c# - ValidationMessageFor together with AddModelError(key, message). What's the key?

Question

I am developing a client-side and server-side validation for a certain viewModel property.

In the .cshtml file I put this:

@Html.DropDownListFor(model => model.EntityType.ParentId, Model.ParentTypeList, "")
@Html.ValidationMessageFor(model => model.EntityType.ParentId)

In the Controller for the business validation

catch (BusinessException e)
{
    ModelState.AddModelError("EntityType.ParentId", Messages.CircularReference);
}

The above works as expected: if an exception is caught, the message appears next to the dropdownlist.

However, I find that this way is not very elegant. In the cshtml, I use a method to generate all the required information about the validation. In the controller, I must know the exact Key string and use it.

Isn't there a better way of doing this?

Answer 1

You could write an extension method that will take a lambda expression for the key instead of a string:

public static class ModelStateExtensions
{
    public static void AddModelError<TModel, TProperty>(
        this ModelStateDictionary modelState, 
        Expression<Func<TModel, TProperty>> ex, 
        string message
    )
    {
        var key = ExpressionHelper.GetExpressionText(ex);
        modelState.AddModelError(key, message);
    }
}

and then use this method:

catch (BusinessException e)
{
    ModelState.AddModelError<MyViewModel, int>(
        x => x.EntityType.ParentId, 
        Messages.CircularReference
    );
}