C# ASP.NET Core ModelBinder not Updating Model

Question

I have created a ModelBinder, which only gets triggered if an object has a [Decimal] attribute assigned, yet for some reason, despite it actually sanitising the data it does not seem to update the posted model.

I wonder if someone could see from my code below, where I maybe going wrong.

Startup.cs

public void ConfigureServices(IServiceCollection serviceCollection)
{
    serviceCollection.AddMvc(config => config.ModelBinderProviders.Insert(0, new DecimalModelBinderProvider()));        
}

DecimalModelBinderProvider.cs

public class DecimalModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext modelBinderProviderContext)
    {
        if (modelBinderProviderContext == null)
        {
            throw new ArgumentNullException(nameof(modelBinderProviderContext));
        }

        if (!modelBinderProviderContext.Metadata.IsComplexType)
        {
            try
            {
                var propertyName = modelBinderProviderContext.Metadata.PropertyName;

                var property = modelBinderProviderContext.Metadata.ContainerType.GetProperty(propertyName);

                if (property != null)
                {
                    var attribute = property.GetCustomAttributes(typeof(DecimalAttribute), false).FirstOrDefault();

                    if (attribute != null)
                    {
                        return new DecimalModelBinder(modelBinderProviderContext.Metadata.ModelType, attribute as IDecimalAttribute);
                    }
                }
            }
            catch (Exception exception)
            {
                var message = exception.Message;

                return null;
            }
        }

        return null;
    }
}

DecimalModelBinder.cs

public class DecimalModelBinder : IModelBinder
{
    private readonly IDecimalAttribute _decimalAttribute;

    private readonly SimpleTypeModelBinder _simpleTypeModelBinder;

    public DecimalModelBinder(Type type, IDecimalAttribute decimalAttribute)
    {
        if (type == null)
        {
            throw new ArgumentNullException(nameof(type));
        }

        _decimalAttribute = decimalAttribute;

        _simpleTypeModelBinder = new SimpleTypeModelBinder(type);
    }

    public Task BindModelAsync(ModelBindingContext modelBindingContext)
    {
        if (modelBindingContext == null)
        {
            throw new ArgumentNullException(nameof(modelBindingContext));
        }

        var valueProviderResult = modelBindingContext.ValueProvider.GetValue(modelBindingContext.ModelName);

        if (valueProviderResult != ValueProviderResult.None)
        {
            modelBindingContext.ModelState.SetModelValue(modelBindingContext.ModelName, valueProviderResult);

            var value = valueProviderResult.FirstValue;

            bool success;

            var result = _decimalAttribute.Decimal(value, out success);

            if (success)
            {
                modelBindingContext.Result = ModelBindingResult.Success(result);

                return Task.CompletedTask;
            }
        }

        return _simpleTypeModelBinder.BindModelAsync(modelBindingContext);
    }
}

IDecimalAttribute.cs

public interface IDecimalAttribute
{
    object Decimal(string value, out bool success);
}

DecimalAttribute.cs

[AttributeUsage(AttributeTargets.Property)]
public class DecimalAttribute : Attribute, IDecimalAttribute
{
    public object Decimal(string value, out bool success)
    {
        var tryModelValue = string.IsNullOrEmpty(value) ? "0.00" : value.Replace("£", "").Replace("%", "");

        decimal @decimal;

        success = decimal.TryParse(tryModelValue, out @decimal);

        return @decimal;
    }
}

Test.cs

public class Test
{
    [Display(Name = "Name", Description = "Name")]
    public string Name { get; set; }

    [Decimal]
    [Display(Name = "Amount", Description = "Amount")]
    public double Amount { get; set; }
}

HomeController

[ValidateAntiForgeryToken]
[HttpPost]
public IActionResult Index(Test test)
{
    if (ModelState.IsValid)
    {

    }

    return View(test);
}

For the purpose of testing I will enter the value £252.83 into the Amount field and submit the form.

If I then place a brakepoint on the line var value = valueProviderResult.FirstValue; I can see that value is £252.83 and if I place a breakpoint on the line modelBindingContext.Result = ModelBindingResult.Success(result); I can see that the result is 252.83M.

However if I step through the code further and place a breakpoint on the line if (ModelState.IsValid) the valid state is false and if I inspect the model test the object Amount is 0.

If anyone can help it would be much appreciated :-)

Answer 1

Try inspect further on the ModelState Error, the Amount property should be invalid and there must be an exception.

I am guessing it should be InvalidCastException. I notice that the Amount property in Test class is Double while you are producing Decimal in your DecimalAttribute.

So the build-in Model Binder that processing the Test class (should be ComplexTypeModelBinder) unable to set the Amount property as it is different type.