Convert ModelState to JSON for Logging

How does it convert ModelState to JSON?

The SerializableError Class provides this functionality.

And more importantly, how can I use what it is using?

using Microsoft.AspNetCore.Mvc;using Newtonsoft.Json;//get key(s) and error message(s) from the ModelStatevar serializableModelState = new SerializableError(ModelState);//convert to a stringvar modelStateJson = JsonConvert.SerializeObject(serializableModelState);//log itlogger.LogInformation("Bad Model State", modelStateJson);

Sample output

{    "Name": ["The Name field is required."]}

I was able to figure this out by digging through the ASP.NET Core MVC source code. The interesting part of source code from the SerializableError class:

foreach (var keyModelStatePair in modelState){    var key = keyModelStatePair.Key;    var errors = keyModelStatePair.Value.Errors;    if (errors != null && errors.Count > 0)    {        var errorMessages = errors.Select(error =>        {            return string.IsNullOrEmpty(error.ErrorMessage) ?                Resources.SerializableError_DefaultError : error.ErrorMessage;        }).ToArray();        Add(key, errorMessages);    }}

Just to get a list of the error messages for each Model Property that failed validation in the manner you want as indicated above using an extension method. i.e ModelState.ToJson(), you need create a static class with a static function ToJson(...). The code example will look something like this.

public static class ModelStateExtensions{    /// <summary>    /// Reads all the error messages in a <see cref="ModelStateDictionary"/> as     /// a collection and returns a JSON <see cref="string"/> of the list.    /// </summary>    /// <param name="modelstate">Current modelstate assuming that you've checked    /// and confirmed that is Invalid using <see     /// cref="ModelStateDictionary.IsValid"/>    /// </param>    /// <returns>    /// Collection of validation errors for the model as a JSON string.    /// </returns>    public static string ToJson(this ModelStateDictionary modelstate)    {        List<string> errors = modelstate.Values                                        .SelectMany(x => x.Errors)                                        .Select(x => x.ErrorMessage)                                        .ToList();        return JsonConvert.SerializeObject(errors);    }}

The ModelState property on every controller is normally a ModelStateDictionary, so if we want an additional method for it, that is the class we need to extend. You can learn more about extension methods in C# by following this Link.

Let now see how to use our extension method in a sample controller action:

public IActionResult Create(UserViewModel model){    if(!ModelState.IsValid)    {        string json = ModelState.ToJson();        // insert code to log json to file here        return BadRequest(ModelState);    }}