Optional argument handling and added tests for ToString(x, f)

README.md

@@ -187,7 +187,7 @@ calling a function will be undefined if:
 | `StartsWith(s, t)` | Tests whether the string `s` starts with substring `t`. |
 | `Substring(s, start, [length])` | Return the substring of string `s` from `start` to the end of the string, or of `length` characters, if this argument is supplied. |
 | `TagOf(o)` | Returns the `TypeTag` field of a captured object (i.e. where `TypeOf(x)` is `'object'`). |
-| `ToString(x, f)` | Applies the format string `f` to the formattable value `x`. |
+| `ToString(x, [format])` | Convert `x` to a string, applying the format string `format` if `x` is `IFormattable`. |
 | `TypeOf(x)` | Returns a string describing the type of expression `x`: a .NET type name if `x` is scalar and non-null, or, `'array'`, `'object'`, `'dictionary'`, `'null'`, or `'undefined'`. |
 | `Undefined()` | Explicitly mark an undefined value. |
 | `UtcDateTime(x)` | Convert a `DateTime` or `DateTimeOffset` into a UTC `DateTime`. |

src/Serilog.Expressions/Expressions/Compilation/Variadics/VariadicCallRewriter.cs

@@ -4,6 +4,7 @@
 
 namespace Serilog.Expressions.Compilation.Variadics
 {
+    // Now a bit of a misnomer - handles variadic `coalesce()`, as well as optional arguments for other functions.
     class VariadicCallRewriter : IdentityTransformer
     {
         static readonly VariadicCallRewriter Instance = new VariadicCallRewriter();
@@ -19,26 +20,37 @@ protected override Expression Transform(CallExpression lx)
             {
                 var operands = lx.Operands
                     .Select(Transform)
-                    .Concat(new[] {new CallExpression(false, Operators.OpUndefined)})
+                    .Concat(new[] {CallUndefined()})
                     .ToArray();
                 return new CallExpression(lx.IgnoreCase, lx.OperatorName, operands);
             }
 
             if (Operators.SameOperator(lx.OperatorName, Operators.OpCoalesce))
             {
                 if (lx.Operands.Length == 0)
-                    return new CallExpression(false, Operators.OpUndefined);
+                    return CallUndefined();
                 if (lx.Operands.Length == 1)
                     return Transform(lx.Operands.Single());
                 if (lx.Operands.Length > 2)
                 {
                     var first = Transform(lx.Operands.First());
-                    return new CallExpression(false, lx.OperatorName, first,
-                        Transform(new CallExpression(false, lx.OperatorName, lx.Operands.Skip(1).ToArray())));
+                    return new CallExpression(lx.IgnoreCase, lx.OperatorName, first,
+                        Transform(new CallExpression(lx.IgnoreCase, lx.OperatorName, lx.Operands.Skip(1).ToArray())));
                 }
             }
 
+            if (Operators.SameOperator(lx.OperatorName, Operators.OpToString) &&
+                lx.Operands.Length == 1)
+            {
+                return new CallExpression(lx.IgnoreCase, lx.OperatorName, lx.Operands[0], CallUndefined());
+            }
+
             return base.Transform(lx);
         }
+
+        static CallExpression CallUndefined()
+        {
+            return new CallExpression(false, Operators.OpUndefined);
+        }
     }
-}
+}

src/Serilog.Expressions/Expressions/Runtime/RuntimeOperators.cs

@@ -4,7 +4,7 @@
 using Serilog.Events;
 using Serilog.Expressions.Compilation.Linq;
 
-// ReSharper disable ForCanBeConvertedToForeach, InvertIf, MemberCanBePrivate.Global, UnusedMember.Global
+// ReSharper disable ForCanBeConvertedToForeach, InvertIf, MemberCanBePrivate.Global, UnusedMember.Global, InconsistentNaming, ReturnTypeCanBeNotNullable
 
 namespace Serilog.Expressions.Runtime
 {
@@ -470,14 +470,25 @@ public static LogEventPropertyValue _Internal_IsNotNull(LogEventPropertyValue? v
 
         public static LogEventPropertyValue? ToString(LogEventPropertyValue? value, LogEventPropertyValue? format)
         {
-            if (!(value is ScalarValue sv && sv.Value is IFormattable formattable) ||
-                !Coerce.String(format, out var fmt))
+            if (!(value is ScalarValue sv) ||
+                sv.Value == null ||
+                !(Coerce.String(format, out var fmt) || format == null || format is ScalarValue { Value: null }))
             {
                 return null;
             }
 
-            // TODO #19: formatting is culture-specific.
-            return new ScalarValue(formattable.ToString(fmt, null));
+            string toString;
+            if (sv.Value is IFormattable formattable)
+            {
+                // TODO #19: formatting is culture-specific.
+                toString = formattable.ToString(fmt, null);
+            }
+            else
+            {
+                toString = sv.Value.ToString();
+            }
+
+            return new ScalarValue(toString);
         }
 
         public static LogEventPropertyValue? UtcDateTime(LogEventPropertyValue? dateTime)

test/Serilog.Expressions.Tests/Cases/expression-evaluation-cases.asv

@@ -218,10 +218,18 @@ if 'string' then 1 else 2            ⇶ 2
 if true then if false then 1 else 2 else 3 ⇶ 2
 
 // ToString
+tostring(16)                         ⇶ '16'
+tostring('test')                     ⇶ 'test'
+tostring({})                         ⇶ undefined()
+tostring([])                         ⇶ undefined()
 tostring(16, '000')                  ⇶ '016'
-tostring('test', '000')              ⇶ undefined()
-tostring(16, undefined())            ⇶ undefined()
-tostring(16, null)                   ⇶ undefined()
+tostring(null)                       ⇶ undefined()
+tostring(undefined())                ⇶ undefined()
+tostring('test', '000')              ⇶ 'test'
+tostring('test', [])                 ⇶ undefined()
+tostring('test', 42)                 ⇶ undefined()
+tostring(16, undefined())            ⇶ '16'
+tostring(16, null)                   ⇶ '16'
 
 // TypeOf
 typeof(undefined())                  ⇶ 'undefined'

test/Serilog.Expressions.Tests/FormatParityTests.cs

@@ -53,7 +53,7 @@ public class FormatParityTests
         // ReSharper disable once UnusedMember.Global
         public static LogEventPropertyValue? ClassicRender(LogEventPropertyValue? messageTemplate, LogEventPropertyValue? properties)
         {
-            if (!(messageTemplate is ScalarValue svt && svt.Value is string smt) ||
+            if (!(messageTemplate is ScalarValue {Value: string smt}) ||
                 !(properties is StructureValue stp))
             {
                 return null;
@@ -69,7 +69,7 @@ public class FormatParityTests
         // ReSharper disable once UnusedMember.Global
         public static LogEventPropertyValue? ClassicRenderings(LogEventPropertyValue? messageTemplate, LogEventPropertyValue? properties)
         {
-            if (!(messageTemplate is ScalarValue svt && svt.Value is string smt) ||
+            if (!(messageTemplate is ScalarValue {Value: string smt}) ||
                 !(properties is StructureValue stp))
             {
                 return null;