Fix source-gen YAML null strings

Fixes #141

Author: xoofx

src/SharpYaml.SourceGenerator/YamlSerializerContextGenerator.cs

@@ -5518,12 +5518,35 @@ private static void EmitReadKnownType(StringBuilder builder, ITypeSymbol typeSym
         var innerIndent = indent + "    ";
         if (IsKnownScalar(typeSymbol))
         {
+            var scalarType = typeSymbol;
+            var acceptsNull = !typeSymbol.IsValueType;
+            if (typeSymbol is INamedTypeSymbol nullableType && nullableType.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T)
+            {
+                scalarType = nullableType.TypeArguments[0];
+                acceptsNull = true;
+            }
+
             builder.Append(innerIndent).AppendLine("if (reader.TokenType != global::SharpYaml.Serialization.YamlTokenType.Scalar)");
             builder.Append(innerIndent).AppendLine("{");
             builder.Append(innerIndent).AppendLine("    throw global::SharpYaml.Serialization.YamlThrowHelper.ThrowExpectedScalar(reader);");
             builder.Append(innerIndent).AppendLine("}");
-            EmitReadScalarAssignment(builder, typeSymbol, valueVarName, indent: innerIndent);
-            builder.Append(innerIndent).AppendLine("reader.Read();");
+            if (acceptsNull)
+            {
+                builder.Append(innerIndent).AppendLine("if (global::SharpYaml.Serialization.YamlScalar.IsNull(reader))");
+                builder.Append(innerIndent).AppendLine("{");
+                builder.Append(innerIndent).AppendLine("    reader.Read();");
+                builder.Append(innerIndent).AppendLine("}");
+                builder.Append(innerIndent).AppendLine("else");
+                builder.Append(innerIndent).AppendLine("{");
+                EmitReadScalarAssignment(builder, scalarType, valueVarName, indent: innerIndent + "    ");
+                builder.Append(innerIndent).AppendLine("    reader.Read();");
+                builder.Append(innerIndent).AppendLine("}");
+            }
+            else
+            {
+                EmitReadScalarAssignment(builder, scalarType, valueVarName, indent: innerIndent);
+                builder.Append(innerIndent).AppendLine("reader.Read();");
+            }
             builder.Append(indent).AppendLine("}");
             return;
         }

src/SharpYaml.Tests/Serialization/YamlSerializerSourceGenerationTests.cs

@@ -507,6 +507,8 @@ internal GeneratedInternalJsonCtorModel(string name, int age)
     public int Age { get; }
 }
 
+internal sealed record GeneratedFunctionDto(string Name, string? Description, string Abbreviation);
+
 internal sealed class GeneratedPopulateChild
 {
     public int Existing { get; set; }
@@ -632,6 +634,7 @@ public TestYamlSerializerContext(YamlSerializerOptions options)
     PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
     DictionaryKeyPolicy = JsonKnownNamingPolicy.CamelCase)]
 [YamlSerializable(typeof(GeneratedWithDefaultOptions))]
+[YamlSerializable(typeof(GeneratedFunctionDto))]
 internal partial class TestYamlSerializerContextWithOptions : YamlSerializerContext
 {
     public TestYamlSerializerContextWithOptions()
@@ -755,6 +758,24 @@ public void GeneratedContext_MergeKey_ExplicitKeyBeforeMergeWins()
         Assert.AreEqual(38, model.Age);
     }
 
+    [TestMethod]
+    public void GeneratedContext_ParameterizedConstructorStringParameterReadsYamlNullAsNull()
+    {
+        var context = TestYamlSerializerContextWithOptions.Default;
+        var yaml = """
+            name: My Function
+            description: null
+            abbreviation: MF
+            """;
+
+        var model = YamlSerializer.Deserialize(yaml, context.GeneratedFunctionDto);
+
+        Assert.IsNotNull(model);
+        Assert.AreEqual("My Function", model.Name);
+        Assert.IsNull(model.Description);
+        Assert.AreEqual("MF", model.Abbreviation);
+    }
+
     [TestMethod]
     public void GeneratedContextProvidesTypedMetadata()
     {