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.

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

@@ -98,7 +98,7 @@ namespace Microsoft.Extensions.FileProviders.Embedded.Manifest
 
         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 @@ namespace Microsoft.AspNetCore.Http.Features
                     }
                     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 @@ namespace Microsoft.AspNetCore.Routing.Patterns
                         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 @@ namespace Microsoft.AspNetCore.Identity
         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 @@ namespace Microsoft.AspNetCore.Identity
         /// <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 @@ namespace Microsoft.AspNetCore.Builder
         /// <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 @@ namespace Microsoft.AspNetCore.Builder
         /// <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 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels
             // 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 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels
 
         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 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels
 
             // 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 @@ namespace Microsoft.AspNetCore.Mvc
         /// <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 @@ namespace Microsoft.AspNetCore.Mvc
 
         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();

src/Mvc/Mvc.Core/src/Formatters/InputFormatter.cs

@@ -159,7 +159,7 @@ namespace Microsoft.AspNetCore.Mvc.Formatters
                     {
                         if (mediaTypes == null)
                         {
-                            mediaTypes = new List<string>();
+                            mediaTypes = new List<string>(SupportedMediaTypes.Count);
                         }
 
                         mediaTypes.Add(mediaType);

src/Mvc/Mvc.Core/src/Formatters/OutputFormatter.cs

@@ -67,7 +67,7 @@ namespace Microsoft.AspNetCore.Mvc.Formatters
                     {
                         if (mediaTypes == null)
                         {
-                            mediaTypes = new List<string>();
+                            mediaTypes = new List<string>(SupportedMediaTypes.Count);
                         }
 
                         mediaTypes.Add(contentType);
@@ -81,7 +81,7 @@ namespace Microsoft.AspNetCore.Mvc.Formatters
                     {
                         if (mediaTypes == null)
                         {
-                            mediaTypes = new List<string>();
+                            mediaTypes = new List<string>(SupportedMediaTypes.Count);
                         }
 
                         mediaTypes.Add(mediaType);

src/Mvc/Mvc.Core/src/Infrastructure/ActionSelector.cs

@@ -135,10 +135,11 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             RouteContext context,
             IReadOnlyList<ActionDescriptor> actions)
         {
-            var candidates = new List<ActionSelectorCandidate>();
+            var actionsCount = actions.Count;
+            var candidates = new List<ActionSelectorCandidate>(actionsCount);
 
             // Perf: Avoid allocations
-            for (var i = 0; i < actions.Count; i++)
+            for (var i = 0; i < actionsCount; i++)
             {
                 var action = actions[i];
                 var constraints = _actionConstraintCache.GetActionConstraints(context.HttpContext, action);
@@ -150,9 +151,10 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             List<ActionDescriptor> results = null;
             if (matches != null)
             {
-                results = new List<ActionDescriptor>(matches.Count);
+                var matchesCount = matches.Count;
+                results = new List<ActionDescriptor>(matchesCount);
                 // Perf: Avoid allocations
-                for (var i = 0; i < matches.Count; i++)
+                for (var i = 0; i < matchesCount; i++)
                 {
                     var candidate = matches[i];
                     results.Add(candidate.Action);

src/Mvc/Mvc.Core/src/ProducesAttribute.cs

@@ -111,7 +111,7 @@ namespace Microsoft.AspNetCore.Mvc
 
         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();

src/Mvc/Mvc.Razor.RuntimeCompilation/src/RazorReferenceManager.cs

@@ -52,7 +52,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation
         // For unit testing
         internal IEnumerable<string> GetReferencePaths()
         {
-            var referencePaths = new List<string>();
+            var referencePaths = new List<string>(_options.AdditionalReferencePaths.Count);
 
             foreach (var part in _partManager.ApplicationParts)
             {

src/Mvc/Mvc.ViewFeatures/src/Infrastructure/DefaultTempDataSerializer.cs

@@ -83,7 +83,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Infrastructure
 
         private static object DeserializeArray(in JsonElement arrayElement)
         {
-            if (arrayElement.GetArrayLength() == 0)
+            int arrayLength = arrayElement.GetArrayLength();
+            if (arrayLength == 0)
             {
                 // We have to infer the type of the array by inspecting it's elements.
                 // If there's nothing to inspect, return a null value since we do not know
@@ -93,7 +94,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Infrastructure
 
             if (arrayElement[0].ValueKind == JsonValueKind.String)
             {
-                var array = new List<string>();
+                var array = new List<string>(arrayLength);
 
                 foreach (var item in arrayElement.EnumerateArray())
                 {
@@ -104,7 +105,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Infrastructure
             }
             else if (arrayElement[0].ValueKind == JsonValueKind.Number)
             {
-                var array = new List<int>();
+                var array = new List<int>(arrayLength);
 
                 foreach (var item in arrayElement.EnumerateArray())
                 {

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentEventHandlerLoweringPass.cs

@@ -170,7 +170,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Components
             // This method is overloaded on string and T, which means that it will put the code in the
             // correct context for intellisense when typing in the attribute.
             var eventArgsType = node.TagHelper.GetEventArgsType();
-            var tokens = new List<IntermediateToken>()
+            var tokens = new List<IntermediateToken>(original.Count + 2)
             {
                 new IntermediateToken()
                 {

src/Razor/Microsoft.AspNetCore.Razor.Tools/src/Application.cs

@@ -96,7 +96,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
         private static string[] ExpandResponseFiles(string[] args)
         {
-            var expandedArgs = new List<string>();
+            var expandedArgs = new List<string>(args.Length);
             foreach (var arg in args)
             {
                 if (!arg.StartsWith("@", StringComparison.Ordinal))

src/Razor/Microsoft.AspNetCore.Razor.Tools/src/DefaultRequestDispatcher.cs

@@ -143,7 +143,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
         /// </summary>
         private void WaitForAnyCompletion(CancellationToken cancellationToken)
         {
-            var all = new List<Task>();
+            var all = new List<Task>(_connections.Count + 3);
             all.AddRange(_connections);
             all.Add(_timeoutTask);
             all.Add(_listenTask);

src/Razor/Microsoft.AspNetCore.Razor.Tools/src/MetadataReaderExtensions.cs

@@ -33,7 +33,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
         
         internal static AssemblyIdentity[] GetReferencedAssembliesOrThrow(this MetadataReader reader)
         {
-            var references = new List<AssemblyIdentity>();
+            var references = new List<AssemblyIdentity>(reader.AssemblyReferences.Count);
 
             foreach (var referenceHandle in reader.AssemblyReferences)
             {

src/Razor/Microsoft.NET.Sdk.Razor/src/FindAssembliesWithReferencesTo.cs

@@ -23,7 +23,7 @@ namespace Microsoft.AspNetCore.Razor.Tasks
 
         public override bool Execute()
         {
-            var referenceItems = new List<AssemblyItem>();
+            var referenceItems = new List<AssemblyItem>(Assemblies.Length);
             foreach (var item in Assemblies)
             {
                 const string FusionNameKey = "FusionName";

src/Razor/Microsoft.NET.Sdk.Razor/src/ReferenceResolver.cs

@@ -120,7 +120,7 @@ namespace Microsoft.AspNetCore.Razor.Tasks
 
                 var metadataReader = peReader.GetMetadataReader();
 
-                var references = new List<AssemblyItem>();
+                var references = new List<AssemblyItem>(metadataReader.AssemblyReferences.Count);
                 foreach (var handle in metadataReader.AssemblyReferences)
                 {
                     var reference = metadataReader.GetAssemblyReference(handle);

src/Razor/Razor/src/TagHelpers/ReadOnlyTagHelperAttributeList.cs

@@ -12,8 +12,6 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
     /// </summary>
     public abstract class ReadOnlyTagHelperAttributeList : ReadOnlyCollection<TagHelperAttribute>
     {
-        private static readonly IReadOnlyList<TagHelperAttribute> EmptyList = new TagHelperAttribute[0];
-
         /// <summary>
         /// Instantiates a new instance of <see cref="ReadOnlyTagHelperAttributeList"/> with an empty
         /// collection.
@@ -146,7 +144,7 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                     matchedAttributes.Add(attribute);
                 }
             }
-            attributes = matchedAttributes ?? EmptyList;
+            attributes = matchedAttributes ?? (IReadOnlyList<TagHelperAttribute>)Array.Empty<TagHelperAttribute>();
 
             return matchedAttributes != null;
         }
@@ -198,4 +196,4 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
             return string.Equals(name, attribute.Name, StringComparison.OrdinalIgnoreCase);
         }
     }
-}
+}

src/Razor/test/Microsoft.AspNetCore.Razor.Test.MvcShim/Microsoft.AspNetCore.Razor.TagHelpers/ReadOnlyTagHelperAttributeList.cs

@@ -12,8 +12,6 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
     /// </summary>
     public abstract class ReadOnlyTagHelperAttributeList : ReadOnlyCollection<TagHelperAttribute>
     {
-        private static readonly IReadOnlyList<TagHelperAttribute> EmptyList = new TagHelperAttribute[0];
-
         /// <summary>
         /// Instantiates a new instance of <see cref="ReadOnlyTagHelperAttributeList"/> with an empty
         /// collection.
@@ -138,7 +136,7 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                     matchedAttributes.Add(Items[i]);
                 }
             }
-            attributes = matchedAttributes ?? EmptyList;
+            attributes = matchedAttributes ?? Array.Empty<TagHelperAttribute>() as IReadOnlyList<TagHelperAttribute>;
 
             return matchedAttributes != null;
         }

src/Security/Authentication/Certificate/src/CertificateAuthenticationHandler.cs

@@ -131,7 +131,7 @@ namespace Microsoft.AspNetCore.Authentication.Certificate
             var certificateIsValid = chain.Build(clientCertificate);
             if (!certificateIsValid)
             {
-                var chainErrors = new List<string>();
+                var chainErrors = new List<string>(chain.ChainStatus.Length);
                 foreach (var validationFailure in chain.ChainStatus)
                 {
                     chainErrors.Add($"{validationFailure.Status} {validationFailure.StatusInformation}");

src/Security/Authentication/JwtBearer/src/JwtBearerHandler.cs

@@ -274,7 +274,7 @@ namespace Microsoft.AspNetCore.Authentication.JwtBearer
         
         private static string CreateErrorDescription(Exception authFailure)
         {
-            IEnumerable<Exception> exceptions;
+            IReadOnlyCollection<Exception> exceptions;
             if (authFailure is AggregateException agEx)
             {
                 exceptions = agEx.InnerExceptions;
@@ -284,7 +284,7 @@ namespace Microsoft.AspNetCore.Authentication.JwtBearer
                 exceptions = new[] { authFailure };
             }
 
-            var messages = new List<string>();
+            var messages = new List<string>(exceptions.Count);
 
             foreach (var ex in exceptions)
             {

src/Servers/HttpSys/src/AuthenticationManager.cs

@@ -107,13 +107,13 @@ namespace Microsoft.AspNetCore.Server.HttpSys
 
         internal static IList<string> GenerateChallenges(AuthenticationSchemes authSchemes)
         {
-            IList<string> challenges = new List<string>();
-
             if (authSchemes == AuthenticationSchemes.None)
             {
-                return challenges;
+                return Array.Empty<string>();
             }
 
+            IList<string> challenges = new List<string>();
+
             // Order by strength.
             if ((authSchemes & AuthenticationSchemes.Kerberos) == AuthenticationSchemes.Kerberos)
             {

src/Servers/HttpSys/src/HttpSysListener.cs

@@ -343,7 +343,7 @@ namespace Microsoft.AspNetCore.Server.HttpSys
                 // Copied from the multi-value headers section of SerializeHeaders
                 if (authChallenges != null && authChallenges.Count > 0)
                 {
-                    pinnedHeaders = new List<GCHandle>();
+                    pinnedHeaders = new List<GCHandle>(authChallenges.Count + 3);
 
                     HttpApiTypes.HTTP_RESPONSE_INFO[] knownHeaderInfo = null;
                     knownHeaderInfo = new HttpApiTypes.HTTP_RESPONSE_INFO[1];

src/Shared/Buffers.Testing/ReadOnlySequenceFactory.cs

@@ -92,7 +92,7 @@ namespace System.Buffers
 
             public override ReadOnlySequence<byte> CreateWithContent(byte[] data)
             {
-                var segments = new List<byte[]>();
+                var segments = new List<byte[]>((data.Length * 2) + 1);
 
                 segments.Add(Array.Empty<byte>());
                 foreach (var b in data)

src/Shared/ObjectMethodExecutor/ObjectMethodExecutor.cs

@@ -157,8 +157,8 @@ namespace Microsoft.Extensions.Internal
             var parametersParameter = Expression.Parameter(typeof(object[]), "parameters");
 
             // Build parameter list
-            var parameters = new List<Expression>();
             var paramInfos = methodInfo.GetParameters();
+            var parameters = new List<Expression>(paramInfos.Length);
             for (int i = 0; i < paramInfos.Length; i++)
             {
                 var paramInfo = paramInfos[i];
@@ -209,8 +209,8 @@ namespace Microsoft.Extensions.Internal
             var parametersParameter = Expression.Parameter(typeof(object[]), "parameters");
 
             // Build parameter list
-            var parameters = new List<Expression>();
             var paramInfos = methodInfo.GetParameters();
+            var parameters = new List<Expression>(paramInfos.Length);
             for (int i = 0; i < paramInfos.Length; i++)
             {
                 var paramInfo = paramInfos[i];

src/Shared/StackTrace/StackFrame/StackTraceHelper.cs

@@ -18,12 +18,10 @@ namespace Microsoft.Extensions.StackTrace.Sources
     {
         public static IList<StackFrameInfo> GetFrames(Exception exception, out AggregateException? error)
         {
-            var frames = new List<StackFrameInfo>();
-
             if (exception == null)
             {
                 error = default;
-                return frames;
+                return Array.Empty<StackFrameInfo>();
             }
 
             var needFileInfo = true;
@@ -33,9 +31,11 @@ namespace Microsoft.Extensions.StackTrace.Sources
             if (stackFrames == null)
             {
                 error = default;
-                return frames;
+                return Array.Empty<StackFrameInfo>();
             }
 
+            var frames = new List<StackFrameInfo>(stackFrames.Length);
+
             List<Exception>? exceptions = null;
 
             for (var i = 0; i < stackFrames.Length; i++)

src/SignalR/common/Http.Connections/src/Internal/HttpConnectionManager.cs

@@ -180,7 +180,7 @@ namespace Microsoft.AspNetCore.Http.Connections.Internal
             // Stop firing the timer
             _nextHeartbeat.Stop();
 
-            var tasks = new List<Task>();
+            var tasks = new List<Task>(_connections.Count);
 
             // REVIEW: In the future we can consider a hybrid where we first try to wait for shutdown
             // for a certain time frame then after some grace period we shutdown more aggressively

src/SignalR/server/Core/src/HubConnectionHandler.cs

@@ -73,8 +73,7 @@ namespace Microsoft.AspNetCore.SignalR
 
                 if (_hubOptions.HubFilters != null)
                 {
-                    hubFilters = new List<IHubFilter>();
+                    hubFilters = new List<IHubFilter>(_hubOptions.HubFilters);
-                    hubFilters.AddRange(_hubOptions.HubFilters);
                 }
             }
             else
@@ -84,8 +83,7 @@ namespace Microsoft.AspNetCore.SignalR
 
                 if (_globalHubOptions.HubFilters != null)
                 {
-                    hubFilters = new List<IHubFilter>();
+                    hubFilters = new List<IHubFilter>(_globalHubOptions.HubFilters);
-                    hubFilters.AddRange(_globalHubOptions.HubFilters);
                 }
             }
 

src/SignalR/server/Core/src/HubOptionsSetup.cs

@@ -56,7 +56,7 @@ namespace Microsoft.AspNetCore.SignalR
 
             if (options.SupportedProtocols == null)
             {
-                options.SupportedProtocols = new List<string>();
+                options.SupportedProtocols = new List<string>(_defaultProtocols.Count);
             }
 
             if (options.StreamBufferCapacity == null)

src/SignalR/server/StackExchangeRedis/src/Internal/DefaultHubMessageSerializer.cs

@@ -10,11 +10,12 @@ namespace Microsoft.AspNetCore.SignalR.Internal
 {
     internal class DefaultHubMessageSerializer
     {
-        private readonly List<IHubProtocol> _hubProtocols = new List<IHubProtocol>();
+        private readonly List<IHubProtocol> _hubProtocols;
 
         public DefaultHubMessageSerializer(IHubProtocolResolver hubProtocolResolver, IList<string> globalSupportedProtocols, IList<string> hubSupportedProtocols)
         {
             var supportedProtocols = hubSupportedProtocols ?? globalSupportedProtocols ?? Array.Empty<string>();
+            _hubProtocols = new List<IHubProtocol>(supportedProtocols.Count);
             foreach (var protocolName in supportedProtocols)
             {
                 var protocol = hubProtocolResolver.GetProtocol(protocolName, (supportedProtocols as IReadOnlyList<string>) ?? supportedProtocols.ToList());

src/SignalR/server/StackExchangeRedis/src/RedisHubLifetimeManager.cs

@@ -465,7 +465,7 @@ namespace Microsoft.AspNetCore.SignalR.StackExchangeRedis
                     {
                         var invocation = _protocol.ReadInvocation((byte[])channelMessage.Message);
 
-                        var tasks = new List<Task>();
+                        var tasks = new List<Task>(subscriptions.Count);
                         foreach (var userConnection in subscriptions)
                         {
                             tasks.Add(userConnection.WriteAsync(invocation.Message).AsTask());
@@ -491,7 +491,7 @@ namespace Microsoft.AspNetCore.SignalR.StackExchangeRedis
                 {
                     var invocation = _protocol.ReadInvocation((byte[])channelMessage.Message);
 
-                    var tasks = new List<Task>();
+                    var tasks = new List<Task>(groupConnections.Count);
                     foreach (var groupConnection in groupConnections)
                     {
                         if (invocation.ExcludedConnectionIds?.Contains(groupConnection.ConnectionId) == true)