# Custom Model Building

Model binding allows controller actions to work directly with model types (passed in as method arguments), rather than HTTP requests. Mapping between incoming request data and application models is handled by model binders. Developers can extend the built-in model binding functionality by implementing **custom model binders** (though typically, you don't need to write your own provider).

The following sample uses the `ModelBinder` attribute on the `Author` model:

```csharp
namespace CustomModelBindingSample.Data
{
    [ModelBinder(BinderType = typeof(AuthorEntityBinder))]
    public class Author
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string GitHub { get; set; }
        public string Twitter { get; set; }
        public string BlogUrl { get; set; }
    }
}
```

The following `AuthorEntityBinder` class binds an `Author` parameter:

```csharp
public class AuthorEntityBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext == null)
            throw new ArgumentNullException(nameof(bindingContext));

        var modelName = bindingContext.ModelName;
        
        // ...
    }
}
```

The `ModelBinder` attribute can be used to apply the `AuthorEntityBinder` to parameters that don't use default conventions:

```csharp
[HttpGet("{id}")]
public IActionResult GetById([ModelBinder(Name = "id")]Author author)
{
    // ...
}
```