Right-size Lists when created (#23714)

Create new instances of List<T> with an appropriate capacity for the items that will be added.
Use Array.Empty<T>() where appropriate, rather than create an empty list and then return it.

Author: martincostello

src/FileProviders/Embedded/src/Manifest/ManifestDirectory.cs

@@ -98,7 +98,7 @@ internal static void ValidateChildrenAndSetParent(ManifestEntry[] children, Mani
 
         private ManifestEntry[] CopyChildren()
         {
-            var list = new List<ManifestEntry>();
+            var list = new List<ManifestEntry>(Children.Count);
             for (int i = 0; i < Children.Count; i++)
             {
                 var child = Children[i];

src/Http/Http/src/Features/RequestCookiesFeature.cs

@@ -80,7 +80,7 @@ public IRequestCookieCollection Cookies
                     }
                     else
                     {
-                        var headers = new List<string>();
+                        var headers = new List<string>(_parsedValues.Count);
                         foreach (var pair in _parsedValues)
                         {
                             headers.Add(new CookieHeaderValue(pair.Key, pair.Value).ToString());

src/Http/Routing/src/Patterns/RoutePatternFactory.cs

@@ -497,7 +497,7 @@ RoutePatternPart VisitPart(RoutePatternPart part)
                         updatedParameterPolicies = new Dictionary<string, List<RoutePatternParameterPolicyReference>>(StringComparer.OrdinalIgnoreCase);
                     }
 
-                    parameterConstraints = new List<RoutePatternParameterPolicyReference>();
+                    parameterConstraints = new List<RoutePatternParameterPolicyReference>(parameter.ParameterPolicies.Count);
                     updatedParameterPolicies.Add(parameter.Name, parameterConstraints);
                 }
 

src/Identity/Core/src/SignInManager.cs

@@ -169,16 +169,22 @@ public virtual async Task<bool> CanSignInAsync(TUser user)
         public virtual async Task RefreshSignInAsync(TUser user)
         {
             var auth = await Context.AuthenticateAsync(IdentityConstants.ApplicationScheme);
-            var claims = new List<Claim>();
+            IList<Claim> claims = Array.Empty<Claim>();
+
             var authenticationMethod = auth?.Principal?.FindFirst(ClaimTypes.AuthenticationMethod);
-            if (authenticationMethod != null)
-            {
-                claims.Add(authenticationMethod);
-            }
             var amr = auth?.Principal?.FindFirst("amr");
-            if (amr != null)
+
+            if (authenticationMethod != null || amr != null)
             {
-                claims.Add(amr);
+                claims = new List<Claim>();
+                if (authenticationMethod != null)
+                {
+                    claims.Add(authenticationMethod);
+                }
+                if (amr != null)
+                {
+                    claims.Add(amr);
+                }
             }
 
             await SignInWithClaimsAsync(user, auth?.Properties, claims);
@@ -203,9 +209,10 @@ public virtual Task SignInAsync(TUser user, bool isPersistent, string authentica
         /// <returns>The task object representing the asynchronous operation.</returns>
         public virtual Task SignInAsync(TUser user, AuthenticationProperties authenticationProperties, string authenticationMethod = null)
         {
-            var additionalClaims = new List<Claim>();
+            IList<Claim> additionalClaims = Array.Empty<Claim>();
             if (authenticationMethod != null)
             {
+                additionalClaims = new List<Claim>();
                 additionalClaims.Add(new Claim(ClaimTypes.AuthenticationMethod, authenticationMethod));
             }
             return SignInWithClaimsAsync(user, authenticationProperties, additionalClaims);

src/Middleware/Localization/src/RequestLocalizationOptions.cs

@@ -122,7 +122,7 @@ public RequestCulture DefaultRequestCulture
         /// <returns>The <see cref="RequestLocalizationOptions"/>.</returns>
         public RequestLocalizationOptions AddSupportedCultures(params string[] cultures)
         {
-            var supportedCultures = new List<CultureInfo>();
+            var supportedCultures = new List<CultureInfo>(cultures.Length);
 
             foreach (var culture in cultures)
             {
@@ -140,7 +140,7 @@ public RequestLocalizationOptions AddSupportedCultures(params string[] cultures)
         /// <returns>The <see cref="RequestLocalizationOptions"/>.</returns>
         public RequestLocalizationOptions AddSupportedUICultures(params string[] uiCultures)
         {
-            var supportedUICultures = new List<CultureInfo>();
+            var supportedUICultures = new List<CultureInfo>(uiCultures.Length);
             foreach (var culture in uiCultures)
             {
                 supportedUICultures.Add(new CultureInfo(culture));

src/Mvc/Mvc.Core/src/ApplicationModels/ApplicationModelFactory.cs

@@ -309,7 +309,7 @@ private static string CreateMixedRoutedActionDescriptorsErrorMessage(
             // Text to show as the attribute route template for conventionally routed actions.
             var nullTemplate = Resources.AttributeRoute_NullTemplateRepresentation;
 
-            var actionDescriptions = new List<string>();
+            var actionDescriptions = new List<string>(actions.Count);
             for (var i = 0; i < actions.Count; i++)
             {
                 var (action, selector) = actions[i];

src/Mvc/Mvc.Core/src/ApplicationModels/ControllerActionDescriptorBuilder.cs

@@ -63,7 +63,7 @@ private static void AddControllerPropertyDescriptors(ActionDescriptor actionDesc
 
         private static void AddParameterDescriptors(ActionDescriptor actionDescriptor, ActionModel action)
         {
-            var parameterDescriptors = new List<ParameterDescriptor>();
+            var parameterDescriptors = new List<ParameterDescriptor>(action.Parameters.Count);
             foreach (var parameter in action.Parameters)
             {
                 var parameterDescriptor = CreateParameterDescriptor(parameter);

src/Mvc/Mvc.Core/src/ApplicationModels/DefaultApplicationModelProvider.cs

@@ -352,7 +352,7 @@ internal ActionModel CreateActionModel(
 
             // This is fairly complicated so that we maintain referential equality between items in
             // ActionModel.Attributes and ActionModel.Attributes[*].Attribute.
-            var applicableAttributes = new List<object>();
+            var applicableAttributes = new List<object>(routeAttributes.Length);
             foreach (var attribute in attributes)
             {
                 if (attribute is IRouteTemplateProvider)

src/Mvc/Mvc.Core/src/BindAttribute.cs

@@ -25,7 +25,7 @@ public class BindAttribute : Attribute, IModelNameProvider, IPropertyFilterProvi
         /// <param name="include">Names of parameters to include in binding.</param>
         public BindAttribute(params string[] include)
         {
-            var items = new List<string>();
+            var items = new List<string>(include.Length);
             foreach (var item in include)
             {
                 items.AddRange(SplitString(item));

src/Mvc/Mvc.Core/src/ConsumesAttribute.cs

@@ -199,7 +199,7 @@ private bool IsApplicable(ActionDescriptor actionDescriptor)
 
         private MediaTypeCollection GetContentTypes(string firstArg, string[] args)
         {
-            var completeArgs = new List<string>();
+            var completeArgs = new List<string>(args.Length + 1);
             completeArgs.Add(firstArg);
             completeArgs.AddRange(args);
             var contentTypes = new MediaTypeCollection();