Content Negotiation

Core Principles

• Respect the Accept header to determine response format
• Default to application/json when no preference is specified
• Support application/problem+json for error responses (RFC 9457)
• Use output formatters for custom content types (CSV, XML, PDF)
• Configure JSON serialization options globally

Patterns

JSON Serialization Configuration

// Program.cs — global JSON options
builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.Converters.Add(
        new JsonStringEnumConverter());
    options.SerializerOptions.DefaultIgnoreCondition =
        JsonIgnoreCondition.WhenWritingNull;
    options.SerializerOptions.PropertyNamingPolicy =
        JsonNamingPolicy.CamelCase;
});

// Controller-specific
builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.Converters.Add(
            new JsonStringEnumConverter());
        options.JsonSerializerOptions.DefaultIgnoreCondition =
            JsonIgnoreCondition.WhenWritingNull;
    });

Adding XML Support

builder.Services.AddControllers()
    .AddXmlDataContractSerializerFormatters();

// Endpoints now respond to:
// Accept: application/json → JSON
// Accept: application/xml  → XML

Custom Output Formatter (CSV)

public sealed class CsvOutputFormatter : TextOutputFormatter
{
    public CsvOutputFormatter()
    {
        SupportedMediaTypes.Add(
            MediaTypeHeaderValue.Parse("text/csv"));
        SupportedEncodings.Add(Encoding.UTF8);
    }

    protected override bool CanWriteType(Type? type)
    {
        return type is not null &&
            (typeof(IEnumerable).IsAssignableFrom(type) ||
             type.IsGenericType);
    }

    public override async Task WriteResponseBodyAsync(
        OutputFormatterWriteContext context,
        Encoding selectedEncoding)
    {
        var response = context.HttpContext.Response;
        var items = (IEnumerable<object>)context.Object!;

        var sb = new StringBuilder();
        var properties = context.ObjectType!
            .GetGenericArguments()[0]
            .GetProperties();

        // Header row
        sb.AppendLine(string.Join(",",
            properties.Select(p => p.Name)));

        // Data rows
        foreach (var item in items)
        {
            var values = properties.Select(p =>
                EscapeCsvValue(p.GetValue(item)?.ToString() ?? ""));
            sb.AppendLine(string.Join(",", values));
        }

        await response.WriteAsync(sb.ToString(), selectedEncoding);
    }

    private static string EscapeCsvValue(string value)
    {
        if (value.Contains(',') || value.Contains('"') ||
            value.Contains('\n'))
            return $"\"{value.Replace("\"", "\"\"")}\"";
        return value;
    }
}

// Registration
builder.Services.AddControllers(options =>
{
    options.OutputFormatters.Add(new CsvOutputFormatter());
});

Format Filter for URL-Based Selection

// Allow ?format=csv or /orders.csv
builder.Services.AddControllers(options =>
{
    options.FormatterMappings.SetMediaTypeMappingForFormat(
        "csv", "text/csv");
    options.FormatterMappings.SetMediaTypeMappingForFormat(
        "xml", "application/xml");
});

[HttpGet]
[FormatFilter]
[Route("/orders.{format?}")]
public async Task<ActionResult<List<OrderResponse>>> GetOrders() { }

Minimal API Content Types

// Minimal API — explicit content type
app.MapGet("/orders/{id}", async (Guid id, ISender sender) =>
{
    var order = await sender.Send(new GetOrderQuery(id));
    return order is not null
        ? Results.Ok(order)
        : Results.NotFound();
}).Produces<OrderResponse>(StatusCodes.Status200OK, "application/json");

// Return specific content type
app.MapGet("/orders/export", async (ISender sender) =>
{
    var csv = await sender.Send(new ExportOrdersCsvQuery());
    return Results.Text(csv, "text/csv", Encoding.UTF8);
});

// Return file
app.MapGet("/orders/{id}/invoice", async (Guid id, ISender sender) =>
{
    var pdf = await sender.Send(new GenerateInvoiceQuery(id));
    return Results.File(
        pdf, "application/pdf", $"invoice-{id}.pdf");
});

ProblemDetails for Errors

builder.Services.AddProblemDetails();

// Automatic problem+json responses for errors
app.UseExceptionHandler();
app.UseStatusCodePages();

// Custom problem details
app.MapGet("/orders/{id}", async (Guid id) =>
{
    return Results.Problem(
        title: "Order Not Found",
        detail: $"No order exists with ID {id}",
        statusCode: StatusCodes.Status404NotFound,
        type: "https://tools.ietf.org/html/rfc9110#section-15.5.5");
});

Accept Header Handling

Client sends:
  Accept: application/json           → JSON response
  Accept: application/xml            → XML response
  Accept: text/csv                   → CSV response
  Accept: */*                        → Default (JSON)
  Accept: application/problem+json   → Error responses

Server responds with:
  Content-Type: application/json; charset=utf-8

Anti-Patterns

• Ignoring the Accept header (always returning JSON)
• Not supporting application/problem+json for errors
• Hardcoding content types in response instead of using formatters
• Missing Content-Type header on responses
• Using custom error formats instead of ProblemDetails

Detect Existing Patterns

1. Check for AddJsonOptions or ConfigureHttpJsonOptions configuration
2. Look for custom OutputFormatter implementations
3. Search for AddXmlDataContractSerializerFormatters calls
4. Check for [Produces] and [Consumes] attributes on controllers
5. Look for AddProblemDetails in Program.cs

Adding to Existing Project

1. Configure JSON options globally with enum conversion and null handling
2. Add ProblemDetails middleware for consistent error responses
3. Add custom formatters for export formats (CSV, XML) if needed
4. Add [Produces] attributes to document supported content types
5. Test with different Accept headers to verify negotiation works

Decision Guide

Content Type	Use Case
application/json	Default for API responses
application/problem+json	Error responses (RFC 9457)
text/csv	Data export
application/xml	Legacy integrations
application/pdf	Report generation
application/octet-stream	File downloads

References

• https://learn.microsoft.com/en-us/aspnet/core/web-api/advanced/formatting
• https://learn.microsoft.com/en-us/aspnet/core/web-api/advanced/custom-formatters