.Net Core Api cannot access original value on model validation failures

I cannot access the original value that didn’t pass the model validation. I would suspect AttemptedValue and/or RawValue in ModelStateEntry to contain the original value, however both properties are null.

For clarification, I wrote a minimalistic api, to showcase the issue.

The model to validate:

public class User
    public string Email { get; set; }

The controller:

public class TestController : ControllerBase
    public string Test([FromBody] User user)
        return user.Email;

The validation filter:

public class ValidationFilterAttribute : ActionFilterAttribute, IOrderedFilter
    public int Order { get; } = int.MinValue;

    override public void OnActionExecuting(ActionExecutingContext context)
        if (!context.ModelState.IsValid)
            ModelStateEntry entry = context.ModelState.ElementAt(0).Value;
            var attemptedVal = entry.AttemptedValue;
            var rawVal = entry.RawValue;
            context.Result = new OkObjectResult(rawVal);

When I call the test method with this model:

    "email": "No email here ;)"

The ValidationFilterAttribute code is called as expected, however the ModelStateEntry does not contain the original value. Both AttemptedValue and RawValue are null:

Visual Studio debugging screenshot


Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

As far as I know, for model binding, the filter will calls context.ModelState.SetModelValue to set value for RawValue and AttemptedValue.

But the SystemTextJsonInputFormatter doesn’t set it to solve this issue, I suggest you could try to build custom extension method and try again.

More details, you could refer to below codes:

Create a new ModelStateJsonInputFormatter class:

public class ModelStateJsonInputFormatter : SystemTextJsonInputFormatter
    public ModelStateJsonInputFormatter(ILogger<ModelStateJsonInputFormatter> logger, JsonOptions options) : 
        base(options ,logger)
    public override async Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context)
        var result = await base.ReadRequestBodyAsync(context);
        foreach (var property in context.ModelType.GetProperties())
            var propValue = property.GetValue(result.Model, null);
            var propAttemptValue = property.GetValue(result.Model, null)?.ToString();
            context.ModelState.SetModelValue(property.Name, propValue, propAttemptValue);
        return result;

Reigster it in startup.cs:
            services.AddControllersWithViews(options => {
                var serviceProvider = services.BuildServiceProvider();
                var modelStateJsonInputFormatter = new ModelStateJsonInputFormatter(
                options.InputFormatters.Insert(0, modelStateJsonInputFormatter);


.Net Core Api cannot access original value on model validation failures

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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