Issue #23 workaround

src/Serilog.Sinks.File/Sinks/File/WriteCountingStream.cs

@@ -47,7 +47,7 @@ public override void Write(byte[] buffer, int offset, int count)
 
         public override void Flush() => _stream.Flush();
         public override bool CanRead => false;
-        public override bool CanSeek => false;
+        public override bool CanSeek => _stream.CanSeek;
         public override bool CanWrite => true;
         public override long Length => _stream.Length;
 
@@ -60,7 +60,7 @@ public override long Position
 
         public override long Seek(long offset, SeekOrigin origin)
         {
-            throw new NotSupportedException();
+            return _stream.Seek(offset, origin);
         }
 
         public override void SetLength(long value)

test/Serilog.Sinks.File.Tests/FileSinkTests.cs

@@ -4,6 +4,8 @@
 using Serilog.Formatting.Json;
 using Serilog.Sinks.File.Tests.Support;
 using Serilog.Tests.Support;
+using System.Text;
+using Serilog.Tests;
 
 namespace Serilog.Sinks.File.Tests
 {
@@ -99,6 +101,75 @@ public void WhenLimitIsNotSpecifiedFileSizeIsNotRestricted()
                 Assert.True(size > maxBytes * 2);
             }
         }
+
+
+        [Fact]
+        public void WhenLimitIsSpecifiedAndEncodingHasPreambleDataIsCorrectlyAppendedToFileSink()
+        {
+            long? maxBytes = 5000;
+            var encoding = Encoding.UTF8;
+
+            Assert.True(encoding.GetPreamble().Length > 0);
+            WriteTwoEventsAndCheckOutputFileLength(maxBytes, encoding);
+        }
+
+        [Fact]
+        public void WhenLimitIsNotSpecifiedAndEncodingHasPreambleDataIsCorrectlyAppendedToFileSink()
+        {
+            long? maxBytes = null;
+            var encoding = Encoding.UTF8;
+
+            Assert.True(encoding.GetPreamble().Length > 0);
+            WriteTwoEventsAndCheckOutputFileLength(maxBytes, encoding);
+        }
+
+        [Fact]
+        public void WhenLimitIsSpecifiedAndEncodingHasNoPreambleDataIsCorrectlyAppendedToFileSink()
+        {
+            long? maxBytes = 5000;
+            var encoding = new UTF8Encoding(false);
+
+            Assert.Equal(0, encoding.GetPreamble().Length);
+            WriteTwoEventsAndCheckOutputFileLength(maxBytes, encoding);
+        }
+
+        [Fact]
+        public void WhenLimitIsNotSpecifiedAndEncodingHasNoPreambleDataIsCorrectlyAppendedToFileSink()
+        {
+            long? maxBytes = null;
+            var encoding = new UTF8Encoding(false);
+
+            Assert.Equal(0, encoding.GetPreamble().Length);
+            WriteTwoEventsAndCheckOutputFileLength(maxBytes, encoding);
+        }
+
+        private static void WriteTwoEventsAndCheckOutputFileLength(long? maxBytes, Encoding encoding)
+        {
+            using (var tmp = TempFolder.ForCaller())
+            {
+                var path = tmp.AllocateFilename("txt");
+                var evt = Some.LogEvent("Irelevant as it will be replaced by the formatter ");
+                var actualEventOutput = "x";
+                var formatter = new FixedOutputFormatter(actualEventOutput);
+                var eventOuputLength = encoding.GetByteCount(actualEventOutput);
+
+                using (var sink = new FileSink(path, formatter, maxBytes, encoding: encoding))
+                {
+                    sink.Emit(evt);
+                }
+                var size = new FileInfo(path).Length;
+                Assert.Equal(encoding.GetPreamble().Length + eventOuputLength, size);
+
+                //write a second event to the same file
+                using (var sink = new FileSink(path, formatter, maxBytes, encoding: encoding))
+                {
+                    sink.Emit(evt);
+                }
+
+                size = new FileInfo(path).Length;
+                Assert.Equal(encoding.GetPreamble().Length + eventOuputLength * 2, size);
+            }
+        }
     }
 }
 

test/Serilog.Sinks.File.Tests/FixedOutputFormatter.cs

@@ -0,0 +1,25 @@
+using Serilog.Formatting;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Serilog.Events;
+using System.IO;
+
+namespace Serilog.Tests
+{
+    public class FixedOutputFormatter : ITextFormatter
+    {
+        string _substitutionText;
+
+        public FixedOutputFormatter(string substitutionText)
+        {
+            _substitutionText = substitutionText;
+        }
+
+        public void Format(LogEvent logEvent, TextWriter output)
+        {
+            output.Write(_substitutionText);
+        }
+    }
+}