Custom MVC1 model binder that handles array properties

ArrayModelBinder.cs

public class ArrayModelBinder : IFilteredPropertyBinder
{
    private static readonly Regex IdRegex = new Regex("(?<=_)[0-9]+(?=_)");

    public bool IsMatch(Type modelType)
    {
        return modelType.IsArray;
    }

    public void BindProperty(IModelBinder binder, ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor)
    {
        var elementType = propertyDescriptor.PropertyType.GetElementType();
        var namePrefix = char.ToLower(propertyDescriptor.Name[0]) + propertyDescriptor.Name.Substring(1);
        var list = new ArrayList();

        var relatedFormValues = GetRelatedFormPairs(namePrefix, bindingContext.ValueProvider);
        var groupedFormValues = GroupByIndex(relatedFormValues);

        foreach (var elementValues in groupedFormValues.OrderBy(x => x.Key))
        {
            var modelBindingContext = new ModelBindingContext
            {
                ValueProvider = new PrefixedValueProvider(namePrefix + "_" + elementValues.Key + "_", elementValues),
                ModelType = elementType
            };
            var model = binder.BindModel(controllerContext, modelBindingContext);

            list.Add(model);
        }

        var array = list.ToArray(elementType);

        propertyDescriptor.SetValue(bindingContext.Model, array);
    }

    static IEnumerable<KeyValuePair<string, ValueProviderResult>> GetRelatedFormPairs(string namePrefix, IEnumerable<KeyValuePair<string, ValueProviderResult>> valueProvider)
    {
        return valueProvider
            .Where(kvp => kvp.Key.StartsWith(namePrefix + "_"));
    }

    static IEnumerable<IGrouping<int, KeyValuePair<string, ValueProviderResult>>> GroupByIndex(IEnumerable<KeyValuePair<string, ValueProviderResult>> formValues)
    {
        return formValues.GroupBy(x => int.Parse(IdRegex.Match(x.Key).Value));
    }
}

IFilteredPropertyBinder.cs

public interface IFilteredPropertyBinder : IPropertyBinder
{
    bool IsMatch(Type modelType);
}

public interface IPropertyBinder
{
    void BindProperty(IModelBinder binder, ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor);
}

SmartBinder.cs

public class SmartBinder : DefaultModelBinder
{
    readonly List<IFilteredModelBinder> modelBinders = new List<IFilteredModelBinder>();
    readonly List<IFilteredPropertyBinder> propertyBinders = new List<IFilteredPropertyBinder>();

    public SmartBinder Add(IFilteredModelBinder binder)
    {
        modelBinders.Add(binder);
        return this;
    }

    public SmartBinder Add(IFilteredPropertyBinder binder)
    {
        propertyBinders.Add(binder);
        return this;
    }

    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        foreach (var binder in modelBinders)
        {
            if (!binder.IsMatch(bindingContext.ModelType)) continue;

            return binder.BindModel(controllerContext, bindingContext);
        }

        return base.BindModel(controllerContext, bindingContext);
    }

    protected override void BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor)
    {
        foreach (var binder in propertyBinders)
        {
            if (!binder.IsMatch(propertyDescriptor.PropertyType)) continue;

            binder.BindProperty(this, controllerContext, bindingContext, propertyDescriptor);
        }

        base.BindProperty(controllerContext, bindingContext, propertyDescriptor);
    }
}

Usage.cs

ModelBinders.Binders.DefaultBinder = new SmartBinder()
    .Add(new ArrayModelBinder());

Very rudimentary implementation of a model binder which will try to populate an array from the posted form values. For this to work your form fields must have the following naming convention: camelCaseArrayPropertyName_arrayIndex_PascalCaseModelPropertyName. For example: people_1_Name.

SmartModelBinder (inspired by Jimmy Bogard's A better Model Binder) allows applying "property" model binders that match a specific criteria.

This was written for an MVC1 system, there may be better alternatives for MVC2+