JSON Patch with System.Text.Json (.NET 10)

using System.Collections.Concurrent;
using System.ComponentModel.DataAnnotations;
using Microsoft.AspNetCore.JsonPatch.SystemTextJson;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddProblemDetails();
var app = builder.Build();

var preferences = new ConcurrentDictionary<string, CustomerPreferences>
{
    ["CUST-1042"] = new() { Language = "en-GB", MarketingEmails = false, Channels = ["email"] },
};

app.MapPatch("/api/customers/{id}/preferences", (string id, JsonPatchDocument<CustomerPreferences> patch) =>
{
    if (!preferences.TryGetValue(id, out var current)) return Results.NotFound();

    var copy = new CustomerPreferences
    {
        Language = current.Language, MarketingEmails = current.MarketingEmails, Channels = [.. current.Channels]
    };

    var errors = new Dictionary<string, string[]>();
    patch.ApplyTo(copy, e => errors[e.Operation?.path ?? "patch"] = [e.ErrorMessage]);

    var results = new List<ValidationResult>();
    if (!Validator.TryValidateObject(copy, new ValidationContext(copy), results, validateAllProperties: true))
        foreach (var r in results)
            errors[r.MemberNames.FirstOrDefault() ?? "patch"] = [r.ErrorMessage ?? "Invalid value."];

    if (errors.Count > 0) return Results.ValidationProblem(errors);

    preferences[id] = copy;
    return Results.Ok(copy);
});

app.Run();

public sealed class CustomerPreferences
{
    [Required, RegularExpression("^[a-z]{2}-[A-Z]{2}$")] public string Language { get; set; } = "en-GB";
    public bool MarketingEmails { get; set; }
    [MaxLength(3)] public List<string> Channels { get; set; } = [];
}

using System.Net.Http.Headers;
using System.Text.Json;
using Microsoft.AspNetCore.JsonPatch.SystemTextJson;

using var http = new HttpClient { BaseAddress = new Uri("https://orders.internal.example.com") };

var patch = new JsonPatchDocument<ShippingPatch>()
    .Test(p => p.Status, "Pending")          // fail if the order moved on meanwhile
    .Replace(p => p.City, "York")
    .Replace(p => p.Postcode, "YO1 7HH")
    .Add(p => p.DeliveryNotes, "Leave with reception");

var content = new StringContent(JsonSerializer.Serialize(patch));
content.Headers.ContentType = new MediaTypeHeaderValue("application/json-patch+json");

using var response = await http.PatchAsync("/api/orders/1001/shipping", content);
Console.WriteLine($"{(int)response.StatusCode}: {await response.Content.ReadAsStringAsync()}");

public sealed class ShippingPatch
{
    public string Street { get; set; } = "";
    public string City { get; set; } = "";
    public string Postcode { get; set; } = "";
    public List<string> DeliveryNotes { get; set; } = [];
    public string Status { get; set; } = "";
}

using System.Text.Json.Nodes;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddProblemDetails();
var app = builder.Build();

var product = new Product("KETTLE-BLUE", "Blue kettle", 39.95m, "Fast-boil 1.7 l kettle");

// PATCH /api/products/KETTLE-BLUE  {"price": 34.95, "description": null}
app.MapPatch("/api/products/{sku}", (string sku, JsonObject changes) =>
{
    if (sku != product.Sku) return Results.NotFound();

    var allowed = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { "name", "price", "description" };
    var unknown = changes.Select(p => p.Key).Where(k => !allowed.Contains(k)).ToArray();
    if (unknown.Length > 0)
        return Results.ValidationProblem(unknown.ToDictionary(k => k, _ => new[] { "This field cannot be patched." }));

    // Absent keys are left alone; a key set to null clears the value. TryGetValue turns a
    // wrongly typed value into a 400; GetValue would throw InvalidOperationException (a 500).
    var errors = new Dictionary<string, string[]>();
    var name = product.Name;
    var price = product.Price;
    var description = product.Description;

    if (changes.TryGetPropertyValue("name", out var nameNode))
    {
        if (nameNode is JsonValue v && v.TryGetValue<string>(out var s) && s.Length > 0) name = s;
        else errors["name"] = ["Must be a non-empty string."];
    }
    if (changes.TryGetPropertyValue("price", out var priceNode))
    {
        if (priceNode is JsonValue v && v.TryGetValue<decimal>(out var p) && p > 0) price = p;
        else errors["price"] = ["Must be a positive number."];
    }
    if (changes.TryGetPropertyValue("description", out var descriptionNode))
    {
        if (descriptionNode is null) description = null;
        else if (descriptionNode is JsonValue v && v.TryGetValue<string>(out var d)) description = d;
        else errors["description"] = ["Must be a string or null."];
    }
    if (errors.Count > 0) return Results.ValidationProblem(errors);

    product = product with { Name = name, Price = price, Description = description };
    return Results.Ok(product);
});

app.Run();

public record Product(string Sku, string Name, decimal Price, string? Description);