From 8505c75b152b46e3de97f7df9f3718c14ec8057c Mon Sep 17 00:00:00 2001
From: Tim Schneider <43130816+DerStimmler@users.noreply.github.com>
Date: Mon, 25 Aug 2025 00:31:23 +0200
Subject: [PATCH 1/3] perf: reuse IResultErrorMapper instances

---
 .../ToAcceptedAtRouteHttpResultTE.cs          |  4 +-
 .../ToAcceptedHttpResultTE.cs                 |  4 +-
 .../ToContentHttpResultStringE.cs             |  2 +-
 .../ToCreatedAtRouteHttpResultTE.cs           |  4 +-
 .../ResultExtensions/ToCreatedHttpResultTE.cs |  2 +-
 .../ToFileHttpResultByteArrayE.cs             |  4 +-
 .../ToFileStreamHttpResultStreamE.cs          |  4 +-
 .../ResultExtensions/ToJsonHttpResultTE.cs    |  4 +-
 .../ToNoContentHttpResultTE.cs                |  4 +-
 .../ResultExtensions/ToOkHttpResultTE.cs      |  4 +-
 .../ToStatusCodeHttpResultTE.cs               |  4 +-
 .../ResultExtensionsGenerator.cs              | 37 +++++++++++++++++++
 .../ToNoContentHttpResultE.cs                 |  4 +-
 .../ToStatusCodeHttpResultE.cs                |  4 +-
 14 files changed, 61 insertions(+), 24 deletions(-)

diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedAtRouteHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedAtRouteHttpResultTE.cs
index a743286..c18526c 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedAtRouteHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedAtRouteHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<AcceptedAtRoute<T>, {{httpResultType}}> ToAcceptedAtRouteHttpResult<T>(this Result<T,{{resultErrorType}}> result, string? routeName = null, Func<T, object>? routeValues = null)
       {
           if (result.IsSuccess) return TypedResults.AcceptedAtRoute(result.Value, routeName, routeValues?.Invoke(result.Value));
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedHttpResultTE.cs
index a12fcf6..fe71561 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToAcceptedHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<Accepted<T>, {{httpResultType}}> ToAcceptedHttpResult<T>(this Result<T,{{resultErrorType}}> result, Func<T, Uri> uri)
       {
           if (result.IsSuccess) return TypedResults.Accepted(uri(result.Value), result.Value);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToContentHttpResultStringE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToContentHttpResultStringE.cs
index 47cddb9..906738c 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToContentHttpResultStringE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToContentHttpResultStringE.cs
@@ -12,7 +12,7 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       {
           if (result.IsSuccess) return TypedResults.Content(result.Value, contentType, contentEncoding, statusCode);
 
-          return new {{mapperClassName}}().Map(result.Error);
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedAtRouteHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedAtRouteHttpResultTE.cs
index c10c879..3b5c8ab 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedAtRouteHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedAtRouteHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<CreatedAtRoute<T>, {{httpResultType}}> ToCreatedAtRouteHttpResult<T>(this Result<T,{{resultErrorType}}> result, string? routeName = null, Func<T, object>? routeValues = null)
       {
           if (result.IsSuccess) return TypedResults.CreatedAtRoute(result.Value, routeName, routeValues?.Invoke(result.Value));
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedHttpResultTE.cs
index a8ced8c..f2a98b6 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToCreatedHttpResultTE.cs
@@ -15,7 +15,7 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
               ? TypedResults.Created(string.Empty, result.Value)
               : TypedResults.Created(uri.Invoke(result.Value), result.Value);
 
-          return new {{mapperClassName}}().Map(result.Error);
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileHttpResultByteArrayE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileHttpResultByteArrayE.cs
index 01a3b6b..2f5052f 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileHttpResultByteArrayE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileHttpResultByteArrayE.cs
@@ -14,8 +14,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
           bool enableRangeProcessing = false)
       {
           if (result.IsSuccess) return TypedResults.File(result.Value, contentType, fileDownloadName, enableRangeProcessing, lastModified, entityTag);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileStreamHttpResultStreamE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileStreamHttpResultStreamE.cs
index 7653832..fb41e69 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileStreamHttpResultStreamE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToFileStreamHttpResultStreamE.cs
@@ -14,8 +14,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
           bool enableRangeProcessing = false) where T : Stream
       {
           if (result.IsSuccess) return TypedResults.Stream(result.Value, contentType, fileDownloadName, lastModified, entityTag, enableRangeProcessing);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToJsonHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToJsonHttpResultTE.cs
index cbb423c..857c1f0 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToJsonHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToJsonHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<JsonHttpResult<T>, {{httpResultType}}> ToJsonHttpResult<T>(this Result<T,{{resultErrorType}}> result, int successStatusCode = 200)
       {
           if (result.IsSuccess) return TypedResults.Json(result.Value, statusCode: successStatusCode);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToNoContentHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToNoContentHttpResultTE.cs
index 2ad7f32..f9944ab 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToNoContentHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToNoContentHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<NoContent, {{httpResultType}}> ToNoContentHttpResult<T>(this Result<T,{{resultErrorType}}> result)
       {
           if (result.IsSuccess) return TypedResults.NoContent();
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToOkHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToOkHttpResultTE.cs
index f21d08c..8a3e723 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToOkHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToOkHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<Ok<T>, {{httpResultType}}> ToOkHttpResult<T>(this Result<T,{{resultErrorType}}> result)
       {
           if (result.IsSuccess) return TypedResults.Ok(result.Value);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToStatusCodeHttpResultTE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToStatusCodeHttpResultTE.cs
index 6de6fab..ee59d4f 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToStatusCodeHttpResultTE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensions/ToStatusCodeHttpResultTE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<StatusCodeHttpResult, {{httpResultType}}> ToStatusCodeHttpResult<T>(this Result<T,{{resultErrorType}}> result, int successStatusCode = 204)
       {
           if (result.IsSuccess) return TypedResults.StatusCode(successStatusCode);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGenerator.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGenerator.cs
index 79c400b..5313fd8 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGenerator.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGenerator.cs
@@ -64,6 +64,9 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
         if (!ResultExtensionsGeneratorValidator.CheckRules(mapperClasses, context))
           return;
 
+        var (fileName, sourceText) = CreateErrorMapperInstancesClass(mapperClasses, requiredNamespaces);
+        context.AddSource(fileName, SourceText.From(sourceText, Encoding.UTF8));
+
         var classBuilders = new List<ClassBuilder>
         {
           new ResultExtensionsClassBuilder(requiredNamespaces, mapperClasses),
@@ -76,6 +79,40 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
     );
   }
 
+  /// <summary>
+  ///   Creates a class to get singleton instances of the various <see cref="IResultErrorMapper{T}" />
+  /// </summary>
+  private static (string FileName, string SourceText) CreateErrorMapperInstancesClass(
+    List<ClassDeclarationSyntax> mapperClasses,
+    HashSet<string> requiredNamespaces
+  )
+  {
+    var sourceBuilder = new StringBuilder();
+
+    sourceBuilder.AppendLine("// <auto-generated/>");
+    sourceBuilder.AppendLine();
+    sourceBuilder.AppendLine("#nullable enable");
+    sourceBuilder.AppendLine();
+    requiredNamespaces
+      .Where(@namespace => !@namespace.StartsWith("global"))
+      .Distinct()
+      .Select(@namespace => $"using {@namespace};")
+      .ToList()
+      .ForEach(@using => sourceBuilder.AppendLine(@using));
+    sourceBuilder.AppendLine();
+
+    sourceBuilder.AppendLine("public static class ErrorMapperInstances {");
+
+    foreach (var mapperName in mapperClasses)
+      sourceBuilder.AppendLine(
+        $"    public static {mapperName.Identifier.Text} {mapperName.Identifier.Text} {{ get; }} = new();"
+      );
+
+    sourceBuilder.AppendLine("}");
+
+    return ("ErrorMapperInstances.g.cs", sourceBuilder.ToString());
+  }
+
   /// <summary>
   ///   Checks if a class implements the <see cref="IResultErrorMapper{T}" /> interface.
   /// </summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToNoContentHttpResultE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToNoContentHttpResultE.cs
index 4183db7..4e5b114 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToNoContentHttpResultE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToNoContentHttpResultE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<NoContent, {{httpResultType}}> ToNoContentHttpResult(this UnitResult<{{resultErrorType}}> result)
       {
           if (result.IsSuccess) return TypedResults.NoContent();
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToStatusCodeHttpResultE.cs b/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToStatusCodeHttpResultE.cs
index d01ab36..026b320 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToStatusCodeHttpResultE.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/UnitResultExtensions/ToStatusCodeHttpResultE.cs
@@ -11,8 +11,8 @@ public string Generate(string mapperClassName, string resultErrorType, string ht
       public static Results<StatusCodeHttpResult, {{httpResultType}}> ToStatusCodeHttpResult(this UnitResult<{{resultErrorType}}> result, int successStatusCode = 204)
       {
           if (result.IsSuccess) return TypedResults.StatusCode(successStatusCode);
-          
-          return new {{mapperClassName}}().Map(result.Error);
+
+          return ErrorMapperInstances.{{mapperClassName}}.Map(result.Error);
       }
 
       /// <summary>

From 5da69a3f50c254c0a827065934c7a3c17971fe24 Mon Sep 17 00:00:00 2001
From: Tim Schneider <43130816+DerStimmler@users.noreply.github.com>
Date: Mon, 25 Aug 2025 01:01:19 +0200
Subject: [PATCH 2/3] feat: add analyzer to check for parameterless constructor
 in mapper class

---
 .../ParameterlessConstructorRuleTests.cs      | 60 +++++++++++++++++++
 .../AnalyzerReleases.Shipped.md               |  8 +++
 .../ResultExtensionsGeneratorValidator.cs     |  2 +-
 .../Rules/ParameterlessConstructorRule.cs     | 45 ++++++++++++++
 README.md                                     |  4 +-
 5 files changed, 116 insertions(+), 3 deletions(-)
 create mode 100644 CSharpFunctionalExtensions.HttpResults.Generators.Tests/Rules/ParameterlessConstructorRuleTests.cs
 create mode 100644 CSharpFunctionalExtensions.HttpResults.Generators/Rules/ParameterlessConstructorRule.cs

diff --git a/CSharpFunctionalExtensions.HttpResults.Generators.Tests/Rules/ParameterlessConstructorRuleTests.cs b/CSharpFunctionalExtensions.HttpResults.Generators.Tests/Rules/ParameterlessConstructorRuleTests.cs
new file mode 100644
index 0000000..9eec9b3
--- /dev/null
+++ b/CSharpFunctionalExtensions.HttpResults.Generators.Tests/Rules/ParameterlessConstructorRuleTests.cs
@@ -0,0 +1,60 @@
+using FluentAssertions;
+using Microsoft.CodeAnalysis;
+
+namespace CSharpFunctionalExtensions.HttpResults.Generators.Tests.Rules;
+
+public class ParameterlessConstructorRuleTests
+{
+  [Fact]
+  public void TestParameterlessConstructor()
+  {
+    var sourceCode = """
+      using Microsoft.AspNetCore.Http;
+      using Microsoft.AspNetCore.Http.HttpResults;
+      using CSharpFunctionalExtensions.HttpResults;
+
+      public class DocumentCreationError
+      {
+        public required string DocumentId { get; init; }
+      }
+
+      // Not parameterless --> error
+      public class DocumentCreationErrorMapper : IResultErrorMapper<DocumentCreationError, Conflict<string>>
+      {
+        public DocumentCreationErrorMapper(string foo) { }
+
+        public Conflict<string> Map(DocumentCreationError error) => TypedResults.Conflict(error.DocumentId);
+      }
+
+      // Explicit parameterless --> No error
+      public class DocumentCreationErrorMapper2 : IResultErrorMapper<string, Conflict<string>>
+      {
+        public DocumentCreationErrorMapper2() { }
+
+        public Conflict<string> Map(string error) => TypedResults.Conflict(error.DocumentId);
+      }
+
+      // Explicit parameterless & with parameters --> No error
+      public class DocumentCreationErrorMapper3 : IResultErrorMapper<int, Conflict<string>>
+      {
+        public DocumentCreationErrorMapper3() { }
+        public DocumentCreationErrorMapper3(string foo) { }
+
+        public Conflict<string> Map(int error) => TypedResults.Conflict(error.DocumentId);
+      }
+      """;
+
+    var diagnostics = ResultExtensionsGeneratorTestHelper.RunGenerator(sourceCode).ToList();
+
+    diagnostics.Count.Should().Be(1);
+
+    var diagnostic = diagnostics[0];
+
+    diagnostic.Id.Should().Be("CFEHTTPR004");
+    diagnostic.Severity.Should().Be(DiagnosticSeverity.Error);
+    diagnostic
+      .GetMessage()
+      .Should()
+      .Be("Class 'DocumentCreationErrorMapper' does not have a parameterless constructor");
+  }
+}
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/AnalyzerReleases.Shipped.md b/CSharpFunctionalExtensions.HttpResults.Generators/AnalyzerReleases.Shipped.md
index 7746192..068e2b2 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/AnalyzerReleases.Shipped.md
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/AnalyzerReleases.Shipped.md
@@ -44,3 +44,11 @@
 | Rule ID     | Category | Severity | Notes                                  |
 |-------------|----------|----------|----------------------------------------|
 | CFEHTTPR003 | Mapping  | Error    | Empty Map getter in IResultErrorMapper |
+
+## Release v1.0.1
+
+### New Rules
+
+| Rule ID     | Category | Severity | Notes                                                   |
+|-------------|----------|----------|---------------------------------------------------------|
+| CFEHTTPR004 | Mapping  | Error    | Missing parameterless constructor in IResultErrorMapper |
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGeneratorValidator.cs b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGeneratorValidator.cs
index d2ef140..15429eb 100644
--- a/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGeneratorValidator.cs
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/ResultExtensionsGeneratorValidator.cs
@@ -9,7 +9,7 @@ namespace CSharpFunctionalExtensions.HttpResults.Generators;
 /// </summary>
 internal static class ResultExtensionsGeneratorValidator
 {
-  private static readonly List<IRule> Rules = [new DuplicateMapperRule()];
+  private static readonly List<IRule> Rules = [new DuplicateMapperRule(), new ParameterlessConstructorRule()];
 
   /// <summary>
   ///   Validates the rules for the generator.
diff --git a/CSharpFunctionalExtensions.HttpResults.Generators/Rules/ParameterlessConstructorRule.cs b/CSharpFunctionalExtensions.HttpResults.Generators/Rules/ParameterlessConstructorRule.cs
new file mode 100644
index 0000000..135d8ce
--- /dev/null
+++ b/CSharpFunctionalExtensions.HttpResults.Generators/Rules/ParameterlessConstructorRule.cs
@@ -0,0 +1,45 @@
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace CSharpFunctionalExtensions.HttpResults.Generators.Rules;
+
+internal class ParameterlessConstructorRule : IRule
+{
+  public DiagnosticDescriptor RuleDescriptor { get; } =
+    new(
+      "CFEHTTPR004",
+      "Missing parameterless constructor in IResultErrorMapper",
+      "Class '{0}' does not have a parameterless constructor",
+      "Mapping",
+      DiagnosticSeverity.Error,
+      true,
+      customTags: ["CompilationEnd"]
+    );
+
+  public IEnumerable<Diagnostic> Check(List<ClassDeclarationSyntax> mapperClasses)
+  {
+    foreach (var mapperClass in mapperClasses)
+    {
+      if (!HasParameterlessConstructor(mapperClass))
+        yield return Diagnostic.Create(
+          RuleDescriptor,
+          mapperClass.Identifier.GetLocation(),
+          mapperClass.Identifier.Text
+        );
+    }
+  }
+
+  private static bool HasParameterlessConstructor(ClassDeclarationSyntax classDeclaration)
+  {
+    var hasExplicitParameterless = classDeclaration
+      .Members.OfType<ConstructorDeclarationSyntax>()
+      .Any(c => c.ParameterList.Parameters.Count == 0);
+
+    if (hasExplicitParameterless)
+      return true;
+
+    var hasAnyConstructors = classDeclaration.Members.OfType<ConstructorDeclarationSyntax>().Any();
+
+    return !hasAnyConstructors;
+  }
+}
diff --git a/README.md b/README.md
index d20cdd6..a69afbe 100644
--- a/README.md
+++ b/README.md
@@ -201,7 +201,7 @@ When using `Result<T,E>` or `UnitResult<E>`, this library uses a Source Generato
         };
     }
     ```
-3. Use the auto generated extension method:
+3. Use the auto-generated extension method:
     ```csharp
     app.MapGet("/users/{id}", (string id, UserRepository repo) =>
         repo.Find(id)       //Result<User,UserNotFoundError>
@@ -222,7 +222,7 @@ When using `Result<T,E>` or `UnitResult<E>`, this library uses a Source Generato
 
 This library includes analyzers to help you use it correctly.
 
-For example, they will notify you if you have multiple mappers for the same custom error type.
+For example, they will notify you if you have multiple mappers for the same custom error type or if your mapper class doesn't have a parameterless constructor.
 
 You can find a complete list of all analyzers [here](https://github.com/co-IT/CSharpFunctionalExtensions.HttpResults/blob/main/CSharpFunctionalExtensions.HttpResults.Generators/AnalyzerReleases.Shipped.md).
 

From d6403e8168e05017e1b56a038d3041e00b79ef4e Mon Sep 17 00:00:00 2001
From: Tim Schneider <43130816+DerStimmler@users.noreply.github.com>
Date: Mon, 25 Aug 2025 01:18:24 +0200
Subject: [PATCH 3/3] ci: disable lychee fragment checks temporarily

---
 .github/workflows/check.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/check.yml b/.github/workflows/check.yml
index 4edbbc5..d37f70d 100644
--- a/.github/workflows/check.yml
+++ b/.github/workflows/check.yml
@@ -52,6 +52,6 @@ jobs:
         uses: lycheeverse/lychee-action@v2
         with:
           fail: true
-          args: --remap '${{ github.event.repository.default_branch }} ${{ github.head_ref || github.ref_name }}' --include-fragments .
+          args: --remap '${{ github.event.repository.default_branch }} ${{ github.head_ref || github.ref_name }}' .
           jobSummary: true
           format: markdown