Question

My application has many models, many of which contain percentage data. These are represented as decimal or decimal? structs in the model. However, not all properties with decimal structs are percentages. Some should be treated like regular decimals.

The percentages need special attention:

• For display, they should use {0:P2} format. (I have this part working.)
• For editing, they should allow the same format as the display, i.e. “95” or “95%” or “95.00 %” all bind to a value of 0.95.

I started down the road of creating a PercentModelBinder that implements IModelBinder, but then realized that you can only apply the ModelBinderAttribute to a class, not a property.

What’s the best way to handle this case where some (but not all) uses of a type need special handling both for display and binding?

Every solution I think of smells badly of overkill, fighting the MVC framework. Surely there is a better way than:

• Creating a custom Percentage struct and using it as the basis for the IModelBinder and EditorTemplates, or
• Reimplementing the default binding behavior of decimal and decimal? and changing the parsing logic based on intimate knowledge of my model, or
• Implementing a custom model binder for each class that contains a percentage property, or
• Using fake proxy properties in the model (i.e. breaking MVC)

Answer 1

One possibility is to write a custom metadata aware attribute:

public class PercentageAttribute : Attribute, IMetadataAware
{
    public void OnMetadataCreated(ModelMetadata metadata)
    {
        metadata.AdditionalValues["percentage"] = metadata.EditFormatString;
    }
}

then decorate your view model properties that represent percentages with it:

public class MyViewModel
{
    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:P2}")]
    [Percentage]
    public decimal? Percentage { get; set; }
}

and inside the custom model binder test for the presence of this value:

public class PercentageModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelMetadata.AdditionalValues.ContainsKey("percentage"))
        {
            var format = (string)bindingContext.ModelMetadata.AdditionalValues["percentage"];
            // TODO: do the custom parsing here
            throw new NotImplementedException();
        }
        else
        {
            // Let the default parsing occur
            return base.BindModel(controllerContext, bindingContext);
        }
    }
}

Now you can register this model binder to all decimals.