Problems with Model binding and validation attributes with asp.net Web API

Question

I am writing a Web API with ASP.NET Web API, and make use of the following View Model. I seem to be having a problem with the data binding when there are two validation attributes on a particular property (i.e. [Required] and [StringLength(10)]).

When posting a JSON value from a client to a controller action of the form:

// POST api/list

public void Post([FromBody] TaskViewModel taskVM)

I observe the following:

• If I remove one of the multiple attributes everything is bound OK;
• If I leave in the multiple attributes, the client recieves a 500 internal server error and the body of the Post method is never reached.

Any ideas why this happens? Cheers

public class TaskViewModel
{

    //Default Constructor
    public TaskViewModel() { }


    public static TaskViewModel MakeTaskViewModel(Task task) 
    {
        return new TaskViewModel(task);
    }

    //Constructor
    private TaskViewModel(Task task)
    {
        this.TaskId = task.TaskID;
        this.Description = task.Description;
        this.StartDate = task.StartDate;
        this.Status = task.Status;
        this.ListID = task.ListID;
    }

    public Guid TaskId { get; set; }

    [Required]
    [StringLength(10)]
    public string Description { get; set; }

    [Required]
    [DataType(DataType.DateTime)]
    public System.DateTime StartDate { get; set; }

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

    public System.Guid ListID { get; set; }       
}

Answer 1

You need to inspect what is inside in the 500 internal server

• make sure that you turn customerror off in your web.config
• If you selfhost web.API you need to set GlobalConfiguration.Configuration.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Always;
• use your browser development console's network tab (in IE, Chrome you can get the console with F12) or if you are using FireFox then use FireBug or a thrid party tool like Fiddler.

Then you can see what went wrong on the server and go further to solve your problem.

In your case this is in the response:

"Message":"An error has occurred.","ExceptionMessage":"Property 'StartDate' on type 'MvcApplication3.Controllers.TaskViewModel' is invalid. Value-typed properties marked as [Required] must also be marked with [DataMember(IsRequired=true)] to be recognized as required. Consider attributing the declaring type with [DataContract] and the property with [DataMember(IsRequired=true)].","ExceptionType":"System.InvalidOperationException"

So your problem is not that you have two attributes but that you've marked your properties with [Required] to solve this the exception tells you what to do.

You need to add [DataMember(IsRequired=true)] to your required properties where the property type is a value type (e.g int, datatime, etc.):

So change your TaskViewModel to:

[DataContract]
public class TaskViewModel
{

    //Default Constructor
    public TaskViewModel() { }

    [DataMember]
    public Guid TaskId { get; set; }

    [Required]
    [DataMember]
    [StringLength(10)]
    public string Description { get; set; }

    [Required]
    [DataMember(IsRequired = true)]
    [DataType(DataType.DateTime)]
    public System.DateTime StartDate { get; set; }

    [Required]
    [DataMember]
    public string Status { get; set; }

    [DataMember]
    public System.Guid ListID { get; set; }
}

Some side notes:

• You need to reference the System.Runtime.Serialization dll in order to use the DataMemberAttribute
• You need to mark your class with [DataContract] and you need to mark all of its properties with [DataMember] not just the required ones.