Asp.Net MVC 2 – Bind a model’s property to a different named value

In response to michaelalm’s answer and request – here’s what I’ve ended up doing. I’ve left the original answer ticked mainly out of courtesy since one of the solutions suggested by Nathan would have worked.

The output of this is a replacement for DefaultModelBinder class which you can either register globally (thereby allowing all model types to take advantage of aliasing) or selectively inherit for custom model binders.

It all starts, predictably with:

/// <summary>
/// Allows you to create aliases that can be used for model properties at
/// model binding time (i.e. when data comes in from a request).
/// 
/// The type needs to be using the DefaultModelBinderEx model binder in 
/// order for this to work.
/// </summary>
[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = true)]
public class BindAliasAttribute : Attribute
{
  public BindAliasAttribute(string alias)
  {
    //ommitted: parameter checking
    Alias = alias;
  }
  public string Alias { get; private set; }
}

And then we get this class:

internal sealed class AliasedPropertyDescriptor : PropertyDescriptor
{
  public PropertyDescriptor Inner { get; private set; }

  public AliasedPropertyDescriptor(string alias, PropertyDescriptor inner)
    : base(alias, null)
  {
    Inner = inner;
  }

  public override bool CanResetValue(object component)
  {
    return Inner.CanResetValue(component);
  }

  public override Type ComponentType
  {
    get { return Inner.ComponentType; }
  }

  public override object GetValue(object component)
  {
    return Inner.GetValue(component);
  }

  public override bool IsReadOnly
  {
    get { return Inner.IsReadOnly; }
  }

  public override Type PropertyType
  {
    get { return Inner.PropertyType; }
  }

  public override void ResetValue(object component)
  {
    Inner.ResetValue(component);
  }

  public override void SetValue(object component, object value)
  {
    Inner.SetValue(component, value);
  }

  public override bool ShouldSerializeValue(object component)
  {
    return Inner.ShouldSerializeValue(component);
  }
}

This proxies a ‘proper’ PropertyDescriptor that is normally found by the DefaultModelBinder but presents its name as the alias.

Next we have the new model binder class:

UPDATED WITH @jsabrooke’s suggestion below

public class DefaultModelBinderEx : DefaultModelBinder
{
  protected override System.ComponentModel.PropertyDescriptorCollection
    GetModelProperties(ControllerContext controllerContext, 
                      ModelBindingContext bindingContext)
  {
    var toReturn = base.GetModelProperties(controllerContext, bindingContext);

    List<PropertyDescriptor> additional = new List<PropertyDescriptor>();

    //now look for any aliasable properties in here
    foreach (var p in 
      this.GetTypeDescriptor(controllerContext, bindingContext)
      .GetProperties().Cast<PropertyDescriptor>())
    {
      foreach (var attr in p.Attributes.OfType<BindAliasAttribute>())
      {
        additional.Add(new AliasedPropertyDescriptor(attr.Alias, p));

        if (bindingContext.PropertyMetadata.ContainsKey(p.Name)
            && !string.Equals(p.Name, attr.Alias, StringComparison.OrdinalIgnoreCase)))
        {
            bindingContext.PropertyMetadata.Add(
                attr.Alias,
                bindingContext.PropertyMetadata[p.Name]);
        }
      }
    }

    return new PropertyDescriptorCollection
      (toReturn.Cast<PropertyDescriptor>().Concat(additional).ToArray());
  }
}

And, then technically, that’s all there is to it. You can now register this DefaultModelBinderEx class as the default using the solution posted as the answer in this SO: Change the default model binder in asp.net MVC, or you can use it as a base for your own model binder.

Once you’ve selected your pattern for how you want the binder to kick in, you simply apply it to a model type as follows:

public class TestModelType
{
    [BindAlias("LPN")]
    //and you can add multiple aliases
    [BindAlias("L")]
    //.. ad infinitum
    public string LongPropertyName { get; set; }
}

The reason I chose this code was because I wanted something that would work with custom type descriptors as well as being able to work with any type. Equally, I wanted the value provider system to be used still in sourcing the model property values. So I’ve changed the meta data that the DefaultModelBinder sees when it starts binding. It’s a slightly more long-winded approach – but conceptually it’s doing at the meta data level exactly what you want it to do.

One potentially interesting, and slightly annoying, side effect will be if the ValueProvider contains values for more than one alias, or an alias and the property by it’s name. In this case, only one of the retrieved values will be used. Difficult to think of a way of merging them all in a type-safe way when you’re just working with objects though. This is similar, though, to supplying a value in both a form post and query string – and I’m not sure exactly what MVC does in that scenario – but I don’t think it’s recommended practise.

Another problem is, of course, that you must not create an alias that equals another alias, or indeed the name of an actual property.

I like to apply my model binders, in general, using the CustomModelBinderAttribute class. The only problem with this can be if you need to derive from the model type and change it’s binding behaviour – since the CustomModelBinderAttribute is inherited in the attribute search performed by MVC.

In my case this is okay, I’m developing a new site framework and am able to push new extensibility into my base binders using other mechanisms to satisfy these new types; but that won’t be the case for everybody.