ASP.NET CORE Model binding: Why the getter of DTO is get called by runtime before the value is set?

Question

I am trying to use the getter to valid the property of DTO, If the property is invalid the exception is thrown, the sample code as follow:

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{

    private readonly ILogger<WeatherForecastController> _logger;

    public WeatherForecastController(ILogger<WeatherForecastController> logger)
    {
        _logger = logger;
    }


    [HttpPost("test")]
    public IActionResult Post([FromBody] Foo f)
    {
        return Ok(f.Bar.Count == 1);
    }
}

public class Foo
{
    private List<string> _bar;
    public List<string> Bar
    {
        get
        {
            if (_bar == null || _bar.Count == 0) throw new System.Exception("at lease one element is required!");
            return _bar;
        }
        set
        {
            _bar = value;
        }
    }
}

use the curl to debug the api

curl --location --request POST 'http://localhost:5000/WeatherForecast/test2' --header 'Content-Type: application/json' --data-raw '{
    "Baz":"HelloWorld"
}'

during the API debugging, I observed that the setter is get called with an empty value of list, even if the corresponding input has value, and then was the getter. As a result, the getter thrown an exception due to the empty list.

It seem that the model binding behavior has some differences between enumerable type and non-enumerable type ( if I changed the Bar from List<string> into string , I would get the value in the setter);

Question:

1. Is there any way to configure asp.net core to bind the property of enumerable type in the way of non-enumerable type, I would like to do the validation in the getter instead of other place

Answer 1

In your request model Foo , you assert that Bar is going to be a collection of strings (eg, an array). However, in your curl request, you are setting Bar to a single string instead of an array containing one string.

Instead, you should use curl --location --request POST 'http://localhost:5000/WeatherForecast/test2' --header 'Content-Type: application/json' --data-raw '{ "Bar": ["HelloWorld"] }' .

If you want to accept either a string or an array for Bar , then you cannot use your DTO class with [FromBody] . Instead, your will have to write your own logic to determine the type of data in the request body and then manually parse the JSON data into the appropriate model.