Microsoft.Extensions.ApiDescription.Server - Nullable query parameters are not generated as nullable in OpenAPI Document

[MattParkerDev]

cc: @captainsafia
Nullable query parameters are not generated as nullable in OpenAPI Document, such as:

group.MapGet("long", (long? myLong) => { })
			.WithName("NullableLong");

becomes:

"get": {
        "tags": [
          "Nullable"
        ],
        "operationId": "NullableLong",
        "parameters": [
          {
            "name": "myLong",
            "in": "query",
            "schema": {
              "type": "integer",
              "format": "int64"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "OK"
          }
        }
      }

I believe the cause is line 745:

aspnetcore/src/OpenApi/src/Services/OpenApiDocumentService.cs

Lines 739 to 761 in 684e739

	private static Type GetTargetType(ApiDescription description, ApiParameterDescription parameter)
	{
	var bindingMetadata = description.ActionDescriptor.EndpointMetadata
	.OfType<IParameterBindingMetadata>()
	.SingleOrDefault(metadata => metadata.Name == parameter.Name);
	var parameterType = parameter.Type is not null
	? Nullable.GetUnderlyingType(parameter.Type) ?? parameter.Type
	: parameter.Type;
	// parameter.Type = typeof(string)
	// parameter.ModelMetadata.Type = typeof(TEnum)
	var requiresModelMetadataFallbackForEnum = parameterType == typeof(string)
	&& parameter.ModelMetadata.ModelType != parameter.Type
	&& parameter.ModelMetadata.ModelType.IsEnum;
	// Enums are exempt because we want to set the IOpenApiSchema.Enum field when feasible.
	// parameter.Type = typeof(TEnum), typeof(TypeWithTryParse)
	// parameter.ModelMetadata.Type = typeof(string)
	var hasTryParse = bindingMetadata?.HasTryParse == true && parameterType is not null && !parameterType.IsEnum;
	var targetType = requiresModelMetadataFallbackForEnum || hasTryParse
	? parameter.ModelMetadata.ModelType
	: parameter.Type;
	targetType ??= typeof(string);
	return targetType;

Is there a historical reason for always unwrapping nullable parameter types?

[captainsafia]

@MattParkerDev Thanks for filing this issue!

I'm closing this out as a dupe of #63029 that tracks who we can address this nullability-related issues by changing how it is modeled in the document. More information about why the issue happens is in the linked issue.