OpenAPI: Fix ModelMetadata null reference exception in emitted XMLComment transformers

src/OpenApi/gen/XmlCommentGenerator.Emitter.cs

@@ -444,7 +444,8 @@ public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransform
             foreach (var parameterDescription in context.Description.ParameterDescriptions)
             {
                 var metadata = parameterDescription.ModelMetadata;
-                if (metadata.MetadataKind == ModelMetadataKind.Property
+                if (metadata is not null
+                    && metadata.MetadataKind == ModelMetadataKind.Property
                     && metadata.ContainerType is { } containerType
                     && metadata.PropertyName is { } propertyName)
                 {

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/OperationTests.Controllers.cs

@@ -96,4 +96,49 @@ await SnapshotTestHelper.VerifyOpenApi(compilation, document =>
             Assert.Equal("The todo to insert into the database.", path3.RequestBody.Description);
         });
     }
+
+    [Fact]
+    public async Task DoesAllowUnusedRouteParameters()
+    {
+        var source = """
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.DependencyInjection;
+
+var builder = WebApplication.CreateBuilder();
+
+builder.Services
+    .AddControllers()
+    .AddApplicationPart(typeof(TestController).Assembly);
+builder.Services.AddOpenApi();
+
+var app = builder.Build();
+
+app.MapControllers();
+
+app.Run();
+
+[ApiController]
+[Route("[controller]")]
+public class TestController : ControllerBase
+{
+    /// <param name="userId">The id of the user.</param>
+    [HttpGet("{userId}")]
+    public string Get()
+    {
+        return "Hello, World!";
+    }
+}
+
+public partial class Program {}
+
+""";
+        var generator = new XmlCommentGenerator();
+        await SnapshotTestHelper.Verify(source, generator, out var compilation);
+        await SnapshotTestHelper.VerifyOpenApi(compilation, document =>
+        {
+            var path = document.Paths["/Test/{userId}"].Operations[HttpMethod.Get];
+            Assert.Equal("The id of the user.", path.Parameters[0].Description);
+        });
+    }
 }

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/snapshots/AddOpenApiTests.CanInterceptAddOpenApi#OpenApiXmlCommentSupport.generated.verified.cs

@@ -1,4 +1,4 @@
-//HintName: OpenApiXmlCommentSupport.generated.cs
+//HintName: OpenApiXmlCommentSupport.generated.cs
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
@@ -426,7 +426,8 @@ public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransform
             foreach (var parameterDescription in context.Description.ParameterDescriptions)
             {
                 var metadata = parameterDescription.ModelMetadata;
-                if (metadata.MetadataKind == ModelMetadataKind.Property
+                if (metadata is not null
+                    && metadata.MetadataKind == ModelMetadataKind.Property
                     && metadata.ContainerType is { } containerType
                     && metadata.PropertyName is { } propertyName)
                 {

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/snapshots/AdditionalTextsTests.CanHandleXmlForSchemasInAdditionalTexts#OpenApiXmlCommentSupport.generated.verified.cs

@@ -1,4 +1,4 @@
-//HintName: OpenApiXmlCommentSupport.generated.cs
+//HintName: OpenApiXmlCommentSupport.generated.cs
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
@@ -455,7 +455,8 @@ public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransform
             foreach (var parameterDescription in context.Description.ParameterDescriptions)
             {
                 var metadata = parameterDescription.ModelMetadata;
-                if (metadata.MetadataKind == ModelMetadataKind.Property
+                if (metadata is not null
+                    && metadata.MetadataKind == ModelMetadataKind.Property
                     && metadata.ContainerType is { } containerType
                     && metadata.PropertyName is { } propertyName)
                 {

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/snapshots/CompletenessTests.SupportsAllXmlTagsOnSchemas#OpenApiXmlCommentSupport.generated.verified.cs

@@ -1,4 +1,4 @@
-//HintName: OpenApiXmlCommentSupport.generated.cs
+//HintName: OpenApiXmlCommentSupport.generated.cs
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
@@ -553,7 +553,8 @@ public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransform
             foreach (var parameterDescription in context.Description.ParameterDescriptions)
             {
                 var metadata = parameterDescription.ModelMetadata;
-                if (metadata.MetadataKind == ModelMetadataKind.Property
+                if (metadata is not null
+                    && metadata.MetadataKind == ModelMetadataKind.Property
                     && metadata.ContainerType is { } containerType
                     && metadata.PropertyName is { } propertyName)
                 {