Add DiagnosticAnalyzer for Arbiter.Mapping.Generators to validate mapper classes by pwelter34 · Pull Request #382 · loresoft/Arbiter

pwelter34 · 2026-04-03T16:04:50Z

Introduces a Roslyn DiagnosticAnalyzer (separate from the source generator pipeline)
that validates [GenerateMapper] classes at compile time, preserving generator cache:

ARB0001: Mapper class must be partial
ARB0002: Mapper class must inherit MapperProfile<TSource, TDestination>
ARB0003: ConfigureMapping contains unsupported statements (not mapping calls)
ARB0004: Destination property in Property() does not exist on destination type
ARB0005: Duplicate destination property mapping
ARB0006: Unrecognized mapping call pattern (not From/Value/Ignore)
ARB0007: Source property in From() does not exist on source type

https://claude.ai/code/session_01SCR7bdSkrCSmNazN911Y2e

…per classes Introduces a Roslyn DiagnosticAnalyzer (separate from the source generator pipeline) that validates [GenerateMapper] classes at compile time, preserving generator cache: - ARB0001: Mapper class must be partial - ARB0002: Mapper class must inherit MapperProfile<TSource, TDestination> - ARB0003: ConfigureMapping contains unsupported statements (not mapping calls) - ARB0004: Destination property in Property() does not exist on destination type - ARB0005: Duplicate destination property mapping - ARB0006: Unrecognized mapping call pattern (not From/Value/Ignore) - ARB0007: Source property in From() does not exist on source type https://claude.ai/code/session_01SCR7bdSkrCSmNazN911Y2e

…roperty existence The Property() and From() lambdas are strongly-typed expressions, so the C# compiler itself will error if a referenced property doesn't exist on the type. These diagnostics were redundant. https://claude.ai/code/session_01SCR7bdSkrCSmNazN911Y2e

Warns when the generator auto-matches properties by name but the source type cannot be implicitly converted to the destination type. Without this, the generated assignment (destination.X = source.X) fails to compile with a confusing error in generated code. Uses Compilation.ClassifyConversion to check implicit convertibility. Skips properties with explicit custom mappings (From/Value/Ignore) and handles Nullable<T> unwrapping for same-underlying-type cases. https://claude.ai/code/session_01SCR7bdSkrCSmNazN911Y2e

Tests cover all diagnostic rules using CSharpCompilation with inline source: - ARB0001: class must be partial (positive and negative) - ARB0002: class must inherit MapperProfile (positive and negative) - ARB0003: unsupported statements in ConfigureMapping (variable declaration, if, foreach, return, non-mapping method calls, mixed valid/invalid) - ARB0004: auto-matched property type mismatch (incompatible types, implicit numeric conversion, nullable same-type, custom-mapped skip, multiple) - ARB0005: duplicate destination property mapping - ARB0006: unrecognized method on property chain - Valid mapper and no-attribute scenarios produce zero diagnostics https://claude.ai/code/session_01SCR7bdSkrCSmNazN911Y2e

Copilot

Pull request overview

Adds a standalone Roslyn DiagnosticAnalyzer to validate [GenerateMapper] mapper profiles at compile time (separate from the incremental generator pipeline), along with unit tests to exercise the analyzer diagnostics.

Changes:

Introduce MapperDiagnosticAnalyzer to report ARB0001–ARB0006 diagnostics for mapper shape and ConfigureMapping contents.
Add MapperDiagnostics descriptor definitions backing those analyzer rules.
Add analyzer-focused tests that compile in-memory sources and run the analyzer, plus a test dependency on Microsoft.CodeAnalysis.CSharp.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
`test/Arbiter.Mapping.Tests/MapperDiagnosticAnalyzerTests.cs`	Adds in-memory compilation harness and tests validating analyzer diagnostics.
`test/Arbiter.Mapping.Tests/Arbiter.Mapping.Tests.csproj`	Adds Roslyn C# package reference needed to compile/run the analyzer in tests.
`src/Arbiter.Mapping.Generators/MapperDiagnostics.cs`	Defines diagnostic descriptors ARB0001–ARB0006.
`src/Arbiter.Mapping.Generators/MapperDiagnosticAnalyzer.cs`	Implements analyzer logic to validate `[GenerateMapper]` mapper classes and `ConfigureMapping`.

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Copilot · 2026-04-03T16:10:24Z

src/Arbiter.Mapping.Generators/MapperDiagnosticAnalyzer.cs

+            // Handle nullable-to-nullable and nullable-to-non-nullable of the same underlying type
+            var sourceUnderlying = GetUnderlyingType(sourceTypeSymbol);
+            var destUnderlying = GetUnderlyingType(destTypeSymbol);
+
+            if (SymbolEqualityComparer.Default.Equals(sourceUnderlying, destUnderlying))
+                continue;
+


The ARB0004 type-compatibility check currently skips cases where the source is nullable and the destination is non-nullable but has the same underlying type (e.g., int? -> int). That conversion is explicit in C# and the generator emits direct assignments for auto-mapped properties (see MapperWriter.WriteSourceExpression path-length==1 branch), so the generated code will not compile. Consider removing this underlying-type equality short-circuit for nullable->non-nullable (or only skipping when the destination is nullable / an implicit conversion exists).

Suggested change

// Handle nullable-to-nullable and nullable-to-non-nullable of the same underlying type

var sourceUnderlying = GetUnderlyingType(sourceTypeSymbol);

var destUnderlying = GetUnderlyingType(destTypeSymbol);

if (SymbolEqualityComparer.Default.Equals(sourceUnderlying, destUnderlying))

continue;

Copilot · 2026-04-03T16:10:24Z

test/Arbiter.Mapping.Tests/MapperDiagnosticAnalyzerTests.cs

+    [Test]
+    public async Task ARB0004_NullableToNonNullableSameType_NoDiagnostic()
+    {
+        var source = """
+            using Arbiter.Mapping;
+
+            namespace TestApp;
+
+            [GenerateMapper]
+            public partial class MyMapper : MapperProfile<Source, Dest> { }
+
+            public class Source { public int? Value { get; set; } }
+            public class Dest { public int Value { get; set; } }
+            """;
+
+        var diagnostics = await RunAnalyzerAsync(source);
+
+        diagnostics.Should().NotContain(d => d.Id == "ARB0004");
+    }


This test asserts that ARB0004 should NOT be reported for nullable->non-nullable auto-matched properties (int? -> int). The generator currently emits direct property access for single-segment paths, so that assignment will not compile without a coalesce/cast. Once ARB0004 is fixed to flag explicit-only conversions, this expectation should be updated (or the generator updated to emit a safe conversion).

Copilot · 2026-04-03T16:10:25Z

src/Arbiter.Mapping.Generators/MapperDiagnostics.cs

+    /// <summary>
+    /// ARB0004: An auto-matched property has incompatible types between source and destination.
+    /// The generated assignment will not compile.
+    /// </summary>
+    public static readonly DiagnosticDescriptor PropertyTypeMismatch = new(
+        id: "ARB0004",
+        title: "Mapped property type mismatch",
+        messageFormat: "Property '{0}' cannot be auto-mapped: source type '{1}' is not implicitly convertible to destination type '{2}'",
+        category: Category,
+        defaultSeverity: DiagnosticSeverity.Warning,
+        isEnabledByDefault: true,
+        description: "An auto-matched property (matched by name) has incompatible types between source and destination. " +
+                     "The generated code will produce a compile error. Use mapping.Property(d => d.Prop).From(...) to provide " +
+                     "an explicit conversion, or .Ignore() to skip the property.");
+


The PR description lists diagnostics ARB0004 (destination property does not exist) and ARB0007 (source property in From() does not exist), but this PR’s implementation defines ARB0004 as a type-mismatch warning for auto-matched properties and does not implement any ARB0007 descriptor or source/destination existence checks. Please either update the PR description to match the shipped diagnostics, or implement the missing diagnostics so the analyzer behavior aligns with the stated contract.

- Add AnalyzerReleases/Shipped.md and Unshipped.md to satisfy RS2008 (release tracking required by EnforceExtendedAnalyzerRules) - Register release tracking files as AdditionalFiles in csproj - Add MA0051 pragma suppression to MapperDiagnosticAnalyzer.cs - Change test helper return type to List<Diagnostic> for assertion compat https://claude.ai/code/session_01SCR7bdSkrCSmNazN911Y2e

claude added 4 commits April 3, 2026 03:10

pwelter34 requested a review from Copilot April 3, 2026 16:05

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Add DiagnosticAnalyzer for Arbiter.Mapping.Generators to validate mapper classes#382

Add DiagnosticAnalyzer for Arbiter.Mapping.Generators to validate mapper classes#382
pwelter34 wants to merge 5 commits intomainfrom
claude/mapping-diagnostic-analyzer-F7SLG

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