How to check if client send me empty json array with .NET Mvc validation?

Question

I have the following view model:

public class EventViewModel
{
    [Required(ErrorMessage = "type can not be empty")]
    [JsonProperty("type")]
    [DisplayName("type")]
    public string Type { get; set; }

    [Required(ErrorMessage = "date can not be empty")]
    [JsonProperty("date")]
    [DisplayName("date")]
    public int Timestamp { get; set; }

    [JsonProperty("data")]
    public JObject Data { get; set; }
}

and the following controller action:

[Route("/api/v1.0/events")]
[HttpPost]
public async Task<IActionResult> Events([FromBody] List<EventViewModel> viewModel)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

I expect the following JSON:

[
 {
  "type" : "open",
  "date" : 1567673607,
        "data" : {
             "message_id" : "lalalalala"
        }
 }
]

But what if a client will send me just:

[]

empty array?

I would like to show validation message. How I can do that? Is it possible?

Update

Maybe the explanation of the problem is not so clear or I don't understand something. I'll try to fix it.

If I have this kind of JSON:

{
 "items" :[
   {
    ...    
   }, 
   ...
   ]  
}

Then it's ok. Not problem. I can write:

public class EventViewModelList
{
    [Required] 
    List<EventViewModel> EventViewModels {get; set;}
}

But my JSON is just array of objects. SO, I can't.

And I can to do something like:

public async Task<IActionResult> Events([Required][FromBody] List<EventViewModel> viewModel)

Because it does not work. Put validation into the controller? No (( Controller - is controller. Validation - is validation.

I think that I need attribute. But controller action level attribute. Like:

 [Route("/api/v1.0/events")]
        [NotEmptyListOfViewModels]
        [HttpPost]
        public async Task<IActionResult> Events([FromBody] List<EventViewModel> viewModel)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

Am I right?

You could create your own validation attribute by implementing `ValidationAttribute` and add the attribute to your property. — NotFound, Jan 27 '20 at 14:08
Does this answer your question? [ModelState is valid with null model](https://stackoverflow.com/questions/19851352/modelstate-is-valid-with-null-model) — rak007, Jan 27 '20 at 14:09

score 1 · Accepted Answer · answered Jan 28 '20 at 06:21

You could write a custom model binder like below:

NotEmptyListOfViewModels

public class NotEmptyListOfViewModels:IModelBinder
{
    public async  Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext == null)
            throw new ArgumentNullException(nameof(bindingContext));

        //Get command model payload (JSON) from the body  
        String valueFromBody;
        using (var streamReader = new StreamReader(bindingContext.HttpContext.Request.Body))
        {
            valueFromBody =await streamReader.ReadToEndAsync();
        }
        var modelInstance = JsonConvert.DeserializeObject<List<EventViewModel>>(valueFromBody);

        if(modelInstance.Count==0)
        {
            bindingContext.ModelState.AddModelError("JsonData", "The json is null !");
        }

        bindingContext.Result = ModelBindingResult.Success(modelInstance);
    }
}

Events action

[Route("/api/v1.0/events")]
[HttpPost]
public async Task<IActionResult> Events([ModelBinder(BinderType = typeof(NotEmptyListOfViewModels))]List<EventViewModel> viewModel)
{
     if (!ModelState.IsValid)
     {
        return BadRequest(ModelState);
     }

     return Ok();
}

score 0 · Answer 2 · answered Jan 27 '20 at 14:11

You can do as follows:

[Route("/api/v1.0/events")]
[HttpPost]
public async Task<IActionResult> Events([FromBody] List<EventViewModel> viewModel)
{
    if(viewModel == null || viewModel.Count == 0)
    {
       ModelState.AddModelError("","List can not be empty or null");
    }

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

    ..............
}

How to check if client send me empty json array with .NET Mvc validation?

Update

2 Answers2