Question

I have a model binder for a custom type Money. The binder works fine, but the built in binding/validation is kicking in and marking data as invalid.

My binder looks like this:

public class MoneyModelBinder : DefaultModelBinder 
{
    protected override void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var money = (Money)bindingContext.Model;
        var value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName + ".Amount").AttemptedValue;
        var currencyCode = bindingContext.ValueProvider.GetValue(bindingContext.ModelName + ".Iso3LetterCode").AttemptedValue;

        Money parsedValue;
        if (String.IsNullOrEmpty(value))
        {
            money.Amount = null;
            return;
        }

        var currency = Currency.FromIso3LetterCode(currencyCode);

        if(!Money.TryParse(value, currency, out parsedValue))
        {
            bindingContext.ModelState.AddModelError("Amount", string.Format("Unable to parse {0} as money", value));
        }
        else
        {
            money.Amount = parsedValue.Amount;
            money.Currency = parsedValue.Currency;
        }
    }
}

When a user types a value like “45,000” into a textbox my binder correctly picks up the value, parses it and sets it into the model.

The problem I have, is that the default validation then kicks in a states The value '45,000' is not valid for Amount, which as it’s a decimal type makes sense, but I’ve already bound the data. How can I prevent the default binders binding data which I’ve already handled?

I’m not sure if this make any difference, but I’m using Html.EditorFor with and editor that looks like this:

@model Money

<div class="input-prepend">
  <span class="add-on">@Model.Currency.Symbol</span>
    @Html.TextBoxFor(m => m.Amount, new{
                            placeholder=string.Format("{0}", Model.Currency), 
                            @class="input-mini",
                            Value=String.Format("{0:n0}", Model.Amount)
                            })
    @Html.HiddenFor(x => x.Iso3LetterCode)
</div>

Answer 1

You can just mark the Amount property as read only:

[ReadOnly(true)]
public decimal? Amount { get; set; }