How to "DRY up" C# attributes in Models and ViewModels?

asp.net-mvc, attributes, c#, dry

Solution

I'll assume that your doing this to take advantage of the HtmlHelpers EditorFor and DisplayFor and don't want the overhead of ceremoniously declaring the same thing 4000 times throughout the application.

The easiest way to DRY this up is to implement your own ModelMetadataProvider. The ModelMetadataProvider is what is reading those attributes and presenting them to the template helpers. MVC2 already provides a DataAnnotationsModelMetadataProvider implementation to get things going so inheriting from that makes things really easy.

To get you started here is a simple example that breaks apart camelcased property names into spaces, FirstName => First Name :

public class ConventionModelMetadataProvider : DataAnnotationsModelMetadataProvider
{
    protected override ModelMetadata CreateMetadata(IEnumerable<Attribute> attributes, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName)
    {
        var metadata = base.CreateMetadata(attributes, containerType, modelAccessor, modelType, propertyName);

        HumanizePropertyNamesAsDisplayName(metadata);

        if (metadata.DisplayName.ToUpper() == "ID")
            metadata.DisplayName = "Id Number";

        return metadata;
    }

    private void HumanizePropertyNamesAsDisplayName(ModelMetadata metadata)
    {
        metadata.DisplayName = HumanizeCamel((metadata.DisplayName ?? metadata.PropertyName));
    }

    public static string HumanizeCamel(string camelCasedString)
    {
        if (camelCasedString == null)
            return "";

        StringBuilder sb = new StringBuilder();

        char last = char.MinValue;
        foreach (char c in camelCasedString)
        {
            if (char.IsLower(last) && char.IsUpper(c))
            {
                sb.Append(' ');
            }
            sb.Append(c);
            last = c;
        }
        return sb.ToString();
    }
}

Then all you have to do is register it like adding your own custom ViewEngine or ControllerFactory inside of Global.asax's Application Start:

ModelMetadataProviders.Current = new ConventionModelMetadataProvider();

Now just to show you I'm not cheating this is the view model I'm using to get the same HtmlHelper.*.For experience as your decorated ViewModel:

    public class FooDisplayViewModel // use for "details" view
    {
        public int Id { get; set; }

        public string FirstName { get; set; }

        public string LastName { get; set; }

        [DataType("EmailAddress")]
        public string EmailAddress { get; set; }

        public int Age { get; set; }

        [DisplayName("Category")]
        public string CategoryName { get; set; }
    }

Problem

This question was inspired by my struggles with ASP.NET MVC, but I think it applies to other situations as well. Let's say I have an ORM-generated Model and two ViewModels (one for a "details" view and one for an "edit" view): Model ``` public class FooModel // ORM generated { public int Id { get; set; } public string FirstName { get; set; } public string LastName { get; set; } public string EmailAddress { get; set; } public int Age { get; set; } public int CategoryId { get; set; } } ``` Display ViewModel ``` public class FooDisplayViewModel // use for "details" view { [DisplayName("ID Number")] public int Id { get; set; } [DisplayName("First Name")] public string FirstName { get; set; } [DisplayName("Last Name")] public string LastName { get; set; } [DisplayName("Email Address")] [DataType("EmailAddress")] public string EmailAddress { get; set; } public int Age { get; set; } [DisplayName("Category")] public string CategoryName { get; set; } } ``` Edit ViewModel ``` public class FooEditViewModel // use for "edit" view { [DisplayName("First Name")] // not DRY public string FirstName { get; set; } [DisplayName("Last Name")] // not DRY public string LastName { get; set; } [DisplayName("Email Address")] // not DRY [DataType("EmailAddress")] // not DRY public string EmailAddress { get; set; } public int Age { get; set; } [DisplayName("Category")] // not DRY public SelectList Categories { get; set; } } ``` Note that the attributes on the ViewModels are not DRY--a lot of information is repeated. Now imagine this scenario multiplied by 10 or 100, and you can see that it can quickly become quite tedious and error prone to ensure consistency across ViewModels (and therefore across Views). How can I "DRY up" this code? Before you answer, "Just put all the attributes on `FooModel`," I've tried that, but it didn't work because I need to keep my ViewModels "flat". In other words, I can't just compose each ViewModel with a Model--I need my ViewModel to have only the properties (and attributes) that should be consumed by the View, and the View can't burrow into sub-properties to get at the values. Update LukLed's answer suggests using inheritance. This definitely reduces the amount of non-DRY code, but it doesn't eliminate it. Note that, in my example above, the `DisplayName` attribute for the `Category` property would need to be written twice because the data type of the property is different between the display and edit ViewModels. This isn't going to be a big deal on a small scale, but as the size and complexity of a project scales up (imagine a lot more properties, more attributes per property, more views per model), there is still the potentially for "repeating yourself" a fair amount. Perhaps I'm taking DRY too far here, but I'd still rather have all my "friendly names", data types, validation rules, etc. typed out only once.

Original source

Related problems