Escape leading @ in property names, since these can in fact be created through ForContext()

Author: nblumhardt

src/Serilog.Formatting.Compact/Formatting/Compact/CompactJsonFormatter.cs

@@ -102,8 +102,15 @@ public static void FormatEvent(LogEvent logEvent, TextWriter output, JsonValueFo
 
             foreach (var property in logEvent.Properties)
             {
+                var name = property.Key;
+                if (name.Length > 0 && name[0] == '@')
+                {
+                    // Escape first '@' by doubling
+                    name = '@' + name;
+                }
+
                 output.Write(',');
-                JsonValueFormatter.WriteQuotedJsonString(property.Key, output);
+                JsonValueFormatter.WriteQuotedJsonString(name, output);
                 output.Write(':');
                 valueFormatter.Format(property.Value, output);
             }

test/Serilog.Formatting.Compact.Tests/CompactJsonFormatterTests.cs

@@ -9,7 +9,7 @@ namespace Serilog.Formatting.Compact.Tests
 {
     public class CompactJsonFormatterTests
     {
-        void AssertValidJson(Action<ILogger> act)
+        JObject AssertValidJson(Action<ILogger> act)
         {
             var output = new StringWriter();
             var formatter = new CompactJsonFormatter();
@@ -22,7 +22,7 @@ void AssertValidJson(Action<ILogger> act)
             var json = output.ToString();
 
             // Unfortunately this will not detect all JSON formatting issues; better than nothing however.
-            JObject.Parse(json);
+            return JObject.Parse(json);
         }
 
         [Fact]
@@ -66,5 +66,17 @@ public void MultipleRenderingsAreDelimited()
         {
             AssertValidJson(log => log.Information("Rendering {First:x8} and {Second:x8}", 1, 2));
         }
+
+        [Fact]
+        public void AtPrefixedPropertyNamesAreEscaped()
+        {
+            // Not possible in message templates, but accepted this way
+            var jobject = AssertValidJson(log => log.ForContext("@Mistake", 42)
+                                                    .Information("Hello"));
+
+            JToken val;
+            Assert.True(jobject.TryGetValue("@@Mistake", out val));
+            Assert.Equal(42, val.ToObject<int>());
+        }
     }
 }