Reduce heap allocation by deferring formatting of log and error messa…

…ges until actually accessed by consumer

src/ConfigCat.Client.Tests/ConfigCatClientTests.cs

@@ -2114,7 +2114,7 @@ public async Task Hooks_MockedClientRaisesEvents()
         var onFetch = (ProjectConfig latestConfig, CancellationToken _) =>
         {
             var logMessage = loggerWrapper.FetchFailedDueToUnexpectedError(errorException);
-            return FetchResult.Failure(latestConfig, RefreshErrorCode.HttpRequestFailure, errorMessage: logMessage.InvariantFormattedMessage, errorException: errorException);
+            return FetchResult.Failure(latestConfig, RefreshErrorCode.HttpRequestFailure, errorMessage: logMessage.ToLazyString(), errorException: errorException);
         };
         this.fetcherMock.Setup(m => m.FetchAsync(It.IsAny<ProjectConfig>(), It.IsAny<CancellationToken>())).ReturnsAsync(onFetch);
 

src/ConfigCatClient/ConfigService/ConfigServiceBase.cs

@@ -96,7 +96,7 @@ public virtual RefreshResult RefreshConfig()
         else
         {
             var logMessage = this.Logger.ConfigServiceCannotInitiateHttpCalls();
-            return RefreshResult.Failure(RefreshErrorCode.OfflineClient, logMessage.InvariantFormattedMessage);
+            return RefreshResult.Failure(RefreshErrorCode.OfflineClient, logMessage.ToLazyString());
         }
     }
 
@@ -133,7 +133,7 @@ public virtual async ValueTask<RefreshResult> RefreshConfigAsync(CancellationTok
         else
         {
             var logMessage = this.Logger.ConfigServiceCannotInitiateHttpCalls();
-            return RefreshResult.Failure(RefreshErrorCode.OfflineClient, logMessage.InvariantFormattedMessage);
+            return RefreshResult.Failure(RefreshErrorCode.OfflineClient, logMessage.ToLazyString());
         }
     }
 

src/ConfigCatClient/Evaluation/EvaluationDetails.cs

@@ -1,5 +1,6 @@
 using System;
 using ConfigCat.Client.Evaluation;
+using ConfigCat.Client.Utils;
 
 namespace ConfigCat.Client;
 
@@ -28,13 +29,13 @@ internal static EvaluationDetails<TValue> FromEvaluateResult<TValue>(string key,
     }
 
     internal static EvaluationDetails<TValue> FromDefaultValue<TValue>(string key, TValue defaultValue, DateTime? fetchTime, User? user,
-        string errorMessage, Exception? errorException = null, EvaluationErrorCode errorCode = EvaluationErrorCode.UnexpectedError)
+        LazyString errorMessage, Exception? errorException = null, EvaluationErrorCode errorCode = EvaluationErrorCode.UnexpectedError)
     {
         var instance = new EvaluationDetails<TValue>(key, defaultValue)
         {
             User = user,
             IsDefaultValue = true,
-            ErrorMessage = errorMessage,
+            errorMessage = errorMessage,
             ErrorException = errorException,
             ErrorCode = errorCode,
         };
@@ -47,6 +48,8 @@ internal static EvaluationDetails<TValue> FromDefaultValue<TValue>(string key, T
         return instance;
     }
 
+    private LazyString errorMessage;
+
     private protected EvaluationDetails(string key)
     {
         Key = key;
@@ -93,7 +96,11 @@ private protected EvaluationDetails(string key)
     /// <summary>
     /// Error message in case evaluation failed.
     /// </summary>
-    public string? ErrorMessage { get; set; }
+    public string? ErrorMessage
+    {
+        get => this.errorMessage.ToString();
+        set => this.errorMessage = value;
+    }
 
     /// <summary>
     /// The <see cref="Exception"/> object related to the error in case evaluation failed (if any).

src/ConfigCatClient/Evaluation/EvaluationHelper.cs

@@ -17,15 +17,15 @@ public static EvaluationDetails<T> Evaluate<T>(this IRolloutEvaluator evaluator,
         {
             logMessage = logger.ConfigJsonIsNotPresent(key, nameof(defaultValue), defaultValue);
             return EvaluationDetails.FromDefaultValue(key, defaultValue, fetchTime: remoteConfig?.TimeStamp, user,
-                logMessage.InvariantFormattedMessage, errorCode: EvaluationErrorCode.ConfigJsonNotAvailable);
+                logMessage.ToLazyString(), errorCode: EvaluationErrorCode.ConfigJsonNotAvailable);
         }
 
         if (!settings.TryGetValue(key, out var setting))
         {
-            var availableKeys = new StringListFormatter(settings.Keys).ToString();
+            var availableKeys = new StringListFormatter(settings.Keys);
             logMessage = logger.SettingEvaluationFailedDueToMissingKey(key, nameof(defaultValue), defaultValue, availableKeys);
             return EvaluationDetails.FromDefaultValue(key, defaultValue, fetchTime: remoteConfig?.TimeStamp, user,
-                logMessage.InvariantFormattedMessage, errorCode: EvaluationErrorCode.SettingKeyMissing);
+                logMessage.ToLazyString(), errorCode: EvaluationErrorCode.SettingKeyMissing);
         }
 
         var evaluateContext = new EvaluateContext(key, setting, user, settings);

src/ConfigCatClient/FetchResult.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
+using ConfigCat.Client.Utils;
 
 namespace ConfigCat.Client;
 
@@ -17,12 +18,12 @@ public static FetchResult NotModified(ProjectConfig config)
         return new FetchResult(config, RefreshErrorCode.None, NotModifiedToken);
     }
 
-    public static FetchResult Failure(ProjectConfig config, RefreshErrorCode errorCode, string errorMessage, Exception? errorException = null)
+    public static FetchResult Failure(ProjectConfig config, RefreshErrorCode errorCode, LazyString errorMessage, Exception? errorException = null)
     {
-        return new FetchResult(config, errorCode, errorMessage, errorException);
+        return new FetchResult(config, errorCode, errorMessage.Value ?? (object)errorMessage, errorException);
     }
 
-    private readonly object? errorMessageOrToken;
+    private readonly object? errorMessageOrToken; // either null or a string or a boxed LazyString or NotModifiedToken
 
     private FetchResult(ProjectConfig config, RefreshErrorCode errorCode, object? errorMessageOrToken, Exception? errorException = null)
     {
@@ -35,10 +36,10 @@ private FetchResult(ProjectConfig config, RefreshErrorCode errorCode, object? er
     public bool IsSuccess => this.errorMessageOrToken is null;
     public bool IsNotModified => ReferenceEquals(this.errorMessageOrToken, NotModifiedToken);
     [MemberNotNullWhen(true, nameof(ErrorMessage))]
-    public bool IsFailure => this.errorMessageOrToken is string;
+    public bool IsFailure => !IsSuccess && !IsNotModified;
 
     public ProjectConfig Config { get; }
     public RefreshErrorCode ErrorCode { get; }
-    public string? ErrorMessage => this.errorMessageOrToken as string;
+    public object? ErrorMessage => !IsNotModified ? this.errorMessageOrToken : null;
     public Exception? ErrorException { get; }
 }

src/ConfigCatClient/FormattableStringFactory.cs

@@ -2,8 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 // Based on: https://github.com/dotnet/runtime/blob/v6.0.13/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/FormattableStringFactory.cs
-// This is a modified version of the built-in type that is only used only internally in the SDK. It allows
-// creating value type instances instead of reference type instances to avoid unnecessary heap allocations.
+// This is a modified version of the built-in type that is only used only internally in the SDK.
+// It creates value type instances instead of reference type instances to avoid unnecessary heap allocations.
 
 #pragma warning disable IDE0161 // Convert to file-scoped namespace
 #pragma warning disable CS0436 // Type conflicts with imported type
@@ -19,7 +19,7 @@ internal static class FormattableStringFactory
         /// Create a <see cref="FormattableString"/> from a composite format string and object
         /// array containing zero or more objects to format.
         /// </summary>
-        public static FormattableString Create(string format, params object?[] arguments)
+        public static ValueFormattableString Create(string format, params object?[] arguments)
         {
             if (format == null)
             {
@@ -31,7 +31,7 @@ public static FormattableString Create(string format, params object?[] arguments
                 throw new ArgumentNullException(nameof(arguments));
             }
 
-            return new FormattableString(format, arguments);
+            return new ValueFormattableString(format, arguments);
         }
     }
 }

src/ConfigCatClient/Hooks/Hooks.cs

@@ -45,8 +45,8 @@ public void RaiseConfigFetched(RefreshResult result, bool isInitiatedByUser)
     public void RaiseConfigChanged(IConfig newConfig)
         => this.events.RaiseConfigChanged(this.client, newConfig);
 
-    public void RaiseError(string message, Exception? exception)
-        => this.events.RaiseError(this.client, message, exception);
+    public void RaiseError(ref FormattableLogMessage message, Exception? exception)
+        => this.events.RaiseError(this.client, ref message, exception);
 
     public event EventHandler<ClientReadyEventArgs>? ClientReady
     {
@@ -84,7 +84,7 @@ public virtual void RaiseClientReady(IConfigCatClient? client, ClientCacheState
         public virtual void RaiseFlagEvaluated(IConfigCatClient? client, EvaluationDetails evaluationDetails) { /* intentional no-op */ }
         public virtual void RaiseConfigFetched(IConfigCatClient? client, RefreshResult result, bool isInitiatedByUser) { /* intentional no-op */ }
         public virtual void RaiseConfigChanged(IConfigCatClient? client, IConfig newConfig) { /* intentional no-op */ }
-        public virtual void RaiseError(IConfigCatClient? client, string message, Exception? exception) { /* intentional no-op */ }
+        public virtual void RaiseError(IConfigCatClient? client, ref FormattableLogMessage message, Exception? exception) { /* intentional no-op */ }
 
         public virtual event EventHandler<ClientReadyEventArgs>? ClientReady { add { /* intentional no-op */ } remove { /* intentional no-op */ } }
         public virtual event EventHandler<FlagEvaluatedEventArgs>? FlagEvaluated { add { /* intentional no-op */ } remove { /* intentional no-op */ } }
@@ -115,9 +115,9 @@ public override void RaiseConfigChanged(IConfigCatClient? client, IConfig newCon
             ConfigChanged?.Invoke(client, new ConfigChangedEventArgs(newConfig));
         }
 
-        public override void RaiseError(IConfigCatClient? client, string message, Exception? exception)
+        public override void RaiseError(IConfigCatClient? client, ref FormattableLogMessage message, Exception? exception)
         {
-            Error?.Invoke(client, new ConfigCatClientErrorEventArgs(message, exception));
+            Error?.Invoke(client, new ConfigCatClientErrorEventArgs(message.InvariantFormattedMessage, exception));
         }
 
         public override event EventHandler<ClientReadyEventArgs>? ClientReady;

src/ConfigCatClient/Hooks/SafeHooksWrapper.cs

@@ -37,8 +37,8 @@ public void RaiseConfigChanged(IConfig newConfig)
         => Hooks.RaiseConfigChanged(newConfig);
 
     [MethodImpl(MethodImplOptions.NoInlining)]
-    public void RaiseError(string message, Exception? exception)
-        => Hooks.RaiseError(message, exception);
+    public void RaiseError(ref FormattableLogMessage message, Exception? exception)
+        => Hooks.RaiseError(ref message, exception);
 
     public static implicit operator SafeHooksWrapper(Hooks? hooks) => hooks is not null ? new SafeHooksWrapper(hooks) : default;
 }

src/ConfigCatClient/HttpConfigFetcher.cs

@@ -90,7 +90,7 @@ private async ValueTask<FetchResult> FetchInternalAsync(ProjectConfig lastConfig
                     {
                         var exception = responseWithBody.Exception;
                         logMessage = this.logger.FetchReceived200WithInvalidBody(exception);
-                        return FetchResult.Failure(lastConfig, RefreshErrorCode.InvalidHttpResponseContent, logMessage.InvariantFormattedMessage, exception);
+                        return FetchResult.Failure(lastConfig, RefreshErrorCode.InvalidHttpResponseContent, logMessage.ToLazyString(), exception);
                     }
 
                     return FetchResult.Success(new ProjectConfig
@@ -105,7 +105,7 @@ private async ValueTask<FetchResult> FetchInternalAsync(ProjectConfig lastConfig
                     if (lastConfig.IsEmpty)
                     {
                         logMessage = this.logger.FetchReceived304WhenLocalCacheIsEmpty((int)response.StatusCode, response.ReasonPhrase);
-                        return FetchResult.Failure(lastConfig, RefreshErrorCode.InvalidHttpResponseWhenLocalCacheIsEmpty, logMessage.InvariantFormattedMessage);
+                        return FetchResult.Failure(lastConfig, RefreshErrorCode.InvalidHttpResponseWhenLocalCacheIsEmpty, logMessage.ToLazyString());
                     }
 
                     return FetchResult.NotModified(lastConfig.With(ProjectConfig.GenerateTimeStamp()));
@@ -115,13 +115,13 @@ private async ValueTask<FetchResult> FetchInternalAsync(ProjectConfig lastConfig
                     logMessage = this.logger.FetchFailedDueToInvalidSdkKey();
 
                     // We update the timestamp for extra protection against flooding.
-                    return FetchResult.Failure(lastConfig.With(ProjectConfig.GenerateTimeStamp()), RefreshErrorCode.InvalidSdkKey, logMessage.InvariantFormattedMessage);
+                    return FetchResult.Failure(lastConfig.With(ProjectConfig.GenerateTimeStamp()), RefreshErrorCode.InvalidSdkKey, logMessage.ToLazyString());
 
                 default:
                     logMessage = this.logger.FetchFailedDueToUnexpectedHttpResponse((int)response.StatusCode, response.ReasonPhrase);
 
                     ReInitializeHttpClient();
-                    return FetchResult.Failure(lastConfig, RefreshErrorCode.UnexpectedHttpResponse, logMessage.InvariantFormattedMessage);
+                    return FetchResult.Failure(lastConfig, RefreshErrorCode.UnexpectedHttpResponse, logMessage.ToLazyString());
             }
         }
         catch (OperationCanceledException) when (this.cancellationTokenSource.IsCancellationRequested)
@@ -153,7 +153,7 @@ private async ValueTask<FetchResult> FetchInternalAsync(ProjectConfig lastConfig
         }
 
         ReInitializeHttpClient();
-        return FetchResult.Failure(lastConfig, errorCode, logMessage.InvariantFormattedMessage, errorException);
+        return FetchResult.Failure(lastConfig, errorCode, logMessage.ToLazyString(), errorException);
     }
 
     private async ValueTask<ResponseWithBody> FetchRequestAsync(string? httpETag, Uri requestUri, sbyte maxExecutionCount = 3)

src/ConfigCatClient/Logging/FormattableLogMessage.cs

@@ -72,6 +72,13 @@ public FormattableLogMessage(string format, string[] argNames, object?[] argValu
     /// </summary>
     public string InvariantFormattedMessage => this.invariantFormattedMessage ??= ToString(CultureInfo.InvariantCulture);
 
+    internal LazyString ToLazyString()
+    {
+        return this.invariantFormattedMessage is { } invariantFormattedMessage
+            ? invariantFormattedMessage
+            : new LazyString(this.format ?? string.Empty, this.argValues);
+    }
+
     /// <summary>
     /// Returns the log message formatted using <see cref="CultureInfo.CurrentCulture"/>.
     /// </summary>

src/ConfigCatClient/Logging/LogMessages.cs

@@ -1,5 +1,6 @@
 using System;
 using ConfigCat.Client.ConfigService;
+using ConfigCat.Client.Utils;
 
 namespace ConfigCat.Client;
 
@@ -17,7 +18,7 @@ public static FormattableLogMessage ConfigJsonIsNotPresent(this LoggerWrapper lo
         $"Config JSON is not present when evaluating setting '{key}'. Returning the `{defaultParamName}` parameter that you specified in your application: '{defaultParamValue}'.",
         "KEY", "DEFAULT_PARAM_NAME", "DEFAULT_PARAM_VALUE");
 
-    public static FormattableLogMessage SettingEvaluationFailedDueToMissingKey(this LoggerWrapper logger, string key, string defaultParamName, object? defaultParamValue, string availableKeys) => logger.LogInterpolated(
+    public static FormattableLogMessage SettingEvaluationFailedDueToMissingKey(this LoggerWrapper logger, string key, string defaultParamName, object? defaultParamValue, StringListFormatter availableKeys) => logger.LogInterpolated(
         LogLevel.Error, 1001,
         $"Failed to evaluate setting '{key}' (the key was not found in config JSON). Returning the `{defaultParamName}` parameter that you specified in your application: '{defaultParamValue}'. Available keys: [{availableKeys}].",
         "KEY", "DEFAULT_PARAM_NAME", "DEFAULT_PARAM_VALUE", "AVAILABLE_KEYS");

src/ConfigCatClient/Logging/LoggerWrapper.cs

@@ -37,7 +37,7 @@ public void Log(LogLevel level, LogEventId eventId, ref FormattableLogMessage me
 
         if (level == LogLevel.Error)
         {
-            this.hooks.RaiseError(message.InvariantFormattedMessage, exception);
+            this.hooks.RaiseError(ref message, exception);
         }
     }
 }

src/ConfigCatClient/RefreshResult.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
+using ConfigCat.Client.Utils;
 
 namespace ConfigCat.Client;
 
@@ -29,17 +30,24 @@ public static RefreshResult Failure(RefreshErrorCode errorCode, string errorMess
             errorException);
     }
 
+    internal static RefreshResult Failure(RefreshErrorCode errorCode, LazyString errorMessage, Exception? errorException = null)
+    {
+        return new RefreshResult(errorCode, errorMessage, errorException);
+    }
+
     internal static RefreshResult From(in FetchResult fetchResult)
     {
         return !fetchResult.IsFailure
             ? Success()
-            : Failure(fetchResult.ErrorCode, fetchResult.ErrorMessage, fetchResult.ErrorException);
+            : new RefreshResult(fetchResult.ErrorCode, fetchResult.ErrorMessage, fetchResult.ErrorException);
     }
 
-    private RefreshResult(RefreshErrorCode errorCode, string? errorMessage, Exception? errorException)
+    private readonly object? errorMessage; // either null or a string or a boxed LazyString
+
+    private RefreshResult(RefreshErrorCode errorCode, object? errorMessage, Exception? errorException)
     {
         ErrorCode = errorCode;
-        ErrorMessage = errorMessage;
+        this.errorMessage = errorMessage;
         ErrorException = errorException;
     }
 
@@ -57,7 +65,7 @@ private RefreshResult(RefreshErrorCode errorCode, string? errorMessage, Exceptio
     /// <summary>
     /// Error message in case the operation failed, otherwise <see langword="null" />.
     /// </summary>
-    public string? ErrorMessage { get; }
+    public string? ErrorMessage => this.errorMessage?.ToString();
 
     /// <summary>
     /// The <see cref="Exception"/> object related to the error in case the operation failed (if any).

src/ConfigCatClient/Utils/LazyString.cs

@@ -0,0 +1,42 @@
+using System.Globalization;
+
+namespace ConfigCat.Client.Utils;
+
+internal struct LazyString
+{
+    private readonly string? format;
+    private object? argsOrValue;
+
+    public LazyString(string? value)
+    {
+        this.format = null;
+        this.argsOrValue = value;
+    }
+
+    public LazyString(string format, params object?[]? args)
+    {
+        this.format = format;
+        this.argsOrValue = args ?? ArrayUtils.EmptyArray<string>();
+    }
+
+    public string? Value => this.argsOrValue as string;
+
+    public override string? ToString()
+    {
+        var argsOrValue = this.argsOrValue;
+        if (argsOrValue is null)
+        {
+            return null;
+        }
+
+        if (argsOrValue is not string value)
+        {
+            var args = (object?[])argsOrValue;
+            this.argsOrValue = value = string.Format(CultureInfo.InvariantCulture, this.format!, args);
+        }
+
+        return value;
+    }
+
+    public static implicit operator LazyString(string? value) => new LazyString(value);
+}