validation rules in mvc 3.0 - asp.net-mvc-3

Question

I have one view model which is common for 3 to 4 views in this model I also define validation rules.Now problem is that in one of that view I want to overwrite that view model validation rules for two to three fields.so what I do? I don't want to make new view model for that view.

Answer 1

From an MVC architecture standpoint - this is exactly why you use view models.
You should create separate view models for each case. Use automapper (available for free on codeplex) to copy the values between your view model and your entity.
Don't even consider a different way, inheritance, etc - this is what ViewModels are for.

Answer 2

Three options I can think of:
Make a separate ViewModel using AutoMapper to handle some of the heavy lifting.
Make a subclass having different validation rules.
Make a custom ValidationAttribute which is context sensitive (Either by overriding the IsValid(Object, ValidationContext)method, or relying other context information from static methods/properties.
For instance, this Required validation attribute would be ignored if the request came from a certain URL:
public class CustomRequiredAttribute : RequiredAttribute
{
public override bool IsValid(object value)
{
if (HttpContext.Current.Request.Url != "urlwhennotrequired")
return base.IsValid(value);
return true;
}
}

Answer 3

If you do go ahead and use inheritance, then make sure that you inherit from abstract class. I think that as the system grows, you are likely to come across a scenario where your abstract class will have to be modified heavily,therefore If I were you, I'd create more view models, even if the code appears to be repetative. In the long term run you'll benefit because you'll be able to modify parts of your applications with as little side affects as possible.

Answer 4

My recommendation is basically what you don't want: create new model classes, but use inheritance to avoid repeating the properties you want. If you are adamantly opposed to creating separate models, you might look into implementing IValidatableObject and have it inspect other properties before validating the properties that you wish to vary.
EDIT:
I don't disagree with Tuliper's answer, but to flesh out my suggestions, consider a scenario in which you want to save a user's data. From one form, you are creating a user; from another, you are simply updating (this is a bit of a stretch but it's for purposes of illustration). The "create" form might require the name of a person referring the user, while the "update" form might not.
Using inheritance, you could do the following:
public class SaveUserModel
{
public int? UserId { get; set; }
...
}
public class CreateUserModel : SaveUserModel
{
[Required]
public string ReferredByName { get; set; }
}
Using IValidatableObject, you could do it this way:
public class SaveUserModel : IValidatableObject
{
public int? UserId { get; set; }
public string ReferredByName { get; set; }
...
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
// if UserId is null, we are creating a user vs. updating
if (UserId != null && string.IsNullOrWhiteSpace(ReferredBySiteUrl))
yield return new ValidationResult("Please specify the name of the person who referred you.", new[] { "ReferredByName" });
}
}
To reiterate, I am not trying to push my answer. I would be inclined to reuse models if they are exactly the same across different views, but generally there are enough differences to warrant simply creating separate models. In the end, any perceived technical debt alleviated by adhering to DRY in this situation would a bit of a wash; models tend to be easy to maintain.