Implement the core logic in the preformatter

Which basically uses the same approach as string.Format method does, but only
for numeric formatted arguments and not serilog onces.

Author: par.dahlman

Common.Logging.Serilog.Tests/SerilogPreformatterTests.cs

@@ -61,7 +61,36 @@ public void Should_Format_Entire_String_If_Only_Numeric_Formatting_Is_Used()
             Assert.That(_resultTemplate, Is.EqualTo(expected));
         }
 
+        [Test]
+        public void Should_Keep_String_Unformatted_If_Serilog_Formatting_Used()
         {
+            /* Setup */
+            const string originalTemplate = "Look at this, a {serilog} formatted string!";
+            var args = new object[] { "serilog" };
+
+            /* Test */
+            var result = _preformatter.TryPreformat(originalTemplate, args, out _resultTemplate, out _resultArgs);
+
+            /* Assert */
+            Assert.That(result, Is.True);
+            Assert.That(_resultTemplate, Is.EqualTo(originalTemplate));
+        }
+
+        [Test]
+        public void Should_Preformat_Numeric_Part_Of_String_But_Keep_Serilog_Formatted_String_And_Arguments()
+        {
+            /* Setup */
+            const string originalTemplate = "This {@string} has {1} numeric match";
+            var args = new object[] { "serilog", "1" };
+            const string expectedTemplate = "This {@string} has 1 numeric match";
+
+
+            /* Test */
+            var result = _preformatter.TryPreformat(originalTemplate, args, out _resultTemplate, out _resultArgs);
+
+            /* Assert */
+            Assert.That(result, Is.True);
+            Assert.That(_resultTemplate, Is.EqualTo(expectedTemplate));
         }
     }
 }

src/SerilogPreformatter.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
+using System.Text;
 using System.Text.RegularExpressions;
 
 namespace Common.Logging.Serilog
@@ -22,17 +23,39 @@ public bool TryPreformat(string templateString, object[] args, out string newTem
             {
                 return true;
             }
+            
+            var templateBuilder = new StringBuilder(templateString);
+            var numericArgs = new List<object>();
+            var matches = _numericFormattedRegex.Matches(templateString);
+
+            for (var i = 0; i < matches.Count; i++)
+            {
+                var currentMatcher = matches[i];
+                var argPosition = GetArgumentPosition(currentMatcher);
+                var currentArg = args[argPosition];
+
+                templateBuilder.Remove(currentMatcher.Index, currentMatcher.Length);
+                templateBuilder.Insert(currentMatcher.Index, currentArg);
+
+                numericArgs.Add(numericArgs);
+            }
+
+            newTemplate = templateBuilder.ToString();
+            newArgs = args
+                .Except(numericArgs)
+                .ToArray();
 
-            newTemplate = string.Format(templateString, args);
-            newArgs = null;
             return true;
         }
 
+        private static int GetArgumentPosition(Match currentMatcher)
         {
+            var nummeric = currentMatcher.Groups
                     .OfType<Group>()
                     .Skip(1)
                     .First()
                     .Value;
+            return int.Parse(nummeric);
         }
     }
 }