FileTarget: Add option WriteHeaderWhenInitialFileNotEmpty

src/NLog/Targets/FileTarget.cs

@@ -486,6 +486,13 @@ public bool ArchiveOldFileOnStartup
             set => _archiveOldFileOnStartup = value;
         }
         private bool? _archiveOldFileOnStartup;
+
+        /// <summary>
+        /// Gets or sets whether to write the Header on initial creation of file appender.
+        /// Independent of file is empty or not.
+        /// Alternative use <see cref="ArchiveOldFileOnStartup"/> to ensure each application session has its own individual log-file.
+        /// </summary>
+        public bool WriteHeaderOnInitialFileOpen { get; set; }
 
         /// <summary>
         /// Gets or sets a value of the file size threshold to archive old log file on startup.
@@ -2493,25 +2500,24 @@ private void WriteHeaderAndBom(BaseFileAppender appender)
             if (Header is null && !WriteBom) return;
 
             var length = appender.GetFileLength();
-            //  Write header and BOM only on empty files or if file info cannot be obtained.
-            if (length is null || length == 0)
+            // File is empty or file info cannot be obtained
+            var isNewOrEmptyFile = length is null || length == 0;
+
+            if (isNewOrEmptyFile && WriteBom)
             {
-                if (WriteBom)
-                {
-                    InternalLogger.Trace("{0}: Write byte order mark from encoding={1}", this, Encoding);
-                    var preamble = Encoding.GetPreamble();
-                    if (preamble.Length > 0)
-                        appender.Write(preamble, 0, preamble.Length);
-                }
-
-                if (Header != null)
+                InternalLogger.Trace("{0}: Write byte order mark from encoding={1}", this, Encoding);
+                var preamble = Encoding.GetPreamble();
+                if (preamble.Length > 0)
+                    appender.Write(preamble, 0, preamble.Length);
+            }
+
+            if (Header != null && (isNewOrEmptyFile || WriteHeaderOnInitialFileOpen))
+            {
+                InternalLogger.Trace("{0}: Write header", this);
+                ArraySegment<byte> headerBytes = GetLayoutBytes(Header);
+                if (headerBytes.Count > 0)
                 {
-                    InternalLogger.Trace("{0}: Write header", this);
-                    ArraySegment<byte> headerBytes = GetLayoutBytes(Header);
-                    if (headerBytes.Count > 0)
-                    {
-                        appender.Write(headerBytes.Array, headerBytes.Offset, headerBytes.Count);
-                    }
+                    appender.Write(headerBytes.Array, headerBytes.Offset, headerBytes.Count);
                 }
             }
         }

tests/NLog.UnitTests/Targets/FileTargetTests.cs

@@ -2237,6 +2237,71 @@ public void RepeatingFooterTest(bool writeFooterOnArchivingOnly)
                     Directory.Delete(tempDir, true);
             }
         }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void WriteHeaderOnStartupTest(bool writeHeaderOnInitialFileOpen)
+        {
+            var logFile = Path.GetTempFileName() + ".txt";
+            try
+            {
+                const string header = "Headerline";
+
+                 // Configure first time
+                var fileTarget = new FileTarget
+                {
+                    FileName = SimpleLayout.Escape(logFile),
+                    LineEnding = LineEndingMode.LF,
+                    Layout = "${message}",
+                    Header = header,
+                    WriteBom = true,
+                    WriteHeaderOnInitialFileOpen = true
+                };
+
+                LogManager.Setup().LoadConfiguration(c => c.ForLogger().WriteTo(fileTarget));
+
+                logger.Debug("aaa");
+                logger.Info("bbb");
+                logger.Warn("ccc");
+
+                LogManager.Configuration = null;
+
+                string headerPart = header + LineEndingMode.LF.NewLineCharacters;
+                string logPart = "aaa\nbbb\nccc\n";
+                AssertFileContents(logFile, headerPart + logPart, Encoding.UTF8, addBom: true);
+
+                // Configure second time
+                fileTarget = new FileTarget
+                {
+                    FileName = SimpleLayout.Escape(logFile),
+                    LineEnding = LineEndingMode.LF,
+                    Layout = "${message}",
+                    Header = header,
+                    WriteBom = true,
+                    WriteHeaderOnInitialFileOpen = writeHeaderOnInitialFileOpen
+                };
+
+                LogManager.Setup().LoadConfiguration(c => c.ForLogger().WriteTo(fileTarget));
+
+                logger.Debug("aaa");
+                logger.Info("bbb");
+                logger.Warn("ccc");
+
+                LogManager.Configuration = null;    // Flush
+
+                if (writeHeaderOnInitialFileOpen)
+                    AssertFileContents(logFile, headerPart + logPart + headerPart + logPart, Encoding.UTF8, addBom: true);
+                else
+                    AssertFileContents(logFile, headerPart + logPart + logPart, Encoding.UTF8, addBom: true);
+            }
+            finally
+            {
+                LogManager.Configuration = null;
+                if (File.Exists(logFile))
+                    File.Delete(logFile);
+            }
+        }
 
         [Theory]
         [InlineData(false)]