Error "InvalidOperationException: Sequence contains more than one matching element" in Swagger when using multiple [FromForm] parameters in Minimal API

[marcominerva]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I have the following Minimal API endpoint:

app.MapPost("/api/people-minimalapi", ([FromForm] Person person, [FromForm] Address address)
    => TypedResults.NoContent())
.WithOpenApi();

public record class Person(string FirstName, string LastName);

public record class Address(string Street, string City, string State, string ZipCode);

When I launch the application, I get the following Exception related to Swagger:

InvalidOperationException: Sequence contains more than one matching element
System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
System.Linq.Enumerable.TryGetSingle<TSource>(IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
System.Linq.Enumerable.SingleOrDefault<TSource>(IEnumerable<TSource> source, Func<TSource, bool> predicate)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOpenApiOperationFromMetadata(ApiDescription apiDescription, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable<ApiDescription> apiDescriptions, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable<ApiDescription> apiDescriptions, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerDocumentWithoutFilters(string documentName, string host, string basePath)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerAsync(string documentName, string host, string basePath)
Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

If instead I use a Controller like this:

[ApiController]
[Route("api/[controller]")]
public class PeopleController : ControllerBase
{
    [HttpPost]
    public IActionResult Save([FromForm] Person person, [FromForm] Address address) => NoContent();
}

Everything works as expected.

Expected Behavior

Swagger should not throw the exception.

Steps To Reproduce

Minimal repro here: https://github.com/marcominerva/FromFormBindingIssue

Exceptions (if any)

InvalidOperationException: Sequence contains more than one matching element
System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
System.Linq.Enumerable.TryGetSingle(IEnumerable source, Func<TSource, bool> predicate, out bool found)
System.Linq.Enumerable.SingleOrDefault(IEnumerable source, Func<TSource, bool> predicate)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOpenApiOperationFromMetadata(ApiDescription apiDescription, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable apiDescriptions, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable apiDescriptions, SchemaRepository schemaRepository)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerDocumentWithoutFilters(string documentName, string host, string basePath)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerAsync(string documentName, string host, string basePath)
Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

.NET Version

8.0.101

Anything else?

No response

[captainsafia]

@marcominerva Thanks for reporting this issue!

I believe the issue is coming from the Microsoft.AspNetCore.OpenAPI <-> Swashbuckle integration layer. In particular here:

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/8f363f7359cb1cb8fa5de5195ec6d97aefaa16b3/src/Swashbuckle.AspNetCore.SwaggerGen/SwaggerGenerator/SwaggerGenerator.cs#L266

I don't have immediate thoughts on the best way to workaround this, at the moment.

[marcominerva]

Thank you @captainsafia for the reply!

So, now I have another question. The last update in the https://github.com/domaindrivendev/Swashbuckle.AspNetCore repository happened on January 2023. Reading this issue: domaindrivendev/Swashbuckle.AspNetCore#2759, what should we expect for the future? I mean, if the situation remain the same, will Swagger integration be removed from the .NET Web API template? And there will be a recommended alternative?

[Havunen]

Have you tested if DotSwashbuckle produces the same error? https://github.com/Havunen/DotSwashbuckle

[vdevc]

The exact same behaviour happens, in minimal api endpoints, if using header (like, e.g. for versioning) and so having endpoints like this one

app.MapPost("/api/people-minimalapi", ([FromHeader(Name = "x-api-version"] string apiVersion)
    => TypedResults.NoContent())
.WithOpenApi();

Have you tested if DotSwashbuckle produces the same error? https://github.com/Havunen/DotSwashbuckle

Yes. The error is present also with DotSwashbuckle @Havunen :)
You can find a repro here.

[captainsafia]

@Havunen You should be able to resolve this in DotSwashbuckle by updating the line of code referenced here to accept either a single parameter from body or multiple parameters from the form.

One thing to note is that I think you'd have to gather the schemas for all the FromForm attributes when constructing the form body.

The exact same behaviour happens, in minimal api endpoints, if using header (like, e.g. for versioning) and so having endpoints like this one

This is surprising to me because I don't know of any code in minimal APIs/ApiExplorer/Swashbuckle that would be causing this off the top of my head but I can look deeper.

[vdevc]

This is surprising to me because I don't know of any code in minimal APIs/ApiExplorer/Swashbuckle that would be causing this off the top of my head but I can look deeper.

Actually the exception seems to be the same

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware: Error: An unhandled exception has occurred while executing the request.

System.InvalidOperationException: Sequence contains more than one matching element
   at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
   at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Func`2 predicate, Boolean& found)
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
   at DotSwashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOpenApiOperationFromMetadata(ApiDescription apiDescription, SchemaRepository schemaRepository)
   at DotSwashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository)
   at DotSwashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
   at DotSwashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
   at DotSwashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerDocumentWithoutFilters(String documentName, String host, String basePath)
   at DotSwashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerAsync(String documentName, String host, String basePath)
   at DotSwashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

[Havunen]

@Havunen You should be able to resolve this in DotSwashbuckle by updating the line of code referenced #53831 (comment) to accept either a single parameter from body or multiple parameters from the form.

that code line does not exist in DotSwashbuckle

Actually the exception seems to be the same

Is this latest version of DotSwashbuckle 3.0.9 ? I cannot reproduce the exception, however there seems to be something wrong with the generated schema so I will check that

[captainsafia]

that code line does not exist in DotSwashbuckle

You're right. It looks like it was removed in this commit.

@marcominerva Given the divergence in the codebases, can you confirm if you repro this issue with DotSwashbuckle? I don't believe what @vdevc reported with FromHeader is related to your FromForm issue.

[vdevc]

Is this latest version of DotSwashbuckle 3.0.9 ? I cannot reproduce the exception, however there seems to be something wrong with the generated schema so I will check that

My check was done on 3.0.8 yesterday. I don't see the 3.0.9 version on nuget yet 🙃

I don't believe what @vdevc reported with FromHeader is related to your FromForm issue.

@captainsafia same doubts here. I am suspecting about the presence of multiple Produces extensions methods. However I haven't been able to reproduce the problem without using the FromHeader parameter.