How do I make an alternative required field in MVC 4 application?

Question

I have a Model which have both phone number and email in MVC 4. It's something like registration. So user can provide either email or cellphone number or both of them. However I can't make a alternative required field of them. My Model is like

public class RegisterModel
    {
        [Required]
        [Display(Name = "User name")]        
        [UsernameValidator(ErrorMessage = "Only Letters and numbers")]
        [System.Web.Mvc.Remote("IsUserExitst", "Account", HttpMethod = "POST", ErrorMessage = "This user name already exists. Try a new one.")]
        public string UserName { get; set; }

        [Required]
        [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
        [DataType(DataType.Password)]
        [Display(Name = "Password")]
        public string Password { get; set; }

        [DataType(DataType.Password)]
        [Display(Name = "Confirm password")]
        [System.ComponentModel.DataAnnotations.Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
        public string ConfirmPassword { get; set; }

        [Required]
        [MaxLength(50)]
        [EmailOrPhoneValidator(ErrorMessage = "Invalid Phone number")]
        [Display(Name = "Phone number")]
        [System.Web.Mvc.Remote("IsMobOrEmailExists", "Account", HttpMethod = "POST", ErrorMessage = "This Phone number or Email already exists. Try a new one.")]
        public string MobileNo { get; set; }


        [Required]
        [MaxLength(50)]
        [EmailAddress(ErrorMessage = "Invalid Email address")]
        [Display(Name = "Email")]
        [System.Web.Mvc.Remote("IsEmailExists", "Account", HttpMethod = "POST", ErrorMessage = "This Email already exists. Try a new one.")]
        public string Email { get; set; }
    }

Here I want to use Email and PhoneNo alternatively. I want to write a class which works as Custom required field. I have gone through this Questions,

1. Question Here the answer provided a solution combined with javascript. But I won't use it. I will write down the class file and implement my logics here.
2. RequiredIf is seems the exact suggestion of my question, but I so tired to try that solution. I am not capable to understand how to import the .dll in my project. and it's developed in Mvc 3
3. Another solution, but here they have combined the alternative properties in a class. So how do I do as my properties are completely separate. So what to do.

Answer 1

I would go with solution 2 and create a RequiredIf validation attribute. No need to import any DLLs since you can just create the class yourself. Here is an example of what the attribute could look like. It's from a RequiredIfTrue attribute I use in my own code and I exchanged the validation logic to look for empty values

public class RequiredIfEmpty : ValidationAttribute
{
    private string _fieldName;

    public RequiredIfEmpty(string fieldName)
    {
        _fieldName = fieldName;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var property = validationContext.ObjectType.GetProperty(_fieldName);
        if (property == null)
        {
            throw new ArgumentException("No property with the name " + _fieldName + " found.");
        }

        string fieldValue = (string)property.GetValue(validationContext.ObjectInstance, null);
        if (String.IsNullOrWhiteSpace(fieldValue))
        {
            return (value != null && !String.IsNullOrWhiteSpace((string)value) ? ValidationResult.Success : new ValidationResult(ErrorMessage));
        }
        else
        {
            return ValidationResult.Success;
        }
    }
}

In your ViewModel you can use the attribute like any other attribute but you have to specify the name of the alternative property

[RequiredIfEmpty("Email", ErrorMessage = "Phone number invalid")]
[MaxLength(50)]
[Display(Name = "Phone number")]
[System.Web.Mvc.Remote("IsMobOrEmailExists", "Account", HttpMethod = "POST", ErrorMessage = "This Phone number or Email already exists. Try a new one.")]
public string MobileNo { get; set; }