Merge pull request #268 from Cysharp/hadashiA/skip-overwrite-by-default

Add attr to not overwrite Member with default value

Author: hadashiA

README.md

@@ -213,7 +213,7 @@ public partial class Person3
 When serializing/deserializing, MemoryPack can invoke a before/after event using the `[MemoryPackOnSerializing]`, `[MemoryPackOnSerialized]`, `[MemoryPackOnDeserializing]`, `[MemoryPackOnDeserialized]` attributes. It can annotate both static and instance (non-static) methods, and public and private methods. 
 
 ```csharp
-[MemoryPackable]
+    [MemoryPackable]
 public partial class MethodCallSample
 {
     // method call order is static -> instance
@@ -546,6 +546,28 @@ public partial class VersionCheck
 
 In use-case, store old data (to file, to redis, etc...) and read to new schema is always ok. In the RPC scenario, schema exists both on the client and the server side, the client must be updated before the server. An updated client has no problem connecting to the old server but an old client can not connect to a new server.
 
+
+By default, when the old data read to new schema, any members not on the data side are initialized with the `default` literal.
+If you want to avoid this and use initial values of field/properties, you can use `[SuppressDefaultInitialization]`.
+
+```cs
+[MemoryPackable]
+public partial class DefaultValue
+{
+    public string Prop1 { get; set; }
+
+    [SuppressDefaultInitialization]
+    public int Prop2 { get; set; } = 111; // < if old data is missing, set `111`.
+    
+    public int Prop3 { get; set; } = 222; // < if old data is missing, set `default`.
+}
+```
+
+ `[SuppressDefaultInitialization]` has following disadvantages:
+- Cannot be used with readonly, init-only, and required modifier.
+=- May not be the best performance due to increased conditional branching. (But it would be negligible.)
+
+
 The next [Serialization info](#serialization-info) section shows how to check for schema changes, e.g., by CI, to prevent accidents.
 
 When using `GenerateType.VersionTolerant`, it supports full version-tolerant.

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 SuppressDefaultInitialization : Attribute;

src/MemoryPack.Generator/DiagnosticDescriptors.cs

@@ -324,4 +324,12 @@ internal static class DiagnosticDescriptors
         category: Category,
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true);
+
+    public static readonly DiagnosticDescriptor SuppressDefaultInitializationMustBeSettable = new(
+        id: "MEMPACK040",
+        title: "Readonly member cannot specify [SuppressDefaultInitialization]",
+        messageFormat: "The MemoryPackable object '{0}' member '{1}' has [SuppressDefaultInitialization], it cannot be readonly, init-only and required.",
+        category: Category,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
 }

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("            ")}}
         };
+{{EmitDeserializeConstructionWithBranching("        ")}}
     READ_END:
 {{readEndBody}}
 """;
@@ -916,10 +917,23 @@ 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, SuppressDefaultInitialization: false })
             .Select(x => $"{indent}@{x.Name} = __{x.Name}"));
     }
 
+    string EmitDeserializeConstructionWithBranching(string indent)
+    {
+        var members = Members
+            .Select((x, i) => (x, i))
+            .Where(v => v.x.SuppressDefaultInitialization);
+
+        var lines = GenerateType is GenerateType.VersionTolerant or GenerateType.CircularReference
+            ? members.Select(v => $"{indent}if (deltas.Length > {v.i} && deltas[{v.i}] != 0) value.@{v.x.Name} = __{v.x.Name};")
+            : members.Select(v => $"{indent}if ({v.i + 1} <= count) value.@{v.x.Name} = __{v.x.Name};");
+
+        return lines.NewLine();
+    }
+
     string EmitUnionTemplate(IGeneratorContext context)
     {
         var classOrInterfaceOrRecord = IsRecord ? "record" : (Symbol.TypeKind == TypeKind.Interface) ? "interface" : "class";

src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs

@@ -375,6 +375,11 @@ public bool Validate(TypeDeclarationSyntax syntax, IGeneratorContext context, bo
                     context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.ReadOnlyFieldMustBeConstructorMember, item.GetLocation(syntax), Symbol.Name, item.Name));
                     noError = false;
                 }
+                else if (item is { SuppressDefaultInitialization: true, IsAssignable: false })
+                {
+                    context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.SuppressDefaultInitializationMustBeSettable, item.GetLocation(syntax), Symbol.Name, item.Name));
+                    noError = false;
+                }
             }
         }
 
@@ -615,6 +620,7 @@ partial class MemberMeta
     public int Order { get; }
     public bool HasExplicitOrder { get; }
     public MemberKind Kind { get; }
+    public bool SuppressDefaultInitialization { get; }
 
     MemberMeta(int order)
     {
@@ -630,6 +636,7 @@ public MemberMeta(ISymbol symbol, IMethodSymbol? constructor, ReferenceSymbols r
         this.Symbol = symbol;
         this.Name = symbol.Name;
         this.Order = sequentialOrder;
+        this.SuppressDefaultInitialization = 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.SuppressDefaultInitialization");
         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 +1,30 @@
+using MemoryPack.Tests.Models;
+
+namespace MemoryPack.Tests;
+
+public class DefaultValueTest
+{
+    [Fact]
+    public void SuppressDefaultInitialization()
+    {
+        var bin = MemoryPackSerializer.Serialize(new DefaultValuePlaceholder { X = 1 });
+        var expected = new HasDefaultValue();
+        var deserializedValue = MemoryPackSerializer.Deserialize<HasDefaultValue>(bin)!;
+        deserializedValue.Y.Should().Be(default);
+        deserializedValue.Z.Should().Be(default);
+        deserializedValue.Y2.Should().Be(expected.Y2);
+        deserializedValue.Z2.Should().Be(expected.Z2);
+    }
+
+    [Fact]
+    public void SuppressDefaultInitialization_VersionTolerant()
+    {
+        var bin = MemoryPackSerializer.Serialize(new DefaultValuePlaceholderWithVersionTolerant { X = 1 });
+        var expected = new HasDefaultValueWithVersionTolerant();
+        var deserializedValue = MemoryPackSerializer.Deserialize<HasDefaultValueWithVersionTolerant>(bin)!;
+        deserializedValue.Y.Should().Be(default);
+        deserializedValue.Z.Should().Be(default);
+        deserializedValue.Y2.Should().Be(expected.Y2);
+        deserializedValue.Z2.Should().Be(expected.Z2);
+    }
+}

tests/MemoryPack.Tests/GeneratorDiagnosticsTest.cs

@@ -642,6 +642,53 @@ public partial class Tester
 
 """);
     }
+
+    [Fact]
+    public void MEMPACK040_SuppressDefaultInitializationMustBeSettable()
+    {
+        Compile(40, """
+using MemoryPack;
+
+[MemoryPackable]
+public partial class Tester
+{
+    [SuppressDefaultInitialization]
+    public required int I1 { get; set; }
+}
+
+""");
+
+        Compile(40, """
+using MemoryPack;
+
+[MemoryPackable]
+public partial class Tester
+{
+    [SuppressDefaultInitialization]
+    public int I1 { get; init; }
+}
+
+""");
+
+        Compile(40, """
+using MemoryPack;
+
+[MemoryPackable]
+public partial class Tester
+{
+    [SuppressDefaultInitialization]
+    public readonly int I1;
+
+    [MemoryPackConstructor]
+    public Tester(int i1)
+    {
+        I1 = i1;
+    }
+}
+
+""");
+
+    }
 }
 
 

tests/MemoryPack.Tests/Models/DefaultValues.cs

@@ -1,69 +1,43 @@
-using System;
-using System.Collections.Generic;
-
 namespace MemoryPack.Tests.Models;
 
-enum TestEnum
-{
-    A, B, C
-}
-
 [MemoryPackable]
 partial class DefaultValuePlaceholder
 {
     public int X { get; set; }
 }
 
-[MemoryPackable]
-partial class FieldDefaultValue
+[MemoryPackable(GenerateType.VersionTolerant, SerializeLayout.Sequential)]
+partial class DefaultValuePlaceholderWithVersionTolerant
 {
-    public int X;
-    public int Y = 12345;
-    public float Z = 678.9f;
-    public string S = "aaaaaaaaa";
-    public bool B = true;
+    public int X { get; set; }
 }
 
-[MemoryPackable]
-partial class PropertyDefaultValue
+[MemoryPackable(GenerateType.VersionTolerant, SerializeLayout.Sequential)]
+partial class HasDefaultValueWithVersionTolerant
 {
-    internal enum NestedEnum
-    {
-        A, B
-    }
+    public int X;
 
-    public int X { get; set; }
-    public int Y { get; set; } = 12345;
+    public int Y = 12345;
     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!;
+
+    [SuppressDefaultInitialization]
+    public int Y2 = 12345;
+
+    [SuppressDefaultInitialization]
+    public float Z2 { get; set; } = 678.9f;
 }
 
 [MemoryPackable]
-partial class CtorParamDefaultValue
+partial class HasDefaultValue
 {
     public int X;
-    public int Y;
-    public float Z;
-    public string S;
-    public bool B;
-    public decimal D;
-    public DateTime StructValue;
 
-    [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;
-    }
+    public int Y = 12345;
+    public float Z { get; set; } = 678.9f;
+
+    [SuppressDefaultInitialization]
+    public int Y2 = 12345;
+
+    [SuppressDefaultInitialization]
+    public float Z2 { get; set; } = 678.9f;
 }