Using the Bind attribute on a ViewModel class in ASP.NET MVC

Question

Why might a developer use the Bind attribute on a ViewModel object in an ASP.NET MVC project and can this have a detrimental effect an application?

[Bind(Include = "Id,Name")]
[MetadataType(typeof (MyViewModelValidation))]
public class MyViewModel
{
    public string CustomerProductUserName { get; set; }

    [Display(Name = "Name")]
    public string Name { get; set; }

}

public class MyViewModelValidation
{
    [HiddenInput(DisplayValue = false)]
    public int Id { get; set; }

    [Required]
    public string Name{ get; set; }
}

Answer 1

First of all, you don't need to create a MetadataType class for a ViewModel. You can use data annotation attributes directly in your ViewModel. MetadataType classes are used for Models automatically generated by EF or other ORMs, so that you can use data annotation attributes without touching the auto-generated code.

The Bind attribute does not have to be used either - unless you want to use Include or Exclude properties of the Bind attribute, to include or exclude properties in your Model in or from binding, respectively.

For example, in the code in your question, only the Id and Name properties will be bound when submitting your Model from your View. Even if you have an input in your View for CustomerProductUserName, when you submit your form, the property will always be null. This can be useful in cases like where you don't want an auto-generated ID field to be included in binding.

Properties excluded from binding are also excluded from validation, because validation is done as part of model binding. Also, you may use the Bind attribute for security reasons; for instance, when you want to make sure nothing but the properties in your model are being posted to the controller.

Answer 2

The purpose of using bind attribute is to prevent attacker from assigning property value while posting of request or control what properties you want to bind.

Let us suppose, you have a class called Member and a create method that saves member. But you do not want user to send a value for MemberType property.

Class Member  
{  
    public int MemberId { get; set; }  
    public string FirstName { get; set; }  
    public string LastName { get; set; }  
    public string MemberType { get; set; }  
}  

[HttpPost]  
Public ActionResult Create(Member member)  
{  
    Save(member);  
}

Let's say for now, you are only offering Regular member type which is the default value. You might think that you can prevent the user to send a value for MemberType property by not allowing input for MemberType Property. But when a user posts the member object, an attacker may intercept the request and send the MemberType value in request, as MemberId=1&FirstName=Chandra&LastName=Malla&MemberType=Premium and save the member as a Premium member. To prevent this, you can decorate Member class with Bind attribute.

[Bind(Include="MemberId,FirstName,LastName")]  
Class Member  
{
    ...

or

[Bind(Exclude="MemberType")]  
Class Member  
{  
    ...

Now if Member object is posted, MemberType property value will not be posted.

If you are using ViewModel, you might not necessarily have to use bind attribute because you can omit MemberType properties in your ViewModel.

Class Member  
{  
    public int MemberId { get; set; }  
    public string FirstName { get; set; }  
    public string LastName { get; set; }  
    public string MemberType { get; set; } 
}  

Class MemberViewModel  
{       
    public int MemberId { get; set; }   
    public string FirstName { get; set; }  
    public string LastName { get; set; }  
}  

[HttpPost]  
Public ActionResult Create(MemberViewModel memberviewmodel)  
{  
    Save(memberviewmodel);  
}

If you do not nicely design your model and/or ViewModel and do not use bind attribute to avoid posting of property you do not want, that might have detrimental effect.

Answer 3

In addition, if you want to prevent refactoring issues when renaming properties from your model you could do something like:

public async Task<ActionResult> Create([Bind(Include = nameof(Foo.Bar1)+","+nameof(Foo.Bar2)+","+nameof(Foo.Bar3))] Foo fooObj)

If you now e.g. rename "Bar1" your Include Bindings will still work.