Custom Conditional Validation For Required fields

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ASP.NET

MVC

C#5.0

validator

Hi I would want to have validations something of this sort

[RequiredCustom(ActionType=(int)Action.Update, ActionType=(int)Action.Delete)]
public string NotesID { get; set; }

[Required]
[RegularExpression("1|2|3|4")]
public int ActionType { get; set; }

I would want to validate this NotesID only when Updation and Deletion is taking place. I don't need any javascript code for unobtrusive and all. I just want server side validation.

Please don't suggest use of separate models I can't do that. Something similar solution will also do.

Let me know if this requires more clarification, Any help would be greatly appreciated.