[release/7.0-rc1] Fix resolving method declaration in separate file (#43323)

* Fix resolving method declaration in separate file

* Update RouteHandlerAnalyzer.cs

Author: captainsafia

src/Framework/AspNetCoreAnalyzers/src/Analyzers/RouteHandlers/RouteHandlerAnalyzer.cs

@@ -77,8 +77,11 @@ public override void Initialize(AnalysisContext context)
                     var foundMethodReferenceBody = false;
                     if (!methodReference.Method.DeclaringSyntaxReferences.IsEmpty)
                     {
-                        var syntaxReference = methodReference.Method.DeclaringSyntaxReferences[0];
-                        var methodOperation = invocation.SemanticModel.GetOperation(syntaxReference.GetSyntax(context.CancellationToken));
+                        var syntaxReference = methodReference.Method.DeclaringSyntaxReferences.Single();
+                        var syntaxNode = syntaxReference.GetSyntax(context.CancellationToken);
+                        var methodOperation = syntaxNode.SyntaxTree == invocation.SemanticModel.SyntaxTree
+                            ? invocation.SemanticModel.GetOperation(syntaxNode, context.CancellationToken)
+                            : null;
                         if (methodOperation is ILocalFunctionOperation { Body: not null } localFunction)
                         {
                             foundMethodReferenceBody = true;

src/Framework/AspNetCoreAnalyzers/test/RouteHandlers/DisallowReturningActionResultsFromMapMethodsTest.cs

@@ -252,5 +252,43 @@ public record Person(string Name);
         AnalyzerAssert.DiagnosticLocation(source.DefaultMarkerLocation, diagnostic.Location);
         Assert.Equal("IActionResult instances should not be returned from a MapPost Delegate parameter. Consider returning an equivalent result from Microsoft.AspNetCore.Http.Results.", diagnostic.GetMessage(CultureInfo.InvariantCulture));
     }
+
+    [Fact]
+    public async Task MinimalAction_ReturningActionResultOfTDeclarationInDifferentFile_ProducesDiagnostics()
+    {
+        // Arrange
+        var source = TestSource.Read(@"
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+
+var webApp = WebApplication.Create();
+webApp.MapPost(""/"", /*MM*/MyController.ActionResultMethod);
+");
+        var controllerSource = @"
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+
+public static class MyController
+{
+    public static async Task<ActionResult<Person>> ActionResultMethod(int id)
+    {
+        await Task.Yield();
+        if (id == 0) return new NotFoundResult();
+        return new Person(""test"");
+    }
+
+    public record Person(string Name);
+}
+";
+
+        // Act
+        var diagnostics = await Runner.GetDiagnosticsAsync(source.Source, controllerSource);
+
+        // Assert
+        var diagnostic = Assert.Single(diagnostics);
+        Assert.Same(DiagnosticDescriptors.DoNotReturnActionResultsFromRouteHandlers, diagnostic.Descriptor);
+        AnalyzerAssert.DiagnosticLocation(source.DefaultMarkerLocation, diagnostic.Location);
+        Assert.Equal("IActionResult instances should not be returned from a MapPost Delegate parameter. Consider returning an equivalent result from Microsoft.AspNetCore.Http.Results.", diagnostic.GetMessage(CultureInfo.InvariantCulture));
+    }
 }