Model Binding

Khái niệm

Model Binding là quá trình ASP.NET Core chuyển đổi dữ liệu từ HTTP request thành các tham số của action method.

┌─────────────────────────────────────────────────────────────────┐
│                    MODEL BINDING SOURCES                         │
├─────────────────────────────────────────────────────────────────┤
│                                                                 │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐  ┌──────────┐       │
│  │   Form   │  │   Route  │  │  Query   │  │   Body   │       │
│  │  Values  │  │  Values  │  │  String  │  │  (JSON)  │       │
│  └────┬─────┘  └────┬─────┘  └────┬─────┘  └────┬─────┘       │
│       │             │             │             │              │
│       └─────────────┴─────────────┴─────────────┘              │
│                         │                                      │
│                         ▼                                      │
│              ┌─────────────────────┐                           │
│              │   Model Binder      │                           │
│              │  (Type Conversion)  │                           │
│              └─────────┬───────────┘                           │
│                        │                                       │
│                        ▼                                       │
│              ┌─────────────────────┐                           │
│              │   Action Method     │                           │
│              │   Parameters        │                           │
│              └─────────────────────┘                           │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Binding Sources

[FromBody]

// JSON request body
[HttpPost]
public IActionResult Create([FromBody] Product product)
{
    // product được deserialize từ JSON body
    return Ok(product);
}

// Request:
// POST /api/products
// Content-Type: application/json
// {"name": "Laptop", "price": 999.99}

[FromQuery]

// Query string parameters
[HttpGet]
public IActionResult GetProducts(
    [FromQuery] int page = 1,
    [FromQuery] int pageSize = 10,
    [FromQuery] string? search = null)
{
    // /api/products?page=2&pageSize=20&search=laptop
    return Ok(new { page, pageSize, search });
}

// Hoặc binding vào object
[HttpGet]
public IActionResult GetProducts([FromQuery] PagingRequest request)
{
    return Ok(request);
}

public class PagingRequest
{
    public int Page { get; set; } = 1;
    public int PageSize { get; set; } = 10;
    public string? Search { get; set; }
}

[FromRoute]

// Route data
[HttpGet("{id}")]
public IActionResult GetById([FromRoute] int id)
{
    // /api/products/123 -> id = 123
    return Ok(id);
}

// Multiple route parameters
[HttpGet("{category}/{id}")]
public IActionResult GetByCategoryAndId(
    [FromRoute] string category,
    [FromRoute] int id)
{
    // /api/products/electronics/123
    return Ok(new { category, id });
}

[FromHeader]

[HttpGet]
public IActionResult GetWithHeader([FromHeader] string apiKey)
{
    // Lấy từ header: X-API-Key: abc123
    return Ok(apiKey);
}

// Hoặc lấy tất cả headers
[HttpGet]
public IActionResult GetHeaders([FromHeader] IHeaderDictionary headers)
{
    return Ok(headers);
}

[FromForm]

// Form data (multipart/form-data)
[HttpPost]
public IActionResult CreateForm([FromForm] ProductForm form)
{
    return Ok(form);
}

public class ProductForm
{
    public string Name { get; set; }
    public decimal Price { get; set; }
    public IFormFile Image { get; set; }
}

Binding Order

ASP.NET Core tìm kiếm giá trị theo thứ tự:

1. Form values - POST form data
2. Route values - URL route parameters
3. Query string - URL query parameters
4. Request body - JSON/XML (cho complex types)

// Không cần attribute cho complex types (mặc định là [FromBody])
[HttpPost]
public IActionResult Create(Product product) // Tự động [FromBody]
{
    return Ok(product);
}

// Cần attribute cho simple types
[HttpGet]
public IActionResult Get(int id) // Tự động [FromQuery]
{
    return Ok(id);
}

Custom Model Binding

Custom Model Binder

public class CommaSeparatedListBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        var value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).FirstValue;
        
        if (string.IsNullOrEmpty(value))
        {
            bindingContext.Result = ModelBindingResult.Success(new List<string>());
            return Task.CompletedTask;
        }

        var list = value.Split(',', StringSplitOptions.RemoveEmptyEntries)
                       .Select(x => x.Trim())
                       .ToList();
        
        bindingContext.Result = ModelBindingResult.Success(list);
        return Task.CompletedTask;
    }
}

// Sử dụng
[HttpGet]
public IActionResult GetTags([ModelBinder(typeof(CommaSeparatedListBinder))] List<string> tags)
{
    // /api/products?tags=csharp,dotnet,web -> ["csharp", "dotnet", "web"]
    return Ok(tags);
}

Model Binder Provider

public class CommaSeparatedListBinderProvider : IModelBinderProvider
{
    public IModelBinder? GetBinder(ModelBinderProviderContext context)
    {
        if (context.Metadata.ModelType == typeof(List<string>))
        {
            return new BinderTypeModelBinder(typeof(CommaSeparatedListBinder));
        }
        return null;
    }
}

// Đăng ký
builder.Services.AddControllers(options =>
{
    options.ModelBinderProviders.Insert(0, new CommaSeparatedListBinderProvider());
});

Binding Complex Types

Nested Objects

public class Address
{
    public string Street { get; set; }
    public string City { get; set; }
    public string Country { get; set; }
}

public class Customer
{
    public string Name { get; set; }
    public Address Address { get; set; }
    public List<string> Tags { get; set; }
}

// JSON binding
// {
//   "name": "John",
//   "address": {
//     "street": "123 Main St",
//     "city": "NYC",
//     "country": "USA"
//   },
//   "tags": ["vip", "active"]
// }

Collections

[HttpPost]
public IActionResult CreateMany([FromBody] List<Product> products)
{
    // [
    //   {"name": "Product 1", "price": 100},
    //   {"name": "Product 2", "price": 200}
    // ]
    return Ok(products.Count);
}

Binding với Minimal APIs

// Tự động binding
app.MapPost("/api/products", (Product product) => Results.Ok(product));

// Binding từ query string
app.MapGet("/api/products", (int page, int pageSize) => 
    Results.Ok(new { page, pageSize }));

// Binding từ route
app.MapGet("/api/products/{id}", (int id) => Results.Ok(id));

// Custom binding
app.MapGet("/api/products", ([FromQuery] PagingRequest request) => 
    Results.Ok(request));

// Binding từ header
app.MapGet("/api/secure", ([FromHeader(Name = "X-API-Key")] string apiKey) =>
{
    if (apiKey != "secret") return Results.Unauthorized();
    return Results.Ok("Authorized");
});

Best Practices

1. Sử dụng đúng binding source

// ✅ Rõ ràng
public IActionResult Get(
    [FromRoute] int id,
    [FromQuery] string? search,
    [FromBody] Product product)

// ❌ Không rõ ràng
public IActionResult Get(int id, string search, Product product)

2. Sử dụng nullable cho optional parameters

// ✅ Good
public IActionResult Get([FromQuery] string? search = null)

// ❌ Bad - sẽ throw nếu không có query param
public IActionResult Get([FromQuery] string search)

3. Validate binding

[HttpPost]
public IActionResult Create([FromBody] Product product)
{
    if (product == null)
    {
        return BadRequest("Invalid request body");
    }
    // ...
}