.Middleware
 {
@@ -23,7 +24,7 @@ public class ImageProcessingContext
         public ImageProcessingContext(
             HttpContext context,
             Stream stream,
-            IDictionary<string, string> commands,
+            CommandCollection commands,
             string contentType,
             string extension)
         {
@@ -47,10 +48,10 @@ public ImageProcessingContext(
         /// <summary>
         /// Gets the parsed collection of processing commands.
         /// </summary>
-        public IDictionary<string, string> Commands { get; }
+        public CommandCollection Commands { get; }
 
         /// <summary>
-        /// Gets the content type for for the processed image.
+        /// Gets the content type for the processed image.
         /// </summary>
         public string ContentType { get; }
 

src/ImageSharp.Web/Middleware/ImageSharpMiddleware.cs

@@ -172,18 +172,15 @@ public ImageSharpMiddleware(
             this.asyncKeyLock = asyncKeyLock;
         }
 
-#pragma warning disable IDE1006 // Naming Styles
         /// <summary>
         /// Performs operations upon the current request.
         /// </summary>
         /// <param name="context">The current HTTP request context.</param>
         /// <returns>The <see cref="Task"/>.</returns>
         public async Task Invoke(HttpContext context)
-#pragma warning restore IDE1006 // Naming Styles
         {
             // We expect to get concrete collection type which removes virtual dispatch concerns and enumerator allocations
-            IDictionary<string, string> parsedCommands = this.requestParser.ParseRequestCommands(context);
-            Dictionary<string, string> commands = parsedCommands as Dictionary<string, string> ?? new Dictionary<string, string>(parsedCommands, StringComparer.OrdinalIgnoreCase);
+            CommandCollection commands = this.requestParser.ParseRequestCommands(context);
 
             if (commands.Count > 0)
             {
@@ -243,7 +240,7 @@ await this.ProcessRequestAsync(
                 commands);
         }
 
-        private void StripUnknownCommands(Dictionary<string, string> commands, int startAtIndex)
+        private void StripUnknownCommands(CommandCollection commands, int startAtIndex)
         {
             var keys = new List<string>(commands.Keys);
             for (int index = startAtIndex; index < keys.Count; index++)
@@ -260,7 +257,7 @@ private async Task ProcessRequestAsync(
             HttpContext context,
             IImageResolver sourceImageResolver,
             ImageContext imageContext,
-            IDictionary<string, string> commands)
+            CommandCollection commands)
         {
             // Create a cache key based on all the components of the requested url
             string uri = GetUri(context, commands);
@@ -511,7 +508,7 @@ private async Task SendResponseAsync(
             }
         }
 
-        private static string GetUri(HttpContext context, IDictionary<string, string> commands)
+        private static string GetUri(HttpContext context, CommandCollection commands)
         {
             var sb = new StringBuilder(context.Request.Host.ToString());
 

src/ImageSharp.Web/Processors/BackgroundColorWebProcessor.cs

@@ -32,7 +32,7 @@ private static readonly IEnumerable<string> ColorCommands
         public FormattedImage Process(
             FormattedImage image,
             ILogger logger,
-            IDictionary<string, string> commands,
+            CommandCollection commands,
             CommandParser parser,
             CultureInfo culture)
         {

src/ImageSharp.Web/Processors/FormatWebProcessor.cs

@@ -46,16 +46,15 @@ public FormatWebProcessor(IOptions<ImageSharpMiddlewareOptions> options)
         public FormattedImage Process(
             FormattedImage image,
             ILogger logger,
-            IDictionary<string, string> commands,
+            CommandCollection commands,
             CommandParser parser,
             CultureInfo culture)
         {
             string extension = commands.GetValueOrDefault(Format);
 
             if (!string.IsNullOrWhiteSpace(extension))
             {
-                IImageFormat format = this.options.Configuration
-                    .ImageFormatsManager.FindFormatByFileExtension(extension);
+                IImageFormat format = this.options.Configuration.ImageFormatsManager.FindFormatByFileExtension(extension);
 
                 if (format != null)
                 {

src/ImageSharp.Web/Processors/IImageWebProcessor.cs

@@ -14,16 +14,16 @@ namespace SixLabors.ImageSharp.Web.Processors
     public interface IImageWebProcessor
     {
         /// <summary>
-        /// Gets the collection of recognized querystring commands.
+        /// Gets the collection of recognized command keys.
         /// </summary>
         IEnumerable<string> Commands { get; }
 
         /// <summary>
-        /// Processes the image based on the querystring commands.
+        /// Processes the image based on the given commands.
         /// </summary>
         /// <param name="image">The image to process.</param>
         /// <param name="logger">The type used for performing logging.</param>
-        /// <param name="commands">The querystring collection containing the processing commands.</param>
+        /// <param name="commands">The ordered collection containing the processing commands.</param>
         /// <param name="parser">The command parser use for parting commands.</param>
         /// <param name="culture">
         /// The <see cref="CultureInfo"/> to use as the current parsing culture.
@@ -32,7 +32,7 @@ public interface IImageWebProcessor
         FormattedImage Process(
             FormattedImage image,
             ILogger logger,
-            IDictionary<string, string> commands,
+            CommandCollection commands,
             CommandParser parser,
             CultureInfo culture);
     }

src/ImageSharp.Web/Processors/QualityWebProcessor.cs

@@ -34,11 +34,11 @@ private static readonly IEnumerable<string> QualityCommands
         public FormattedImage Process(
             FormattedImage image,
             ILogger logger,
-            IDictionary<string, string> commands,
+            CommandCollection commands,
             CommandParser parser,
             CultureInfo culture)
         {
-            if (commands.ContainsKey(Quality))
+            if (commands.Contains(Quality))
             {
                 // The encoders clamp any values so no validation is required.
                 int quality = parser.ParseValue<int>(commands.GetValueOrDefault(Quality), culture);