Support explicit default value from ctor

VYaml.SourceGenerator.Roslyn3/MemberMeta.cs

@@ -9,16 +9,20 @@ class MemberMeta
     public string Name { get; }
     public string FullTypeName { get; }
     public ITypeSymbol MemberType { get; }
-    public INamedTypeSymbol? CustomFormatter { get; }
-    public string? CustomFormatterName { get; }
     public bool IsField { get; }
     public bool IsProperty { get; }
     public bool IsSettable { get; }
-    public bool IsConstructorParameter { get; }
     public int Order { get; }
     public bool HasExplicitOrder { get; }
     public bool HasKeyNameAlias { get; }
 
+    public bool IsConstructorParameter { get; set; }
+    public bool HasExplicitDefaultValueFromConstructor { get; set; }
+    public object? ExplicitDefaultValueFromConstructor { get; set; }
+
+    public INamedTypeSymbol? CustomFormatter { get; }
+    public string? CustomFormatterName { get; }
+
     public string KeyName => keyName ??= KeyNameHelper.ToCamelCase(Name);
     public byte[] KeyNameUtf8Bytes => keyNameUtf8Bytes ??= System.Text.Encoding.UTF8.GetBytes(KeyName);
 

VYaml.SourceGenerator.Roslyn3/VYamlSourceGenerator.cs

@@ -482,7 +482,19 @@ static bool TryEmitDeserializeMethod(
         codeWriter.Append("return new ");
         if (selectedConstructor != null)
         {
-            var parameters = string.Join(",", constructedMembers.Select(x => $"__{x.Name}__"));
+            var parameters = string.Join(",", constructedMembers.Select(x =>
+            {
+                if (x.HasExplicitDefaultValueFromConstructor)
+                {
+                    return x.ExplicitDefaultValueFromConstructor switch
+                    {
+                        null => $"__{x.Name}__ = null",
+                        string stringValue => $"__{x.Name}__ = \"{stringValue}\"",
+                        { } anyValue => $"__{x.Name}__ = {anyValue}"
+                    };
+                }
+                return $"__{x.Name}__";
+            }));
             codeWriter.Append($"{typeMeta.TypeName}({parameters})", false);
         }
         else
@@ -603,6 +615,12 @@ static bool TryGetConstructor(
                 .FirstOrDefault(member => parameter.Name.Equals(member.Name, StringComparison.OrdinalIgnoreCase));
             if (matchedMember != null)
             {
+                matchedMember.IsConstructorParameter = true;
+                if (parameter.HasExplicitDefaultValue)
+                {
+                    matchedMember.HasExplicitDefaultValueFromConstructor = true;
+                    matchedMember.ExplicitDefaultValueFromConstructor = parameter.ExplicitDefaultValue;
+                }
                 parameterMembers.Add(matchedMember);
             }
             else

VYaml.Tests/Serialization/SerializerTest.cs

@@ -2,12 +2,22 @@
 using NUnit.Framework;
 using VYaml.Internal;
 using VYaml.Serialization;
+using VYaml.Tests.TypeDeclarations;
 
 namespace VYaml.Tests.Serialization
 {
     [TestFixture]
     public class SerializerTest
     {
+        [Test]
+        public void Deserialize_ExplicitDefaultValueFromConstructor()
+        {
+            var yamlBytes = StringEncoding.Utf8.GetBytes("valueSet: 22");
+            var value = YamlSerializer.Deserialize<WithDefaultValue>(yamlBytes);
+            Assert.That(value.Value, Is.EqualTo(12));
+            Assert.That(value.ValueSet, Is.EqualTo(22));
+        }
+
         [Test]
         public void DeserializeMultipleDocuments()
         {

VYaml.Tests/TypeDeclarations/Simple.cs

@@ -64,6 +64,20 @@ public partial class WithValueTuple
         public ValueTuple<int, int, int, int, int, int, int> Seven { get; set; }
     }
 
+    [YamlObject]
+    public partial class WithDefaultValue
+    {
+        public readonly int Value;
+        public readonly int ValueSet;
+
+        [YamlConstructor]
+        public WithDefaultValue(int valueSet, int value = 12)
+        {
+            ValueSet = valueSet;
+            Value = value;
+        }
+    }
+
     public enum SimpleEnum
     {
         A,

VYaml.Unity/Assets/VYaml/Tests/Serialization/SerializerTest.cs

@@ -2,12 +2,22 @@
 using NUnit.Framework;
 using VYaml.Internal;
 using VYaml.Serialization;
+using VYaml.Tests.TypeDeclarations;
 
 namespace VYaml.Tests.Serialization
 {
     [TestFixture]
     public class SerializerTest
     {
+        [Test]
+        public void Deserialize_ExplicitDefaultValueFromConstructor()
+        {
+            var yamlBytes = StringEncoding.Utf8.GetBytes("valueSet: 22");
+            var value = YamlSerializer.Deserialize<WithDefaultValue>(yamlBytes);
+            Assert.That(value.Value, Is.EqualTo(12));
+            Assert.That(value.ValueSet, Is.EqualTo(22));
+        }
+
         [Test]
         public void DeserializeMultipleDocuments()
         {

VYaml.Unity/Assets/VYaml/Tests/TypeDeclarations/Simple.cs

@@ -64,6 +64,20 @@ public partial class WithValueTuple
         public ValueTuple<int, int, int, int, int, int, int> Seven { get; set; }
     }
 
+    [YamlObject]
+    public partial class WithDefaultValue
+    {
+        public readonly int Value;
+        public readonly int ValueSet;
+
+        [YamlConstructor]
+        public WithDefaultValue(int valueSet, int value = 12)
+        {
+            ValueSet = valueSet;
+            Value = value;
+        }
+    }
+
     public enum SimpleEnum
     {
         A,