How to Write ISchemaFilter for ProblemDetails in ASP.NET Core 3 & Swashbuckle 5?

Question

I'm using ASP.NET Core 3.0 with Swashbuckle 5. I'm trying to write a ISchemaFilter for ProblemDetails in ASP.NET Core 3.0. ProblemDetails is returned for a lot of different status codes e.g. 400, 401, 403, 406, 415, 500 etc. I want to provide a different example of ProblemDetails depending on the status code. How can I achieve this? Here is some code I've written to get started, just need to fill in the blank:

public class ProblemDetailsSchemaFilter : ISchemaFilter
{
    private static readonly OpenApiObject Status400ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.1"),
        ["title"] = new OpenApiString("Bad Request"),
        ["status"] = new OpenApiInteger(StatusCodes.Status400BadRequest),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
        ["errors"] = new OpenApiObject()
        {
            ["property1"] = new OpenApiArray()
            {
                new OpenApiString("The property field is required"),
            },
        },
    };

    private static readonly OpenApiObject Status401ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7235#section-3.1"),
        ["title"] = new OpenApiString("Unauthorized"),
        ["status"] = new OpenApiInteger(StatusCodes.Status401Unauthorized),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status403ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.3"),
        ["title"] = new OpenApiString("Forbidden"),
        ["status"] = new OpenApiInteger(StatusCodes.Status403Forbidden),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status404ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.4"),
        ["title"] = new OpenApiString("Not Found"),
        ["status"] = new OpenApiInteger(StatusCodes.Status404NotFound),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status406ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.6"),
        ["title"] = new OpenApiString("Not Acceptable"),
        ["status"] = new OpenApiInteger(StatusCodes.Status406NotAcceptable),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status409ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.8"),
        ["title"] = new OpenApiString("Conflict"),
        ["status"] = new OpenApiInteger(StatusCodes.Status409Conflict),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status415ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.13"),
        ["title"] = new OpenApiString("Unsupported Media Type"),
        ["status"] = new OpenApiInteger(StatusCodes.Status415UnsupportedMediaType),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status422ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc4918#section-11.2"),
        ["title"] = new OpenApiString("Unprocessable Entity"),
        ["status"] = new OpenApiInteger(StatusCodes.Status422UnprocessableEntity),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject Status500ProblemDetails = new OpenApiObject()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.6.1"),
        ["title"] = new OpenApiString("Internal Server Error"),
        ["status"] = new OpenApiInteger(StatusCodes.Status500InternalServerError),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    public void Apply(OpenApiSchema schema, SchemaFilterContext context)
    {
        if (context.ApiModel.Type == typeof(ProblemDetails))
        {
            // TODO: Set the default and example based on the status code.
            // schema.Default = ???;
            // schema.Example = ???;
        }
    }
}

Answer 1

I've taken your examples and converted them into an operation filter where you have access to the response.

You can use the following OperationFilter:

using System.Collections.Generic;
using System.Net.Mime;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.WebUtilities;
using Microsoft.OpenApi.Any;
using Microsoft.OpenApi.Models;
using Swashbuckle.AspNetCore.SwaggerGen;

public class ProblemDetailsOperationFilter : IOperationFilter
{
    private static readonly OpenApiObject _status400ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.1"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status400BadRequest)),
        ["status"] = new OpenApiInteger(StatusCodes.Status400BadRequest),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
        ["errors"] = new OpenApiObject
        {
            ["property1"] = new OpenApiArray
            {
                new OpenApiString("The property field is required"),
            },
        },
    };

    private static readonly OpenApiObject _status401ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7235#section-3.1"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status401Unauthorized)),
        ["status"] = new OpenApiInteger(StatusCodes.Status401Unauthorized),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status403ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.3"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status403Forbidden)),
        ["status"] = new OpenApiInteger(StatusCodes.Status403Forbidden),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status404ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.4"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status404NotFound)),
        ["status"] = new OpenApiInteger(StatusCodes.Status404NotFound),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status406ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.6"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status406NotAcceptable)),
        ["status"] = new OpenApiInteger(StatusCodes.Status406NotAcceptable),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status409ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.8"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status409Conflict)),
        ["status"] = new OpenApiInteger(StatusCodes.Status409Conflict),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status415ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.5.13"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status415UnsupportedMediaType)),
        ["status"] = new OpenApiInteger(StatusCodes.Status415UnsupportedMediaType),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status422ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc4918#section-11.2"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status422UnprocessableEntity)),
        ["status"] = new OpenApiInteger(StatusCodes.Status422UnprocessableEntity),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    private static readonly OpenApiObject _status500ProblemDetails = new()
    {
        ["type"] = new OpenApiString("https://tools.ietf.org/html/rfc7231#section-6.6.1"),
        ["title"] = new OpenApiString(ReasonPhrases.GetReasonPhrase(StatusCodes.Status500InternalServerError)),
        ["status"] = new OpenApiInteger(StatusCodes.Status500InternalServerError),
        ["traceId"] = new OpenApiString("00-982607166a542147b435be3a847ddd71-fc75498eb9f09d48-00"),
    };

    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        var problemDetails = new Dictionary<string, IOpenApiAny>
        {
            { StatusCodes.Status400BadRequest.ToString(), _status400ProblemDetails },
            { StatusCodes.Status401Unauthorized.ToString(), _status401ProblemDetails },
            { StatusCodes.Status403Forbidden.ToString(), _status403ProblemDetails },
            { StatusCodes.Status404NotFound.ToString(), _status404ProblemDetails },
            { StatusCodes.Status406NotAcceptable.ToString(), _status406ProblemDetails },
            { StatusCodes.Status409Conflict.ToString(), _status409ProblemDetails },
            { StatusCodes.Status415UnsupportedMediaType.ToString(), _status415ProblemDetails },
            { StatusCodes.Status422UnprocessableEntity.ToString(), _status422ProblemDetails },
            { StatusCodes.Status500InternalServerError.ToString(), _status500ProblemDetails },
        };

        foreach (var operationResponse in operation.Responses)
        {
            if (problemDetails.TryGetValue(operationResponse.Key, out var problemDetail))
                operationResponse.Value.Content[MediaTypeNames.Application.Json].Example = problemDetail;
        }
    }
}

Register it with

services.AddSwaggerGen(x =>
{
    ...
    x.OperationFilter<ProblemDetailsOperationFilter>();
    ...
});

Answer 2

Here is a simple workaround like below:

1.Model:

public class ProblemDetails
{
    public int ID { get; set; }
    public string Description { get; set; }
}

2.ProblemDetailsSchemaFilter:

public void Apply(OpenApiSchema schema, SchemaFilterContext context)
{
    if (context.ApiModel.Type == typeof(ProblemDetails))
    {
        // TODO: Set the default and example based on the status code.
        schema.Default = new OpenApiObject
        {
            ["ID"] = new OpenApiInteger(2),
            ["Description"] = new OpenApiString("test")
        };
        schema.Example = new OpenApiObject
        {
            ["ID"] = new OpenApiInteger(1),
            ["Description"] = new OpenApiString("test test")
        };
    }
}

3.Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new OpenApiInfo { Title = "My API", Version = "v1" });
        c.SchemaFilter<ProblemDetailsSchemaFilter>();
    });
}

You could display schema.Example on Swagger UI:

And generate the json by using schema.Default: