Model Binding

Learning objectives#

• Bind simple and complex parameters
• Bind lists using indexed field names
• Read conversion errors from ModelState
• Prevent over-posting with [Bind] or view models

Explanation#

For each action parameter, the default model binder looks for a value with the same name in the form fields, then route values, then the query string. Names are matched case-insensitively.

For a complex type (a class), the binder creates the object and fills each public property with a matching name: a field named Price fills product.Price. Nested properties use dots (Address.City) and lists use indexes (Lines[0].Qty).

Values that cannot be converted (Price=abc) do not throw. The property keeps its default value and ModelState gets an error: “The value 'abc' is not valid for Price.”

The binder fills every property it finds a value for — including ones you did not put in the form, such as IsAdmin. This is over-posting. Use [Bind(Include = "Name,Price")] or, better, a view model that only has the editable fields.

UpdateModel(entity) and TryUpdateModel(entity, new[] { "Name", "Price" }) bind into an existing object, which is handy for edit pages.

Syntax#

1public ActionResult Search(string q, int page = 1)     // ?q=pen&page=2
2public ActionResult Create(Product product)          // form fields → properties
3public ActionResult Save(List<OrderLine> lines)      // lines[0].Qty, lines[1].Qty
4public ActionResult Edit([Bind(Include = "Name,Price")] Product product)

Code example: Binding a product, its lines and a bad value#

1// Models/Product.cs
2using System.Collections.Generic;
3

4namespace ShopSite.Models
5{
6    public class Product
7    {
8        public int Id { get; set; }
9        public string Name { get; set; }
10        public decimal Price { get; set; }
11        public bool IsFeatured { get; set; }          // must NOT be set by customers
12        public List<string> Tags { get; set; }
13    }
14}
15

16// Controllers/BindingController.cs
17using System.Linq;
18using System.Web.Mvc;
19using ShopSite.Models;
20

21namespace ShopSite.Controllers
22{
23    public class BindingController : Controller
24    {
25        // POST /Binding/Create/7
26        [HttpPost]
27        public ActionResult Create(int id, [Bind(Include = "Name,Price,Tags")] Product product)
28        {
29            var errors = ModelState
30                .Where(kv => kv.Value.Errors.Any())
31                .Select(kv => 
quot;{kv.Key}: {kv.Value.Errors[0].ErrorMessage}");
32

33            return Content(
34                
quot;id (from route) = {id}\n" +
35                
quot;Name = {product.Name}, Price = {product.Price}, IsFeatured = {product.IsFeatured}\n" +
36                
quot;Tags = {string.Join(" | ", product.Tags ?? new System.Collections.Generic.List<string>())}\n" +
37                
quot;Valid = {ModelState.IsValid}; " + string.Join("; ", errors),
38                "text/plain");
39        }
40    }
41}

POST /Binding/Create/7 name=Gel+Pen&price=20.50&isFeatured=true&tags[0]=blue&tags[1]=smooth
200 OK
id (from route) = 7
Name = Gel Pen, Price = 20.50, IsFeatured = False
Tags = blue | smooth
Valid = True;

POST /Binding/Create/7 Name=Ruler&Price=abc
200 OK
id (from route) = 7
Name = Ruler, Price = 0, IsFeatured = False
Tags =
Valid = False; Price: The value 'abc' is not valid for Price.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. The id comes from the route segment /7; the other values come from the form body.
2. Field names are matched case-insensitively, so name and price fill Name and Price, and tags[0], tags[1] become a two-item list.
3. isFeatured=true was posted, but [Bind(Include = …)] does not list IsFeatured, so it stays false — the over-posting attempt fails.
4. Price=abc cannot be converted to decimal: Price keeps 0 and ModelState records the error, so IsValid is false.

Real-world use case#

Common mistakes#

Best practices#

• Bind into view models that contain only editable fields
• Generate inputs with strongly typed helpers so names match
• Check ModelState.IsValid in every POST action
• Use TryUpdateModel with an explicit property list for edits

Remember this#

Practice exercise#

1// Controllers/CartController.cs
2using System.Collections.Generic;
3using System.Linq;
4using System.Web.Mvc;
5

6namespace ShopSite.Controllers
7{
8    public class OrderLine
9    {
10        public string Sku { get; set; }
11        public int Qty { get; set; }
12    }
13

14    public class CartController : Controller
15    {
16        [HttpPost]
17        public ActionResult Total(List<OrderLine> lines)
18        {
19            var count = lines?.Count ?? 0;
20            var total = lines?.Sum(l => l.Qty) ?? 0;
21            return Content(
quot;{count} lines, {total} items", "text/plain");
22        }
23    }
24}

POST /Cart/Total lines[0].Sku=PEN&lines[0].Qty=3&lines[1].Sku=BOOK&lines[1].Qty=2
200 OK
2 lines, 5 items

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