Support custom timestamp format

Support customising the format of the timestamps.
Resolves #311.

src/Logging.XUnit/XUnitLogger.cs

@@ -13,7 +13,7 @@ namespace MartinCostello.Logging.XUnit;
 /// </summary>
 public partial class XUnitLogger : ILogger
 {
-    //// Based on https://github.com/aspnet/Logging/blob/master/src/Microsoft.Extensions.Logging.Console/ConsoleLogger.cs
+    //// Based on https://github.com/dotnet/runtime/blob/65067052e433eda400c5e7cc9f7b21c84640f901/src/libraries/Microsoft.Extensions.Logging.Console/src/ConsoleLogger.cs#L41-L66
 
     /// <summary>
     /// The padding to use for log levels.
@@ -36,6 +36,11 @@ public partial class XUnitLogger : ILogger
     [ThreadStatic]
     private static StringBuilder? _logBuilder;
 
+    /// <summary>
+    /// The format string used to format the timestamp in log messages.
+    /// </summary>
+    private readonly string _timestampFormat;
+
     /// <summary>
     /// Gets or sets the filter to use.
     /// </summary>
@@ -52,6 +57,7 @@ private XUnitLogger(string name, XUnitLoggerOptions? options)
 
         _filter = options?.Filter ?? (static (_, _) => true);
         _messageSinkMessageFactory = options?.MessageSinkMessageFactory ?? (message => new DiagnosticMessage(message));
+        _timestampFormat = options?.TimestampFormat ?? "u";
         IncludeScopes = options?.IncludeScopes ?? false;
     }
 
@@ -156,7 +162,8 @@ public virtual void WriteMessage(LogLevel logLevel, int eventId, string? message
         logBuilder.Append(Name);
         logBuilder.Append('[');
         logBuilder.Append(eventId);
-        logBuilder.AppendLine("]");
+        logBuilder.Append(']');
+        logBuilder.AppendLine();
 
         if (IncludeScopes)
         {
@@ -184,11 +191,17 @@ public virtual void WriteMessage(LogLevel logLevel, int eventId, string? message
             logBuilder.Append(exception.ToString());
         }
 
-        string formatted = logBuilder.ToString();
+        // Prefix the formatted message so it renders like this:
+        // [{timestamp}] {logLevelString}{message}
+        logBuilder.Insert(0, logLevelString);
+        logBuilder.Insert(0, "] ");
+        logBuilder.Insert(0, Clock().ToString(_timestampFormat, CultureInfo.CurrentCulture));
+        logBuilder.Insert(0, '[');
+
+        string line = logBuilder.ToString();
 
         try
         {
-            var line = $"[{Clock():u}] {logLevelString}{formatted}";
             if (outputHelper != null)
             {
                 outputHelper.WriteLine(line);

src/Logging.XUnit/XUnitLoggerOptions.cs

@@ -33,4 +33,12 @@ public XUnitLoggerOptions()
     /// Gets or sets a value indicating whether to include scopes.
     /// </summary>
     public bool IncludeScopes { get; set; }
+
+    /// <summary>
+    /// Gets or sets format string used to format the timestamp in log messages. Defaults to <c>u</c>.
+    /// </summary>
+#if NET7_0_OR_GREATER
+    [StringSyntax(StringSyntaxAttribute.DateTimeFormat)]
+#endif
+    public string? TimestampFormat { get; set; }
 }

tests/Logging.XUnit.Tests/IntegrationTests.cs

@@ -177,6 +177,7 @@ public static void Can_Configure_xunit_For_ILoggerFactory_With_Options_Factory()
         var options = new XUnitLoggerOptions()
         {
             Filter = (_, level) => level >= LogLevel.Error,
+            TimestampFormat = "yyyy-MM-dd HH:mm:ss.fff",
         };
 
         var logger = BootstrapFactory((builder) => builder.AddXUnit(mock.Object, () => options));