From 621ecb38034dd24175be840c03d2726ca26e8f10 Mon Sep 17 00:00:00 2001
From: Andreas Ravnestad <andreas.ravnestad@gmail.com>
Date: Thu, 20 Aug 2026 10:37:26 +0200
Subject: [PATCH] fix: Handle mapper interface generation for internal types

---
 .../ExpressionTranslator.cs                   | 18 ++++++----
 .../Mappers/IIssue399Mapper.cs                | 10 ++++++
 .../Mappers/Issue399Mapper.cs                 | 24 +++++++++++++
 .../WhenMapperInterfaceUsesInternalTypes.cs   | 34 +++++++++++++++++++
 4 files changed, 80 insertions(+), 6 deletions(-)
 create mode 100644 src/Mapster.Tool.Tests/Mappers/IIssue399Mapper.cs
 create mode 100644 src/Mapster.Tool.Tests/Mappers/Issue399Mapper.cs
 create mode 100644 src/Mapster.Tool.Tests/WhenMapperInterfaceUsesInternalTypes.cs

diff --git a/src/ExpressionTranslator/ExpressionTranslator.cs b/src/ExpressionTranslator/ExpressionTranslator.cs
index 6c219f86..7a1e3e36 100644
--- a/src/ExpressionTranslator/ExpressionTranslator.cs
+++ b/src/ExpressionTranslator/ExpressionTranslator.cs
@@ -56,6 +56,15 @@ private void ResetIndentLevel()
                 _indentLevel++;
         }
 
+        private bool IsGeneratedTypeInternal()
+        {
+            if (Definitions == null)
+                return false;
+
+            return Definitions.IsInternal ||
+                   Definitions.Implements?.Any(it => !it.GetTypeInfo().IsVisible) == true;
+        }
+
         public static ExpressionTranslator Create(Expression node, ExpressionDefinitions? definitions = null)
         {
             var translator = new ExpressionTranslator(definitions);
@@ -1201,7 +1210,7 @@ public Expression VisitLambda(LambdaExpression node, LambdaType type, string? me
                 if (type == LambdaType.PublicLambda)
                 {
                     var name = methodName ?? "Main";
-                    if (!isInternal)
+                    if (!isInternal && !IsGeneratedTypeInternal())
                         isInternal = node.ReturnType.GetTypeInfo().IsNotPublic ||
                                      node.Parameters.Any(it => it.Type.GetTypeInfo().IsNotPublic);
                     WriteModifierNextLine(isInternal ? "internal" : "public");
@@ -1235,7 +1244,7 @@ public Expression VisitLambda(LambdaExpression node, LambdaType type, string? me
                 var name = methodName ?? "Main";
                 if (type == LambdaType.PublicMethod || type == LambdaType.ExtensionMethod)
                 {
-                    if (!isInternal)
+                    if (!isInternal && !IsGeneratedTypeInternal())
                         isInternal = node.ReturnType.GetTypeInfo().IsNotPublic ||
                                      node.Parameters.Any(it => it.Type.GetTypeInfo().IsNotPublic);
                     WriteModifierNextLine(isInternal ? "internal" : "public");
@@ -1891,10 +1900,7 @@ public override string ToString()
                         Indent();
                     }
 
-                    var isInternal = Definitions.IsInternal;
-                    if (!isInternal)
-                        isInternal = Definitions.Implements?.Any(it =>
-                            !it.GetTypeInfo().IsInterface && !it.GetTypeInfo().IsPublic) ?? false;
+                    var isInternal = IsGeneratedTypeInternal();
                     WriteModifierNextLine(isInternal ? "internal" : "public");
                     Write("partial ", Definitions.IsRecordType ? "record " : "class ", Definitions.TypeName);
                     if (Definitions.IsRecordType && ctorParams?.Count > 0)
diff --git a/src/Mapster.Tool.Tests/Mappers/IIssue399Mapper.cs b/src/Mapster.Tool.Tests/Mappers/IIssue399Mapper.cs
new file mode 100644
index 00000000..9d4dee1c
--- /dev/null
+++ b/src/Mapster.Tool.Tests/Mappers/IIssue399Mapper.cs
@@ -0,0 +1,10 @@
+using System.Linq.Expressions;
+
+namespace Mapster.Tool.Tests.Mappers;
+
+[Mapper]
+internal interface IIssue399Mapper
+{
+    Expression<Func<Issue399Source, Issue399Destination>> ProjectToDestination { get; }
+    Issue399Destination Map(Issue399Source source);
+}
diff --git a/src/Mapster.Tool.Tests/Mappers/Issue399Mapper.cs b/src/Mapster.Tool.Tests/Mappers/Issue399Mapper.cs
new file mode 100644
index 00000000..bdcd5a94
--- /dev/null
+++ b/src/Mapster.Tool.Tests/Mappers/Issue399Mapper.cs
@@ -0,0 +1,24 @@
+using System;
+using System.Linq.Expressions;
+using Mapster.Tool.Tests;
+using Mapster.Tool.Tests.Mappers;
+
+namespace Mapster.Tool.Tests.Mappers
+{
+    internal partial class Issue399Mapper : IIssue399Mapper
+    {
+        public Expression<Func<Issue399Source, Issue399Destination>> ProjectToDestination => p1 => new Issue399Destination()
+        {
+            Id = p1.Id,
+            Name = p1.Name
+        };
+        public Issue399Destination Map(Issue399Source p2)
+        {
+            return p2 == null ? null : new Issue399Destination()
+            {
+                Id = p2.Id,
+                Name = p2.Name
+            };
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Mapster.Tool.Tests/WhenMapperInterfaceUsesInternalTypes.cs b/src/Mapster.Tool.Tests/WhenMapperInterfaceUsesInternalTypes.cs
new file mode 100644
index 00000000..00b09672
--- /dev/null
+++ b/src/Mapster.Tool.Tests/WhenMapperInterfaceUsesInternalTypes.cs
@@ -0,0 +1,34 @@
+using FluentAssertions;
+using Mapster.Tool.Tests.Mappers;
+
+namespace Mapster.Tool.Tests;
+
+/// <summary>
+/// Tests for https://github.com/MapsterMapper/Mapster/issues/399
+/// </summary>
+public class WhenMapperInterfaceUsesInternalTypes : TestBase
+{
+    [Fact]
+    public void MapperInterfaceImplementationUsesInternalTypes()
+    {
+        IIssue399Mapper mapper = new Issue399Mapper();
+        var source = new Issue399Source { Id = 1, Name = "Test" };
+
+        var destination = mapper.Map(source);
+
+        destination.Id.Should().Be(source.Id);
+        destination.Name.Should().Be(source.Name);
+    }
+}
+
+internal class Issue399Source
+{
+    public int Id { get; init; }
+    public string? Name { get; init; }
+}
+
+internal class Issue399Destination
+{
+    public int Id { get; init; }
+    public string? Name { get; init; }
+}