Add attr to not overwrite Member with default value

Author: hadashiA

src/MemoryPack.Core/Attributes.cs

@@ -153,3 +153,6 @@ public sealed class MemoryPackOnDeserializedAttribute : Attribute
 public sealed class GenerateTypeScriptAttribute : Attribute
 {
 }
+
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
+public sealed class SkipOverwriteByDefaultAttribute : Attribute;

src/MemoryPack.Generator/MemoryPackGenerator.Emitter.cs

@@ -539,14 +539,15 @@ private string EmitDeserializeBody()
 
     SET:
         {{(!IsUseEmptyConstructor ? "goto NEW;" : "")}}
-{{Members.Where(x => x.Symbol != null).Where(x => x.IsAssignable).Select(x => $"        {(IsUseEmptyConstructor ? "" : "// ")}value.@{x.Name} = __{x.Name};").NewLine()}}
+{{Members.Where(x => x.IsAssignable).Select(x => $"        {(IsUseEmptyConstructor ? "" : "// ")}value.@{x.Name} = __{x.Name};").NewLine()}}
         goto READ_END;
 
     NEW:
         value = {{EmitConstructor()}}
         {
 {{EmitDeserializeConstruction("            ")}}
         };
+{{Members.Select((x, i) => (x, i)).Where(v => v.x.SkipOverwriteByDefault && v.x.IsAssignable).Select(v => $"        if ({v.i + 1} <= count) value.@{v.x.Name} = __{v.x.Name};").NewLine()}}
     READ_END:
 {{readEndBody}}
 """;
@@ -916,7 +917,7 @@ string EmitDeserializeConstruction(string indent)
     {
         // all value is deserialized, __Name is exsits.
         return string.Join("," + Environment.NewLine, Members
-            .Where(x => x.IsSettable && !x.IsConstructorParameter)
+            .Where(x => x is { IsSettable: true, IsConstructorParameter: false, SkipOverwriteByDefault: false })
             .Select(x => $"{indent}@{x.Name} = __{x.Name}"));
     }
 

src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs

@@ -615,6 +615,7 @@ partial class MemberMeta
     public int Order { get; }
     public bool HasExplicitOrder { get; }
     public MemberKind Kind { get; }
+    public bool SkipOverwriteByDefault { get; }
 
     MemberMeta(int order)
     {
@@ -630,6 +631,7 @@ public MemberMeta(ISymbol symbol, IMethodSymbol? constructor, ReferenceSymbols r
         this.Symbol = symbol;
         this.Name = symbol.Name;
         this.Order = sequentialOrder;
+        this.SkipOverwriteByDefault = symbol.ContainsAttribute(references.SkipOverwriteDefaultAttribute);
         var orderAttr = symbol.GetAttribute(references.MemoryPackOrderAttribute);
         if (orderAttr != null)
         {

src/MemoryPack.Generator/ReferenceSymbols.cs

@@ -22,6 +22,7 @@ public class ReferenceSymbols
     public INamedTypeSymbol MemoryPackOnSerializedAttribute { get; }
     public INamedTypeSymbol MemoryPackOnDeserializingAttribute { get; }
     public INamedTypeSymbol MemoryPackOnDeserializedAttribute { get; }
+    public INamedTypeSymbol SkipOverwriteDefaultAttribute { get; }
     public INamedTypeSymbol GenerateTypeScriptAttribute { get; }
     public INamedTypeSymbol IMemoryPackable { get; }
 
@@ -46,6 +47,7 @@ public ReferenceSymbols(Compilation compilation)
         MemoryPackOnSerializedAttribute = GetTypeByMetadataName("MemoryPack.MemoryPackOnSerializedAttribute");
         MemoryPackOnDeserializingAttribute = GetTypeByMetadataName("MemoryPack.MemoryPackOnDeserializingAttribute");
         MemoryPackOnDeserializedAttribute = GetTypeByMetadataName("MemoryPack.MemoryPackOnDeserializedAttribute");
+        SkipOverwriteDefaultAttribute = GetTypeByMetadataName("MemoryPack.SkipOverwriteByDefaultAttribute");
         GenerateTypeScriptAttribute = GetTypeByMetadataName(MemoryPackGenerator.GenerateTypeScriptAttributeFullName);
         IMemoryPackable = GetTypeByMetadataName("MemoryPack.IMemoryPackable`1").ConstructUnboundGenericType();
         KnownTypes = new WellKnownTypes(this);
@@ -161,7 +163,7 @@ public class WellKnownTypes
 
             { "System.Collections.Generic.KeyValuePair<,>", "global::MemoryPack.Formatters.KeyValuePairFormatter<TREPLACE>" },
             { "System.Lazy<>", "global::MemoryPack.Formatters.LazyFormatter<TREPLACE>" },
-            
+
             // TupleFormatters
             { "System.Tuple<>", "global::MemoryPack.Formatters.TupleFormatter<TREPLACE>" },
             { "System.Tuple<,>", "global::MemoryPack.Formatters.TupleFormatter<TREPLACE>" },

tests/MemoryPack.Tests/DefaultValueTest.cs

@@ -0,0 +1,28 @@
+using MemoryPack.Tests.Models;
+
+namespace MemoryPack.Tests;
+
+public class DefaultValueTest
+{
+    [Fact]
+    public void FieldDefaultValue()
+    {
+        var bin = MemoryPackSerializer.Serialize(new DefaultValuePlaceholder { X = 1 });
+        var expected = new FieldDefaultValue();
+        var deserializedValue = MemoryPackSerializer.Deserialize<FieldDefaultValue>(bin)!;
+        deserializedValue.Y.Should().Be(default);
+        deserializedValue.Z.Should().Be(expected.Z);
+        deserializedValue.FromMethod.Should().Be(expected.FromMethod);
+    }
+
+    [Fact]
+    public void PropertyDefaultValue()
+    {
+        var bin = MemoryPackSerializer.Serialize(new DefaultValuePlaceholder { X = 1 });
+        var expected = new PropertyDefaultValue();
+        var deserializedValue = MemoryPackSerializer.Deserialize<PropertyDefaultValue>(bin)!;
+        deserializedValue.Y.Should().Be(default);
+        deserializedValue.Z.Should().Be(expected.Z);
+        deserializedValue.FromMethod.Should().Be(expected.FromMethod);
+    }
+}

tests/MemoryPack.Tests/Models/DefaultValues.cs

@@ -1,69 +1,47 @@
-using System;
-using System.Collections.Generic;
-
 namespace MemoryPack.Tests.Models;
 
-enum TestEnum
-{
-    A, B, C
-}
-
 [MemoryPackable]
 partial class DefaultValuePlaceholder
 {
     public int X { get; set; }
 }
 
+static class StaticMethod
+{
+    public static int GetNumberCalls;
+
+    public static int GetNumber()
+    {
+        GetNumberCalls++;
+        return 100;
+    }
+}
+
 [MemoryPackable]
 partial class FieldDefaultValue
 {
     public int X;
     public int Y = 12345;
+
+    [SkipOverwriteByDefault]
     public float Z = 678.9f;
-    public string S = "aaaaaaaaa";
-    public bool B = true;
+
+    [SkipOverwriteByDefault]
+    public int FromMethod = StaticMethod.GetNumber();
 }
 
 [MemoryPackable]
 partial class PropertyDefaultValue
 {
-    internal enum NestedEnum
-    {
-        A, B
-    }
-
-    public int X { get; set; }
+    public int X { get; init; }
     public int Y { get; set; } = 12345;
+
+    [SkipOverwriteByDefault]
     public float Z { get; set; } = 678.9f;
-    public string S { get; set; } = "aaaaaaaaa";
-    public bool B { get; set; } = true;
-    public List<string> Alpha { get; set; } = new List<string>(new HashSet<string>());
-    public TestEnum E { get; set; } = TestEnum.A;
-    public NestedEnum E2 { get; set; } = NestedEnum.A;
-    public (TestEnum, List<string>) Tuple { get; set; } = (TestEnum.A, new List<string>(new HashSet<string>()));
-    public DateTime Struct { get; set; } = default!;
-}
 
-[MemoryPackable]
-partial class CtorParamDefaultValue
-{
-    public int X;
-    public int Y;
-    public float Z;
-    public string S;
-    public bool B;
-    public decimal D;
-    public DateTime StructValue;
+    [SkipOverwriteByDefault]
+    public float W { get; init; } = 678.9f;
 
-    [MemoryPackConstructor]
-    public CtorParamDefaultValue(int x, int y = 12345, float z = 678.9f, string s = "aaaaaa", bool b = true, decimal d = 99M, DateTime structValue = default)
-    {
-        X = x;
-        Y = y;
-        Z = z;
-        S = s;
-        B = b;
-        D = d;
-        StructValue = structValue;
-    }
+    [SkipOverwriteByDefault]
+    public int FromMethod { get; set; } = StaticMethod.GetNumber();
 }