Empty string not being converted to null when passing JSON object to controller

Question

In ASP.NET Core 2.1, I'm noticing basically the opposite is happening to this question: string.empty converted to null when passing JSON object to MVC Controller

When I send a JSON object back to a controller, that has properties with empty strings, they are no longer automatically being converted to null.

For example, take this JS object:

 var person = {firstName:"", lastName:""}; 

        $http.post("/app/index", person)
            .then(function (response) {
                // Success
            }, function (error) {
                // Failure
            })

When bound to the model in the controller, the properties with empty strings are remaining as empty strings:

    //
    // POST: /App
    [HttpPost]
    public async Task<ActionResult> Index([FromBody] AppFormDTO data)
    {
       ....
    }

Which when saving this object to the database, I'd prefer to have null values instead of a bunch of empty strings.

Did they change the DefaultModelBinder in ASP.NET Core 2.1? And if so, how do I change it back to how it was in MVC5 -- where string.empty was automatically converted to null?

UPDATE, to add my AppFormDTO model, for reference:

public class AppFormDTO
{
    public string firstName{ get; set; }
    public string lastName{ get; set; }
}

Right, but in ASP.NET MVC5, when an empty string was passed, it was automatically converted to null. I'm converting an MVC5 project to ASP.NET Core 2.1, and I'm wondering if I can retain that feature in the new framework? — Daniel Congrove
– Daniel Congrove, Commented Sep 7, 2018 at 13:13
You should pass null value or can use property attribute if value is empty then it will be null — Md. Abdul Alim
– Md. Abdul Alim, Commented Sep 7, 2018 at 13:17
possible duplicate of stackoverflow.com/questions/51376618/… — Antoine V
– Antoine V, Commented Sep 7, 2018 at 13:18

Jebathon · Accepted Answer · 2018-09-07 13:18:14Z

0

Add this to your PERSON model for firstName/lastName:

[DisplayFormat(ConvertEmptyStringToNull = false)]
... the model values

answered Sep 7, 2018 at 13:18

Jebathon

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1 Comment

Daniel Congrove Over a year ago

I've tried adding this set to both false and true, doesn't seem to make a difference, it's still coming through as an empty string instead of null.

Antoine V · Accepted Answer · 2018-09-07 14:44:35Z

0

Following the source code of ASP.NET Core

 if (bindingContext.ModelType == typeof(string))
 {
      // Already have a string. No further conversion required but handle ConvertEmptyStringToNull.
      if (bindingContext.ModelMetadata.ConvertEmptyStringToNull && string.IsNullOrWhiteSpace(value))
      {
           model = null;
      }
      else
      {
           model = value;
      }
 }

The behavior by default is set to empty string to null. Recheck your model AppFormDTO if you putted ConvertEmptyStringToNull = false which is not correct.

answered Sep 7, 2018 at 14:44

Antoine V

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1 Comment

Daniel Congrove Over a year ago

I added my AppFormDTO model for reference, which doesn't use the ConvertEmptyStringToNull attribute. Are you able to get it to work correctly? I'm thinking default model binding for a JSON object is overriding this. Specifically the JSONInputFormatter mentioned in the docs (learn.microsoft.com/en-us/aspnet/core/mvc/models/…), but I don't know see how to modify it.

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