Handle ModelState Validation in ASP.NET Web API

Question

I was wondering how I can achieve model validation with ASP.NET Web API. I have my model like so:

public class Enquiry {     [Key]     public int EnquiryId { get; set; }     [Required]     public DateTime EnquiryDate { get; set; }     [Required]     public string CustomerAccountNumber { get; set; }     [Required]     public string ContactName { get; set; } } 

I then have a Post action in my API Controller:

public void Post(Enquiry enquiry) {     enquiry.EnquiryDate = DateTime.Now;     context.DaybookEnquiries.Add(enquiry);     context.SaveChanges(); } 

How do I add if(ModelState.IsValid) and then handle the error message to pass down to the user?

Answer 1

For separation of concern, I would suggest you use action filter for model validation, so you don't need to care much how to do validation in your api controller:

using System.Net; using System.Net.Http; using System.Web.Http.Controllers; using System.Web.Http.Filters;  namespace System.Web.Http.Filters {     public class ValidationActionFilter : ActionFilterAttribute     {         public override void OnActionExecuting(HttpActionContext actionContext)         {             var modelState = actionContext.ModelState;              if (!modelState.IsValid)                 actionContext.Response = actionContext.Request                      .CreateErrorResponse(HttpStatusCode.BadRequest, modelState);         }     } } 

Answer 2

Maybe not what you were looking for, but perhaps nice for someone to know:

If you are using .net Web Api 2 you could just do the following:

if (!ModelState.IsValid)      return BadRequest(); 

Depending on the model errors, you get this result:

{    Message: "The request is invalid."    ModelState: {        model.PropertyA: [             "The PropertyA field is required."        ],        model.PropertyB: [              "The PropertyB field is required."        ]    } }