ASP.NET mvc 3 model binding with list of strings

Question

I am currently using model binding and ASP.NET MVC 3 and .NET 4.0.

View Model Class:

public class BasicViewModel 
{
    [Display(Name = @"Names")]
    [Required(ErrorMessage = @"Names is required")]
    [DisplayFormat(ConvertEmptyStringToNull = true)]
    List<string> Names { get; set; }

    [Display(Name = @"Email")]
    [Required(ErrorMessage = @"Email is required")]
    string Email { get; set; }

}

Controller

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult NameEmail( BasicViewModel basicModel)
{
      // some manipulation of data
}

View in cshtml file (razor view engine)

 // model declared here using @model BasivViewModel
 // only required part shown labels part of code removed
 @Html.TextBoxFor(model => model.Names)
 ...
 @Html.TextBoxFor(model => model.Email)
 ...

The model binding provided by ASP.NET MVC binds the string Email to null if it is empty but binds the List Names to empty string (""). I want it to be null. I made the binding work using JavaScript by parsing the values of form fields on click of submit button. But i want the asp.net model binding to do this. Furthermore, it would be great if there is some field in Data Annotations class like Required for this functionality. I tried this Null Display Text Property and refer to the remarks section. Is there a solution or is this how it is implemented?. I am not sure whether i have understood this part of model binding correctly.

with lots of duplicate : consider stackoverflow.com/questions/12487188/… — Vishal Sharma
@vishalsharma may i know how the above link solves the problem? I know how to model bind to the list. It would be great if you could provide some explanation on how the link solves the problem. — Sudarsan GP
hanselman.com/blog/… read this one.. if you don't understand tell me i'll surely help — Vishal Sharma
@vishalsharma from the article it is clear that the index for the list needs to be continuous. But i guess i am missing the link between that article and how the List Names binds to empty string instead of null which is not the case in string email. — Sudarsan GP
can you please tell me what values are available in request.form during posting .. — Vishal Sharma

vbn vbn · Accepted Answer · 2014-04-14T12:27:38

By default, if the field, representing an array, is in the html, the controller will receive an array of length 0. However, to make the array null, you can define a custom ModelBinder.

public class MyModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType == typeof(List<string>))
        {
            HttpRequestBase request = controllerContext.HttpContext.Request;
            // Check to see if any of the elements the array is not empty and
            // returns null if they all are.
            return request.Form.GetValues("Names").Any(x => !string.IsNullOrWhiteSpace(x)) ?
                base.BindModel(controllerContext, bindingContext) :
                null;
            //You can also remove empty element from the array as well, by using
            // a where clause
        }
        return base.BindModel(controllerContext, bindingContext);
    }
}

Alternatively, you can also implement IModelBinder instead of DefaultModelBinder.

The next step is to register the custom binder in your Application_Start function in the Global.asax.cs file.

ModelBinders.Binders.Add(typeof(List<string>), new MyModelBinder());

This basically tells the mvc engine to use the MyModelBinder whenever the field is List<string>

To know more about modelbinder, goolge "MVC custom model binding". Let me know you go :)

ASP.NET mvc 3 model binding with list of strings

1 Answers