How to validate for illegal fields in Model Validation - asp.net-core

Question

I have a .NET Core 2.2 web-api that accepts a PersonDto, it is getting validated with Model Validation, but it does not check for illegal fields. It only checks if matching fields are valid.
I want to make sure that the supplied JSON contains only the fields that are in my Dto (Class).
public class PersonDto
{
public string firstname { get; set; }
public string lastname { get; set; }
}
My controller looks simplified like this:
public async Task<ActionResult<Person>> Post([FromBody] PersonDto personDto)
{
// do things
}
I send it incorrect fields (name does not exist in my dto) and the ModelState is valid.
{
"name": "Diego"
}
I expected the Model Validation to complain that the field "Name" does not exist.
How can I check for illegal fields?

Answer 1

You could use ActionFilter and Reflection to compare the request body content to the model fields. If there are unexpected fields, manually add model errors and the ModelState.IsValid will be false.
1.Create an ActionFilter
public class CompareFieldsActionFilter : ActionFilterAttribute
{
public override void OnActionExecuting(ActionExecutingContext context)
{
//get all fields name
var listOfFieldNames = typeof(PersonDto).GetProperties().Select(f => f.Name).ToList();
var request = context.HttpContext.Request;
request.Body.Position = 0;
using (var reader = new StreamReader(request.Body))
{
//get request body content
var bodyString = reader.ReadToEnd();
//transfer content to json
JObject json = JObject.Parse(bodyString);
//if json contains fields that do not exist in Model, add model error
foreach (JProperty property in json.Children())
{
if (!listOfFieldNames.Contains(property.Name))
{
context.ModelState.AddModelError("Filed", "Field does not exist");
}
}
}
base.OnActionExecuting(context);
}
}
2.Use the filter on your action:
[HttpPost]
[CompareFieldsActionFilter]
public async Task<ActionResult<Person>> Post([FromBody] PersonDto personDto)
{
if (ModelState.IsValid)
{
// do things
}
// do things
}