Make sure the JSON has a valid structure.

When setting the excludeAdditionalProperties and excludeStandardColumns configuration properties to true to the logEvent column, it produces an invalid JSON, with an extra comma before the Properties or Rendering property.
Example: {, "Properties": {...}}

Author: cirilomegg

src/Serilog.Sinks.MSSqlServer/Sinks/MSSqlServer/JsonLogEventFormatter.cs

@@ -1,4 +1,4 @@
-// Copyright 2018 Serilog Contributors
+// Copyright 2018 Serilog Contributors
 // 
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -29,6 +29,7 @@ namespace Serilog.Sinks.MSSqlServer
     internal class JsonLogEventFormatter : ITextFormatter
     {
         static readonly JsonValueFormatter ValueFormatter = new JsonValueFormatter(typeTagName: null);
+        private const string COMMA_DELIMITER = ",";
 
         MSSqlServerSinkTraits traits;
 
@@ -52,6 +53,8 @@ public void Format(LogEvent logEvent, TextWriter output)
 
             output.Write("{");
 
+            string precedingDelimiter = "";
+
             if (traits.columnOptions.LogEvent.ExcludeStandardColumns == false)
             {
                 // The XML Properties column has never included the Standard Columns, but prior
@@ -63,7 +66,6 @@ public void Format(LogEvent logEvent, TextWriter output)
                 // whether Standard Columns are written (specifically, the subset of Standard
                 // columns that were output by the external JsonFormatter class).
 
-                string precedingDelimiter = "";
                 var store = traits.columnOptions.Store;
 
                 WriteIfPresent(StandardColumn.TimeStamp);
@@ -77,7 +79,7 @@ void WriteIfPresent(StandardColumn col)
                     if(store.Contains(col))
                     {
                         output.Write(precedingDelimiter);
-                        precedingDelimiter = ",";
+                        precedingDelimiter = COMMA_DELIMITER;
                         var colData = traits.GetStandardColumnNameAndValue(col, logEvent);
                         JsonValueFormatter.WriteQuotedJsonString(colData.Key, output);
                         output.Write(":");
@@ -88,7 +90,11 @@ void WriteIfPresent(StandardColumn col)
             }
 
             if (logEvent.Properties.Count != 0)
+            {
+                output.Write(precedingDelimiter);
                 WriteProperties(logEvent.Properties, output);
+                precedingDelimiter = COMMA_DELIMITER;
+            }
 
             var tokensWithFormat = logEvent.MessageTemplate.Tokens
                 .OfType<PropertyToken>()
@@ -98,6 +104,7 @@ void WriteIfPresent(StandardColumn col)
 
             if (tokensWithFormat.Length != 0)
             {
+                output.Write(precedingDelimiter);
                 WriteRenderings(tokensWithFormat, logEvent.Properties, output);
             }
 
@@ -106,13 +113,13 @@ void WriteIfPresent(StandardColumn col)
 
         static void WriteProperties(IReadOnlyDictionary<string, LogEventPropertyValue> properties, TextWriter output)
         {
-            output.Write(",\"Properties\":{");
+            output.Write("\"Properties\":{");
 
             string precedingDelimiter = "";
             foreach (var property in properties)
             {
                 output.Write(precedingDelimiter);
-                precedingDelimiter = ",";
+                precedingDelimiter = COMMA_DELIMITER;
                 JsonValueFormatter.WriteQuotedJsonString(property.Key, output);
                 output.Write(':');
                 ValueFormatter.Format(property.Value, output);
@@ -123,13 +130,13 @@ static void WriteProperties(IReadOnlyDictionary<string, LogEventPropertyValue> p
 
         static void WriteRenderings(IEnumerable<IGrouping<string, PropertyToken>> tokensWithFormat, IReadOnlyDictionary<string, LogEventPropertyValue> properties, TextWriter output)
         {
-            output.Write(",\"Renderings\":{");
+            output.Write("\"Renderings\":{");
 
             string precedingDelimiter = "";
             foreach (var ptoken in tokensWithFormat)
             {
                 output.Write(precedingDelimiter);
-                precedingDelimiter = ",";
+                precedingDelimiter = COMMA_DELIMITER;
 
                 JsonValueFormatter.WriteQuotedJsonString(ptoken.Key, output);
                 output.Write(":[");
@@ -138,7 +145,7 @@ static void WriteRenderings(IEnumerable<IGrouping<string, PropertyToken>> tokens
                 foreach (var format in ptoken)
                 {
                     output.Write(fdelim);
-                    fdelim = ",";
+                    fdelim = COMMA_DELIMITER;
 
                     output.Write("{\"Format\":");
                     JsonValueFormatter.WriteQuotedJsonString(format.Format, output);