Percentage properties in MVC 3

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

Answer 1

Bind to a string in your view model. Then when do the conversion back to the real model.

Answer 2

Instead of representing your percentage properties using decimal primitives (see Primitive Obsession), why don't you create a Percentage type to wrap your desired functionality? You should have much more flexibility doing it that way...

Answer 3

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.