.NET Core 2.0如何为自定义Accept头配置内置XML/JSON格式化器

Great question! To let clients request HATEOAS links using custom Accept headers like application/json+hateoas or application/xml+hateoas, we need to extend the default formatters and add conditional logic in controllers. Here's a step-by-step implementation:

Step 1: Configure Custom Media Types in ConfigureServices #

First, we'll register the custom media types with the existing JSON and XML formatters so the API recognizes them. Update your AddMvc configuration like this:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options =>
    {
        // Map custom formats to their respective media types
        options.FormatterMappings.SetMediaTypeMappingForFormat(
            "json+hateoas", MediaTypeHeaderValue.Parse("application/json+hateoas"));
        options.FormatterMappings.SetMediaTypeMappingForFormat(
            "xml+hateoas", MediaTypeHeaderValue.Parse("application/xml+hateoas"));

        // Add custom media type to the default JSON formatter
        var jsonFormatter = options.OutputFormatters.OfType<JsonOutputFormatter>().First();
        jsonFormatter.SupportedMediaTypes.Add(
            MediaTypeHeaderValue.Parse("application/json+hateoas"));

        // Add custom media type to the default XML formatter
        var xmlFormatter = options.OutputFormatters.OfType<XmlDataContractSerializerOutputFormatter>().First();
        xmlFormatter.SupportedMediaTypes.Add(
            MediaTypeHeaderValue.Parse("application/xml+hateoas"));
    });
}

Step 2: Add Conditional HATEOAS Logic in Controllers #

Create a helper method to check if the client requested HATEOAS, then modify your response to include links when needed:

private bool ShouldIncludeHateoas(HttpRequest request)
{
    var acceptHeader = request.Headers["Accept"].ToString();
    return acceptHeader.Contains("application/json+hateoas") 
           || acceptHeader.Contains("application/xml+hateoas");
}

[HttpGet("{id}")]
public IActionResult Get(int id)
{
    // Fetch your resource (example: a User object)
    var user = _userService.GetById(id);

    if (ShouldIncludeHateoas(Request))
    {
        // Add HATEOAS navigation links
        user.Links = new List<Link>
        {
            new Link
            {
                Rel = "self",
                Href = Url.Action("Get", new { id = user.Id }),
                Method = "GET"
            },
            new Link
            {
                Rel = "delete",
                Href = Url.Action("Delete", new { id = user.Id }),
                Method = "DELETE"
            }
        };
    }

    return Ok(user);
}

Step 3: Define a Link Class #

Make sure you have a simple Link class to represent navigation links, and update your resource class to include a nullable Links property:

public class Link
{
    public string Rel { get; set; } // Relationship name (e.g., "self", "update")
    public string Href { get; set; } // URL to the related resource
    public string Method { get; set; } // HTTP method for the link
}

// Example resource class with optional Links property
public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public string Email { get; set; }
    public List<Link> Links { get; set; } // Will be null if HATEOAS isn't requested
}

Key Notes #

• The custom media types reuse the default formatters, so serialization/deserialization works exactly like standard JSON/XML—only the links are added conditionally.
• For XML serialization, add [DataContract] and [DataMember] attributes to your Link and resource classes if you haven't already, to ensure proper XML output.
• The Links property will be omitted from responses when HATEOAS isn't requested (since it's null), keeping your standard responses clean.

内容的提问来源于stack exchange，提问作者Daniel