Custom Validation Attribute error message not showing with ValidationMessageFor

I have this custom validation :

public class CollectionNotEmptyAttribute : ValidationAttribute
    private const string errorMessage = "'{0}' must have at least one element.";

    public CollectionNotEmptyAttribute()
        : base(errorMessage)


    public override bool IsValid(object value)
        var collection = value as ICollection;
        if (collection != null)
            return collection.Count > 0;

        return false;

    public override string FormatErrorMessage(string name)
        return String.Format(this.ErrorMessageString, name);

My viewmodel

public class ProjectViewModel
    public ProjectViewModel()
        this.Users = new Collection<UserProjectViewModel>();

    public int ProjectID { get; set; }
    public Collection<UserProjectViewModel> Users { get; set; }

My View

@Html.ValidationMessageFor(m => m.Users)

The validation is working fine, Model.IsValid returning false if collection count below 1, but the error message is not showing.

Any help will be appreciated.


I believe you should override other IsValid method:

protected virtual ValidationResult IsValid(
   Object value,
   ValidationContext validationContext

since it allows you to return ValidationResult with proper error message.

The one you overrode just determines whether result is valid or not.

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