Manipulate model value before passing it to DefaultModelBinder.BindModel

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Some decimal and decimal? properties in my view model are marked as "Percent" data type, along with other data annotations, for example:

[DataType("Percent")]
[Display(Name = "Percent of foo completed")]
[Range(0, 1)]
public decimal? FooPercent { get; set; }

I'd like to permit the user some flexibility in how they enter the data, i.e. with or without the percent sign, intermediate spaces, etc. But I still want to use the DefaultModelBinder behavior to get all of its functionality such as checking the RangeAttribute and adding the appropriate validation messages.

Is there a way to parse and change the model value, then pass it along? Here is what I am trying, but am getting a runtime exception. (Ignore the actual parsing logic; this is not its final form. I'm just interested in the model replacement question at this point.)

public class PercentModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext,
                                     ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelMetadata.DataTypeName == "Percent")
        {
            ValueProviderResult result =
                bindingContext.ValueProvider.GetValue(
                    bindingContext.ModelName);
            if (result != null)
            {
                string stringValue =
                    (string)result.ConvertTo(typeof(string));
                decimal decimalValue;
                if (!string.IsNullOrWhiteSpace(stringValue) &&
                    decimal.TryParse(
                        stringValue.TrimEnd(new char[] { '%', ' ' }),
                        out decimalValue))
                {
                    decimalValue /= 100.0m;

                    // EXCEPTION : This property setter is obsolete, 
                    // because its value is derived from 
                    // ModelMetadata.Model now.
                    bindingContext.Model = decimalValue;
                }
            }
        }

        return base.BindModel(controllerContext, bindingContext);
    }
}
1

There are 1 answers

1
Dave Mateer On BEST ANSWER

Never mind, this was a fundamental misunderstanding of where validation happens in the MVC cycle. After spending some time in the MVC source code, I see how this works.

In case it is helpful to others, here is what is working for me:

[DataType("Percent")]
[Display(Name = "Percent of foo completed")]
[Range(0.0d, 1.0d, ErrorMessage="The field {0} must be between {1:P0} and {2:P0}.")]
public decimal? FooPercent { get; set; }

And in the binder, you just return the value:

public class PercentModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext,
                                     ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelMetadata.DataTypeName == "Percent")
        {
            ValueProviderResult result =
                bindingContext.ValueProvider.GetValue(
                    bindingContext.ModelName);
            if (result != null)
            {
                string stringValue =
                    (string)result.ConvertTo(typeof(string));
                decimal decimalValue;
                if (!string.IsNullOrWhiteSpace(stringValue) &&
                    decimal.TryParse(
                        stringValue.TrimEnd(new char[] { '%', ' ' }),
                        out decimalValue))
                {
                    return decimalValue / 100.0m;
                }
            }
        }

        return base.BindModel(controllerContext, bindingContext);
    }
}