Reduce allocations during HTML generation

- aspnet#3918
- precompute size of `StringBuilder` in `ExpressionHelper`
- reduce `string` allocations in `ViewDataEvaluator`
 - also get rid of `Enumeration` state machines
- reduce the size of a few objects; use more expression-valued properties
 - e.g. don't store `_modelType` in `ModelExplorer`
- add `EmptyArray<TElement>`; make empty arrays consistently `static`
- avoid `string.Split()` in HTML and tag helpers

nits:
- make `ExpressionHelperTest` tests more stringent
- correct `Message` for an `ArgumentNullException`
- remove excess `using`s in classes I touched (but often ended up leaving otherwise unchanged)
- improve doc comments
- remove `ToString()` call on a `string`
- avoid encoding `string.Empty`
- fix test file name
- remove useless variables
- correct spelling
- improve whitespace

src/Microsoft.AspNetCore.Mvc.Abstractions/ModelBinding/ModelStateDictionary.cs

@@ -169,7 +169,7 @@ public ModelStateEntry this[string key]
             }
         }
 
-        // Flag that indiciates if TooManyModelErrorException has already been added to this dictionary.
+        // Flag that indicates if TooManyModelErrorException has already been added to this dictionary.
         private bool HasRecordedMaxModelError { get; set; }
 
         /// <summary>
@@ -625,7 +625,7 @@ private void AddModelErrorCore(string key, Exception exception)
         }
 
         /// <summary>
-        /// Removes all keys and values from ths instance of <see cref="ModelStateDictionary"/>.
+        /// Removes all keys and values from this instance of <see cref="ModelStateDictionary"/>.
         /// </summary>
         public void Clear()
         {

src/Microsoft.AspNetCore.Mvc.Abstractions/ModelBinding/ModelStateEntry.cs

@@ -11,6 +11,7 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding
     public abstract class ModelStateEntry
     {
         private ModelErrorCollection _errors;
+
         /// <summary>
         /// Gets the raw value from the request associated with this entry.
         /// </summary>

src/Microsoft.AspNetCore.Mvc.Core/BindAttribute.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using Microsoft.AspNetCore.Mvc.Internal;
 #if NETSTANDARD1_6
 using System.Reflection;
 #endif
@@ -83,7 +84,7 @@ private static IEnumerable<string> SplitString(string original)
         {
             if (string.IsNullOrEmpty(original))
             {
-                return new string[0];
+                return EmptyArray<string>.Instance;
             }
 
             var split = original.Split(',').Select(piece => piece.Trim()).Where(piece => !string.IsNullOrEmpty(piece));

src/Microsoft.AspNetCore.Mvc.Core/ChallengeResult.cs

@@ -20,7 +20,7 @@ public class ChallengeResult : ActionResult
         /// Initializes a new instance of <see cref="ChallengeResult"/>.
         /// </summary>
         public ChallengeResult()
-            : this(new string[] { })
+            : this(EmptyArray<string>.Instance)
         {
         }
 
@@ -51,7 +51,7 @@ public ChallengeResult(IList<string> authenticationSchemes)
         /// <param name="properties"><see cref="AuthenticationProperties"/> used to perform the authentication
         /// challenge.</param>
         public ChallengeResult(AuthenticationProperties properties)
-            : this(new string[] { }, properties)
+            : this(EmptyArray<string>.Instance, properties)
         {
         }
 

src/Microsoft.AspNetCore.Mvc.Core/ConsumesAttribute.cs

@@ -53,7 +53,7 @@ public ConsumesAttribute(string contentType, params string[] otherContentTypes)
         // The value used is a non default value so that it avoids getting mixed with other action constraints
         // with default order.
         /// <inheritdoc />
-        int IActionConstraint.Order { get; } = ConsumesActionConstraintOrder;
+        int IActionConstraint.Order => ConsumesActionConstraintOrder;
 
         /// <summary>
         /// Gets or sets the supported request content types. Used to select an action when there would otherwise be

src/Microsoft.AspNetCore.Mvc.Core/Filters/MiddlewareFilterAttribute.cs

@@ -35,7 +35,7 @@ public MiddlewareFilterAttribute(Type configurationType)
         public int Order { get; set; }
 
         /// <inheritdoc />
-        public bool IsReusable { get; } = true;
+        public bool IsReusable => true;
 
         /// <inheritdoc />
         public IFilterMetadata CreateInstance(IServiceProvider serviceProvider)

src/Microsoft.AspNetCore.Mvc.Core/ForbidResult.cs

@@ -20,7 +20,7 @@ public class ForbidResult : ActionResult
         /// Initializes a new instance of <see cref="ForbidResult"/>.
         /// </summary>
         public ForbidResult()
-            : this(new string[] { })
+            : this(EmptyArray<string>.Instance)
         {
         }
 
@@ -51,7 +51,7 @@ public ForbidResult(IList<string> authenticationSchemes)
         /// <param name="properties"><see cref="AuthenticationProperties"/> used to perform the authentication
         /// challenge.</param>
         public ForbidResult(AuthenticationProperties properties)
-            : this(new string[] { }, properties)
+            : this(EmptyArray<string>.Instance, properties)
         {
         }
 

src/Microsoft.AspNetCore.Mvc.Core/Formatters/FormatterMappings.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.AspNetCore.Mvc.Core;
-using Microsoft.Extensions.Primitives;
 using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.Mvc.Formatters
@@ -18,7 +17,7 @@ public class FormatterMappings
             new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
         /// <summary>
-        /// Sets mapping for the format to specified media type. 
+        /// Sets mapping for the format to specified media type.
         /// If the format already exists, the media type will be overwritten with the new value.
         /// </summary>
         /// <param name="format">The format value.</param>
@@ -39,7 +38,7 @@ public void SetMediaTypeMappingForFormat(string format, string contentType)
         }
 
         /// <summary>
-        /// Sets mapping for the format to specified media type. 
+        /// Sets mapping for the format to specified media type.
         /// If the format already exists, the media type will be overwritten with the new value.
         /// </summary>
         /// <param name="format">The format value.</param>

src/Microsoft.AspNetCore.Mvc.Core/Internal/ClientValidatorCache.cs

@@ -11,8 +11,6 @@ namespace Microsoft.AspNetCore.Mvc.Internal
 {
     public class ClientValidatorCache
     {
-        private readonly IReadOnlyList<IClientModelValidator> EmptyArray = new IClientModelValidator[0];
-
         private readonly ConcurrentDictionary<ModelMetadata, CacheEntry> _cacheEntries = new ConcurrentDictionary<ModelMetadata, CacheEntry>();
 
         public IReadOnlyList<IClientModelValidator> GetValidators(ModelMetadata metadata, IClientModelValidatorProvider validatorProvider)
@@ -106,7 +104,7 @@ private IReadOnlyList<IClientModelValidator> ExtractValidators(List<ClientValida
 
             if (count == 0)
             {
-                return EmptyArray;
+                return EmptyArray<IClientModelValidator>.Instance;
             }
 
             var validators = new IClientModelValidator[count];

src/Microsoft.AspNetCore.Mvc.Core/Internal/ControllerActionInvokerCache.cs

@@ -3,7 +3,6 @@
 
 using System.Collections.Concurrent;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.Linq;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.Filters;
@@ -13,8 +12,6 @@ namespace Microsoft.AspNetCore.Mvc.Internal
 {
     public class ControllerActionInvokerCache
     {
-        private readonly IFilterMetadata[] EmptyFilterArray = new IFilterMetadata[0];
-
         private readonly IActionDescriptorCollectionProvider _collectionProvider;
         private readonly IFilterProvider[] _filterProviders;
 
@@ -134,7 +131,7 @@ private IFilterMetadata[] GetFilters(ActionContext actionContext, List<FilterIte
 
             if (count == 0)
             {
-                return EmptyFilterArray;
+                return EmptyArray<IFilterMetadata>.Instance;
             }
             else
             {

src/Microsoft.AspNetCore.Mvc.Core/Internal/EmptyArray.cs

@@ -0,0 +1,19 @@
+// 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.
+
+#if !NET451
+using System;
+#endif
+
+namespace Microsoft.AspNetCore.Mvc.Internal
+{
+    public static class EmptyArray<TElement>
+    {
+        public static TElement[] Instance { get; } =
+#if NET451
+            new TElement[0];
+#else
+            Array.Empty<TElement>();
+#endif
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/Internal/FileResultExecutorBase.cs

@@ -41,7 +41,7 @@ private void SetContentDispositionHeader(ActionContext context, FileResult resul
         private void SetContentType(ActionContext context, FileResult result)
         {
             var response = context.HttpContext.Response;
-            response.ContentType = result.ContentType.ToString();
+            response.ContentType = result.ContentType;
         }
 
         protected static ILogger CreateLogger<T>(ILoggerFactory factory)

src/Microsoft.AspNetCore.Mvc.Core/Internal/PrefixContainer.cs

@@ -14,7 +14,6 @@ namespace Microsoft.AspNetCore.Mvc.Internal
     /// </summary>
     public class PrefixContainer
     {
-        private static readonly string[] EmptyArray = new string[0];
         private static readonly char[] Delimiters = new char[] { '[', '.' };
 
         private readonly ICollection<string> _originalValues;
@@ -31,7 +30,7 @@ public PrefixContainer(ICollection<string> values)
 
             if (_originalValues.Count == 0)
             {
-                _sortedValues = EmptyArray;
+                _sortedValues = EmptyArray<string>.Instance;
             }
             else
             {

src/Microsoft.AspNetCore.Mvc.Core/Internal/ValidatorCache.cs

@@ -11,8 +11,6 @@ namespace Microsoft.AspNetCore.Mvc.Internal
 {
     public class ValidatorCache
     {
-        private readonly IReadOnlyList<IModelValidator> EmptyArray = new IModelValidator[0];
-
         private readonly ConcurrentDictionary<ModelMetadata, CacheEntry> _cacheEntries = new ConcurrentDictionary<ModelMetadata, CacheEntry>();
 
         public IReadOnlyList<IModelValidator> GetValidators(ModelMetadata metadata, IModelValidatorProvider validatorProvider)
@@ -105,7 +103,7 @@ private IReadOnlyList<IModelValidator> ExtractValidators(List<ValidatorItem> ite
 
             if (count == 0)
             {
-                return EmptyArray;
+                return EmptyArray<IModelValidator>.Instance;
             }
 
             var validators = new IModelValidator[count];

src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/ArrayModelBinder.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using Microsoft.AspNetCore.Mvc.Internal;
 
 namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
 {
@@ -36,7 +37,7 @@ protected override object CreateEmptyCollection(Type targetType)
         {
             Debug.Assert(targetType == typeof(TElement[]), "GenericModelBinder only creates this binder for arrays.");
 
-            return new TElement[0];
+            return EmptyArray<TElement>.Instance;
         }
 
         /// <inheritdoc />

src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/FormCollectionModelBinder.cs

@@ -7,7 +7,6 @@
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;
 using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
@@ -36,7 +35,7 @@ public async Task BindModelAsync(ModelBindingContext bindingContext)
             {
                 model = new EmptyFormCollection();
             }
-            
+
             bindingContext.Result = ModelBindingResult.Success(model);
         }
 

src/Microsoft.AspNetCore.Mvc.Localization/LocalizedHtmlString.cs

@@ -3,9 +3,9 @@
 
 using System;
 using System.IO;
-using System.Text;
 using System.Text.Encodings.Web;
 using Microsoft.AspNetCore.Html;
+using Microsoft.AspNetCore.Mvc.Internal;
 
 namespace Microsoft.AspNetCore.Mvc.Localization
 {
@@ -14,11 +14,6 @@ namespace Microsoft.AspNetCore.Mvc.Localization
     /// </summary>
     public class LocalizedHtmlString : IHtmlContent
     {
-#if NETSTANDARD1_4
-        private static readonly object[] EmptyArguments = Array.Empty<object>();
-#else
-        private static readonly object[] EmptyArguments = new object[0];
-#endif
         private readonly object[] _arguments;
 
         /// <summary>
@@ -27,7 +22,7 @@ public class LocalizedHtmlString : IHtmlContent
         /// <param name="name">The name of the string resource.</param>
         /// <param name="value">The string resource.</param>
         public LocalizedHtmlString(string name, string value)
-            : this(name, value, isResourceNotFound: false, arguments: EmptyArguments)
+            : this(name, value, isResourceNotFound: false, arguments: EmptyArray<string>.Instance)
         {
         }
 
@@ -38,7 +33,7 @@ public LocalizedHtmlString(string name, string value)
         /// <param name="value">The string resource.</param>
         /// <param name="isResourceNotFound">A flag that indicates if the resource is not found.</param>
         public LocalizedHtmlString(string name, string value, bool isResourceNotFound)
-            : this(name, value, isResourceNotFound, arguments: EmptyArguments)
+            : this(name, value, isResourceNotFound, arguments: EmptyArray<string>.Instance)
         {
         }
 

src/Microsoft.AspNetCore.Mvc.Razor/Internal/CompilerCacheResult.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Linq.Expressions;
 using System.Reflection;
+using Microsoft.AspNetCore.Mvc.Internal;
 using Microsoft.AspNetCore.Mvc.Razor.Compilation;
 using Microsoft.Extensions.Primitives;
 
@@ -22,7 +23,7 @@ public struct CompilerCacheResult
         /// <param name="relativePath">Path of the view file relative to the application base.</param>
         /// <param name="compilationResult">The <see cref="Compilation.CompilationResult"/>.</param>
         public CompilerCacheResult(string relativePath, CompilationResult compilationResult)
-            : this(relativePath, compilationResult, new IChangeToken[0])
+            : this(relativePath, compilationResult, EmptyArray<IChangeToken>.Instance)
         {
         }
 
@@ -86,6 +87,5 @@ public CompilerCacheResult(IList<IChangeToken> expirationTokens)
         /// Gets a delegate that creates an instance of the <see cref="IRazorPage"/>.
         /// </summary>
         public Func<IRazorPage> PageFactory { get; }
-
     }
 }

src/Microsoft.AspNetCore.Mvc.Razor/RazorViewEngine.cs

@@ -7,6 +7,7 @@
 using System.Globalization;
 using System.Linq;
 using System.Text.Encodings.Web;
+using Microsoft.AspNetCore.Mvc.Internal;
 using Microsoft.AspNetCore.Mvc.Razor.Internal;
 using Microsoft.AspNetCore.Mvc.ViewEngines;
 using Microsoft.Extensions.Caching.Memory;
@@ -31,8 +32,6 @@ public class RazorViewEngine : IRazorViewEngine
 
         private const string ControllerKey = "controller";
         private const string AreaKey = "area";
-        private static readonly ViewLocationCacheItem[] EmptyViewStartLocationCacheItems =
-            new ViewLocationCacheItem[0];
         private static readonly TimeSpan _cacheExpirationDuration = TimeSpan.FromMinutes(20);
 
         private readonly IRazorPageFactoryProvider _pageFactory;
@@ -415,7 +414,7 @@ private ViewLocationCacheResult CreateCacheResult(
                 // Only need to lookup _ViewStarts for the main page.
                 var viewStartPages = isMainPage ?
                     GetViewStartPages(relativePath, expirationTokens) :
-                    EmptyViewStartLocationCacheItems;
+                    EmptyArray<ViewLocationCacheItem>.Instance;
 
                 return new ViewLocationCacheResult(
                     new ViewLocationCacheItem(factoryResult.RazorPageFactory, relativePath),

src/Microsoft.AspNetCore.Mvc.TagHelpers/Internal/GlobbingUrlBuilder.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.Internal;
 using Microsoft.Extensions.Caching.Memory;
 using Microsoft.Extensions.FileProviders;
 using Microsoft.Extensions.FileSystemGlobbing;
@@ -18,13 +19,6 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers.Internal
     /// </summary>
     public class GlobbingUrlBuilder
     {
-        private static readonly IReadOnlyList<string> EmptyList =
-#if NET451
-            new string[0];
-#else
-            Array.Empty<string>();
-#endif
-
         // Valid whitespace characters defined by the HTML5 spec.
         private static readonly char[] ValidAttributeWhitespaceChars =
             new[] { '\t', '\n', '\u000C', '\r', ' ' };
@@ -116,7 +110,7 @@ private IReadOnlyList<string> ExpandGlobbedUrl(string include, string exclude)
         {
             if (string.IsNullOrEmpty(include))
             {
-                return EmptyList;
+                return EmptyArray<string>.Instance;
             }
 
             var cacheKey = new GlobbingUrlKey(include, exclude);
@@ -130,7 +124,7 @@ private IReadOnlyList<string> ExpandGlobbedUrl(string include, string exclude)
             var includeEnumerator = includeTokenizer.GetEnumerator();
             if (!includeEnumerator.MoveNext())
             {
-                return EmptyList;
+                return EmptyArray<string>.Instance;
             }
 
             var options = new MemoryCacheEntryOptions();