feat: add MapDerivedType for existing target type mapping

docs/docs/configuration/derived-type-mapping.md

@@ -24,38 +24,44 @@ This can be configured with the `MapDerivedTypeAttribute`:
       [MapDerivedType(typeof(Apple), typeof(AppleDto))] // for c# language level < 11
       // highlight-end
       public static partial FruitDto MapFruit(Fruit source);
-  }
-
-  abstract class Fruit {}
-  class Banana : Fruit {}
-  class Apple : Fruit {}
-
-  abstract class FruitDto {}
-  class BananaDto : FruitDto {}
-  class AppleDto : FruitDto {}
-  ```
-
-  </TabItem>
-  <TabItem value="generated" label="Generated code" default>
-
-  ```csharp
-  [Mapper]
-  public static partial class ModelMapper
-  {
-      public static partial FruitDto MapFruit(Fruit source)
-      {
-          return source switch
-          {
-              Banana x => MapToBananaDto(x),
-              Apple x => MapToAppleDto(x),
-              _ => throw new System.ArgumentException($"Cannot map {source.GetType()} to FruitDto as there is no known derived type mapping", nameof(source)),
-          };
-      }
-
-      // ... implementations of MapToBananaDto and MapToAppleDto
-  }
-  ```
-
+
+      // highlight-start
+      [MapDerivedType<Banana, BananaDto>] // for c# language level ≥ 11
+      [MapDerivedType(typeof(Apple), typeof(AppleDto))] // for c# language level < 11
+      // highlight-end
+      public static partial void MapToFruit(Fruit source, FruitDto target);
+}
+
+abstract class Fruit {}
+class Banana : Fruit {}
+class Apple : Fruit {}
+
+abstract class FruitDto {}
+class BananaDto : FruitDto {}
+class AppleDto : FruitDto {}
+```
+
+</TabItem>
+<TabItem value="generated" label="Generated code" default>
+
+```csharp
+[Mapper]
+public static partial class ModelMapper
+{
+    public static partial FruitDto MapFruit(Fruit source)
+    {
+        return source switch
+        {
+            Banana x => MapToBananaDto(x),
+            Apple x => MapToAppleDto(x),
+            _ => throw new System.ArgumentException($"Cannot map {source.GetType()} to FruitDto as there is no known derived type mapping", nameof(source)),
+        };
+    }
+
+    // ... implementations of MapToBananaDto and MapToAppleDto
+}
+```
+
   </TabItem>
 </Tabs>
 

src/Riok.Mapperly/Descriptors/MappingBuilders/DerivedTypeMappingBuilder.cs

@@ -1,6 +1,7 @@
 using Microsoft.CodeAnalysis;
 using Riok.Mapperly.Configuration;
 using Riok.Mapperly.Descriptors.Mappings;
+using Riok.Mapperly.Descriptors.Mappings.ExistingTarget;
 using Riok.Mapperly.Diagnostics;
 using Riok.Mapperly.Helpers;
 
@@ -19,24 +20,47 @@ public static class DerivedTypeMappingBuilder
             : new DerivedTypeSwitchMapping(ctx.Source, ctx.Target, derivedTypeMappings);
     }
 
+    public static IExistingTargetMapping? TryBuildExistingTargetMapping(MappingBuilderContext ctx)
+    {
+        var derivedTypeMappings = TryBuildExistingTargetContainedMappings(ctx);
+        return derivedTypeMappings == null ? null : new DerivedExistingTargetTypeSwitchMapping(ctx.Source, ctx.Target, derivedTypeMappings);
+    }
+
     public static IReadOnlyCollection<INewInstanceMapping>? TryBuildContainedMappings(
         MappingBuilderContext ctx,
         bool duplicatedSourceTypesAllowed = false
     )
     {
         return ctx.Configuration.DerivedTypes.Count == 0
             ? null
-            : BuildContainedMappings(ctx, ctx.Configuration.DerivedTypes, duplicatedSourceTypesAllowed);
+            : BuildContainedMappings(ctx, ctx.Configuration.DerivedTypes, ctx.FindOrBuildMapping, duplicatedSourceTypesAllowed);
+    }
+
+    private static IReadOnlyCollection<IExistingTargetMapping>? TryBuildExistingTargetContainedMappings(
+        MappingBuilderContext ctx,
+        bool duplicatedSourceTypesAllowed = false
+    )
+    {
+        return ctx.Configuration.DerivedTypes.Count == 0
+            ? null
+            : BuildContainedMappings(
+                ctx,
+                ctx.Configuration.DerivedTypes,
+                (source, target, options, _) => ctx.FindOrBuildExistingTargetMapping(source, target, options),
+                duplicatedSourceTypesAllowed
+            );
     }
 
-    private static IReadOnlyCollection<INewInstanceMapping> BuildContainedMappings(
+    private static IReadOnlyCollection<TMapping> BuildContainedMappings<TMapping>(
         MappingBuilderContext ctx,
         IReadOnlyCollection<DerivedTypeMappingConfiguration> configs,
+        Func<ITypeSymbol, ITypeSymbol, MappingBuildingOptions, Location?, TMapping?> findOrBuildMapping,
         bool duplicatedSourceTypesAllowed
     )
+        where TMapping : ITypeMapping
     {
         var derivedTypeMappingSourceTypes = new HashSet<ITypeSymbol>(SymbolEqualityComparer.Default);
-        var derivedTypeMappings = new List<INewInstanceMapping>(configs.Count);
+        var derivedTypeMappings = new List<TMapping>(configs.Count);
         Func<ITypeSymbol, bool> isAssignableToSource = ctx.Source is ITypeParameterSymbol sourceTypeParameter
             ? t => ctx.SymbolAccessor.DoesTypeSatisfyTypeParameterConstraints(sourceTypeParameter, t, ctx.Source.NullableAnnotation)
             : t => ctx.SymbolAccessor.HasImplicitConversion(t, ctx.Source);
@@ -67,7 +91,7 @@ bool duplicatedSourceTypesAllowed
                 continue;
             }
 
-            var mapping = ctx.FindOrBuildMapping(
+            var mapping = findOrBuildMapping(
                 sourceType,
                 targetType,
                 MappingBuildingOptions.KeepUserSymbol | MappingBuildingOptions.MarkAsReusable | MappingBuildingOptions.ClearDerivedTypes,

src/Riok.Mapperly/Descriptors/MappingBuilders/ExistingTargetMappingBuilder.cs

@@ -9,6 +9,7 @@ public class ExistingTargetMappingBuilder(MappingCollection mappings)
     private static readonly IReadOnlyCollection<BuildExistingTargetMapping> _builders = new BuildExistingTargetMapping[]
     {
         NullableMappingBuilder.TryBuildExistingTargetMapping,
+        DerivedTypeMappingBuilder.TryBuildExistingTargetMapping,
         DictionaryMappingBuilder.TryBuildExistingTargetMapping,
         SpanMappingBuilder.TryBuildExistingTargetMapping,
         MemoryMappingBuilder.TryBuildExistingTargetMapping,

src/Riok.Mapperly/Descriptors/Mappings/DerivedExistingTargetTypeSwitchMapping.cs

@@ -0,0 +1,82 @@
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Riok.Mapperly.Descriptors.Mappings.ExistingTarget;
+using Riok.Mapperly.Emit.Syntax;
+using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
+using static Riok.Mapperly.Emit.Syntax.SyntaxFactoryHelper;
+
+namespace Riok.Mapperly.Descriptors.Mappings;
+
+/// <summary>
+/// A derived type mapping maps one base type or interface to another
+/// by implementing a switch statement over known types and performs the provided mapping for each type.
+/// </summary>
+public class DerivedExistingTargetTypeSwitchMapping(
+    ITypeSymbol sourceType,
+    ITypeSymbol targetType,
+    IReadOnlyCollection<IExistingTargetMapping> existingTargetTypeMappings
+) : ExistingTargetMapping(sourceType, targetType)
+{
+    private const string SourceName = "source";
+    private const string TargetName = "target";
+    private const string GetTypeMethodName = nameof(GetType);
+
+    public override IEnumerable<StatementSyntax> Build(TypeMappingBuildContext ctx, ExpressionSyntax target)
+    {
+        var sourceExpression = TupleExpression(CommaSeparatedList(Argument(ctx.Source), Argument(target)));
+        var caseSections = existingTargetTypeMappings.Select(x => BuildSwitchSection(ctx, x));
+        var defaultSection = BuildDefaultSwitchSection(ctx, target);
+
+        yield return ctx.SyntaxFactory
+            .SwitchStatement(sourceExpression, caseSections, defaultSection)
+            .AddLeadingLineFeed(ctx.SyntaxFactory.Indentation);
+    }
+
+    private SwitchSectionSyntax BuildSwitchSection(TypeMappingBuildContext ctx, IExistingTargetMapping mapping)
+    {
+        var (sectionCtx, sourceVariableName) = ctx.WithNewScopedSource(SourceName);
+        var targetVariableName = sectionCtx.NameBuilder.New(TargetName);
+        sectionCtx = sectionCtx.AddIndentation();
+
+        // (A source, B target)
+        var positionalTypeMatch = PositionalPatternClause(
+            CommaSeparatedList(
+                Subpattern(DeclarationPattern(mapping.SourceType, sourceVariableName)),
+                Subpattern(DeclarationPattern(mapping.TargetType, targetVariableName))
+            )
+        );
+        var pattern = RecursivePattern().WithPositionalPatternClause(positionalTypeMatch);
+
+        // case (A source, B target):
+        var caseLabel = CasePatternSwitchLabel(pattern).AddLeadingLineFeed(sectionCtx.SyntaxFactory.Indentation);
+
+        // break;
+        var statementContext = sectionCtx.AddIndentation();
+        var breakStatement = BreakStatement().AddLeadingLineFeed(statementContext.SyntaxFactory.Indentation);
+        var target = IdentifierName(targetVariableName);
+        var statements = mapping.Build(statementContext, target).Append(breakStatement);
+
+        return SwitchSection(caseLabel, statements);
+    }
+
+    private SwitchSectionSyntax BuildDefaultSwitchSection(TypeMappingBuildContext ctx, ExpressionSyntax target)
+    {
+        // default:
+        var sectionCtx = ctx.SyntaxFactory.AddIndentation();
+        var defaultCaseLabel = DefaultSwitchLabel().AddLeadingLineFeed(sectionCtx.Indentation);
+
+        // throw new ArgumentException(msg, nameof(ctx.Source)),
+        var sourceType = Invocation(MemberAccess(ctx.Source, GetTypeMethodName));
+        var targetType = Invocation(MemberAccess(target, GetTypeMethodName));
+        var statementContext = sectionCtx.AddIndentation();
+        var throwExpression = ThrowArgumentExpression(
+                InterpolatedString($"Cannot map {sourceType} to {targetType} as there is no known derived type mapping"),
+                ctx.Source
+            )
+            .AddLeadingLineFeed(statementContext.Indentation);
+
+        var statements = new StatementSyntax[] { ExpressionStatement(throwExpression) };
+
+        return SwitchSection(defaultCaseLabel, statements);
+    }
+}

src/Riok.Mapperly/Descriptors/Mappings/TypeMappingBuildContext.cs

@@ -45,20 +45,26 @@ SyntaxFactoryHelper syntaxFactory
     /// builds the name of the source in this new scope
     /// and creates a new context with the new source.
     /// </summary>
+    /// <param name="sourceName">The name for the new scoped source.</param>
     /// <returns>The new context and the scoped name of the source.</returns>
-    public (TypeMappingBuildContext Context, string SourceName) WithNewScopedSource() => WithNewScopedSource(IdentifierName);
+    public (TypeMappingBuildContext Context, string SourceName) WithNewScopedSource(string sourceName = DefaultSourceName) =>
+        WithNewScopedSource(IdentifierName, sourceName);
 
     /// <summary>
     /// Creates a new scoped name builder,
     /// builds the name of the source in this new scope
     /// and creates a new context with the new source.
     /// </summary>
     /// <param name="sourceBuilder">A function to build the source access for the new context.</param>
+    /// <param name="sourceName">The name for the new scoped source.</param>
     /// <returns>The new context and the scoped name of the source.</returns>
-    public (TypeMappingBuildContext Context, string SourceName) WithNewScopedSource(Func<string, ExpressionSyntax> sourceBuilder)
+    public (TypeMappingBuildContext Context, string SourceName) WithNewScopedSource(
+        Func<string, ExpressionSyntax> sourceBuilder,
+        string sourceName = DefaultSourceName
+    )
     {
         var scopedNameBuilder = NameBuilder.NewScope();
-        var scopedSourceName = scopedNameBuilder.New(DefaultSourceName);
+        var scopedSourceName = scopedNameBuilder.New(sourceName);
         var ctx = new TypeMappingBuildContext(sourceBuilder(scopedSourceName), ReferenceHandler, scopedNameBuilder, SyntaxFactory);
         return (ctx, scopedSourceName);
     }

src/Riok.Mapperly/Emit/Syntax/SyntaxFactoryHelper.Pattern.cs

@@ -1,3 +1,4 @@
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Riok.Mapperly.Helpers;
@@ -16,6 +17,12 @@ public static PatternSyntax OrPattern(IEnumerable<ExpressionSyntax?> values) =>
     public static IsPatternExpressionSyntax IsPattern(ExpressionSyntax expression, PatternSyntax pattern) =>
         IsPatternExpression(expression, SpacedToken(SyntaxKind.IsKeyword), pattern);
 
+    public static DeclarationPatternSyntax DeclarationPattern(ITypeSymbol type, string designation) =>
+        SyntaxFactory.DeclarationPattern(
+            FullyQualifiedIdentifier(type).AddTrailingSpace(),
+            SingleVariableDesignation(Identifier(designation))
+        );
+
     private static BinaryPatternSyntax BinaryPattern(SyntaxKind kind, PatternSyntax left, PatternSyntax right)
     {
         var binaryPattern = SyntaxFactory.BinaryPattern(kind, left, right);

src/Riok.Mapperly/Emit/Syntax/SyntaxFactoryHelper.Switch.cs

@@ -23,4 +23,28 @@ public static SwitchExpressionArmSyntax SwitchArm(PatternSyntax pattern, Express
     }
 
     public static WhenClauseSyntax SwitchWhen(ExpressionSyntax condition) => WhenClause(SpacedToken(SyntaxKind.WhenKeyword), condition);
+
+    public SwitchStatementSyntax SwitchStatement(
+        ExpressionSyntax governingExpression,
+        IEnumerable<SwitchSectionSyntax> sections,
+        SwitchSectionSyntax defaultSection
+    )
+    {
+        return SyntaxFactory.SwitchStatement(
+            default,
+            TrailingSpacedToken(SyntaxKind.SwitchKeyword),
+            Token(SyntaxKind.None),
+            governingExpression,
+            Token(SyntaxKind.None),
+            LeadingLineFeedToken(SyntaxKind.OpenBraceToken),
+            List(sections.Append(defaultSection)),
+            LeadingLineFeedToken(SyntaxKind.CloseBraceToken)
+        );
+    }
+
+    public static SwitchSectionSyntax SwitchSection(SwitchLabelSyntax labelSyntax, IEnumerable<StatementSyntax> statements) =>
+        SyntaxFactory.SwitchSection().WithLabels(SingletonList(labelSyntax)).WithStatements(List(statements));
+
+    public static CasePatternSwitchLabelSyntax CasePatternSwitchLabel(PatternSyntax pattern) =>
+        SyntaxFactory.CasePatternSwitchLabel(TrailingSpacedToken(SyntaxKind.CaseKeyword), pattern, null, Token(SyntaxKind.ColonToken));
 }

test/Riok.Mapperly.IntegrationTests/Mapper/StaticTestMapper.cs

@@ -91,6 +91,15 @@ public static void MapExistingList(List<string> src, List<int> dst)
 
         public static partial TTarget MapGeneric<TSource, TTarget>(TSource source);
 
+#if NET7_0_OR_GREATER
+        [MapDerivedType<ExistingObjectTypeA, ExistingObjectTypeA>]
+        [MapDerivedType<ExistingObjectTypeB, ExistingObjectTypeB>]
+#else
+        [MapDerivedType(typeof(ExistingObjectTypeA), typeof(ExistingObjectTypeA))]
+        [MapDerivedType(typeof(ExistingObjectTypeB), typeof(ExistingObjectTypeB))]
+#endif
+        public static partial void MapToDerivedExisting(ExistingObjectBase source, ExistingObjectBase target);
+
         [MapEnum(EnumMappingStrategy.ByName)]
         public static partial TestEnumDtoByName MapToEnumDtoByName(TestEnum v);
 

test/Riok.Mapperly.IntegrationTests/Models/ExistingObjectBase.cs

@@ -0,0 +1,7 @@
+namespace Riok.Mapperly.IntegrationTests.Models
+{
+    public class ExistingObjectBase
+    {
+        public int Value { get; set; }
+    }
+}

test/Riok.Mapperly.IntegrationTests/Models/ExistingObjectTypeA.cs

@@ -0,0 +1,7 @@
+namespace Riok.Mapperly.IntegrationTests.Models
+{
+    public class ExistingObjectTypeA : ExistingObjectBase
+    {
+        public int ValueA { get; set; }
+    }
+}

test/Riok.Mapperly.IntegrationTests/Models/ExistingObjectTypeB.cs

@@ -0,0 +1,7 @@
+namespace Riok.Mapperly.IntegrationTests.Models
+{
+    public class ExistingObjectTypeB : ExistingObjectBase
+    {
+        public int ValueB { get; set; }
+    }
+}

test/Riok.Mapperly.IntegrationTests/_snapshots/StaticMapperTest.SnapshotGeneratedSource_NET6_0.verified.cs

@@ -569,6 +569,23 @@ object x when typeof(TTarget).IsAssignableFrom(typeof(object)) => (TTarget)(obje
             };
         }
 
+        public static partial void MapToDerivedExisting(global::Riok.Mapperly.IntegrationTests.Models.ExistingObjectBase source, global::Riok.Mapperly.IntegrationTests.Models.ExistingObjectBase target)
+        {
+            switch (source, target)
+            {
+                case (global::Riok.Mapperly.IntegrationTests.Models.ExistingObjectTypeA source1, global::Riok.Mapperly.IntegrationTests.Models.ExistingObjectTypeA target1):
+                    target1.ValueA = DirectInt(source1.ValueA);
+                    target1.Value = DirectInt(source1.Value);
+                    break;
+                case (global::Riok.Mapperly.IntegrationTests.Models.ExistingObjectTypeB source1, global::Riok.Mapperly.IntegrationTests.Models.ExistingObjectTypeB target1):
+                    target1.ValueB = DirectInt(source1.ValueB);
+                    target1.Value = DirectInt(source1.Value);
+                    break;
+                default:
+                    throw new System.ArgumentException($"Cannot map {source.GetType()} to {target.GetType()} as there is no known derived type mapping", nameof(source));
+            }
+        }
+
         public static partial global::Riok.Mapperly.IntegrationTests.Dto.TestEnumDtoByName MapToEnumDtoByName(global::Riok.Mapperly.IntegrationTests.Models.TestEnum v)
         {
             return v switch