Fluent Validation ensuring a list has at least one item with property value of somevalue

Assume I have the following viewmodel:

public class TaskViewModel{
  public MTask Task {get;set;}
  public List<DocIdentifier> Documents {get;set;}
  .....
}

public class DocIdentifier{
  public string DocID {get;set;}
  public bool Selected {get;set;}
}

And here's the Fluent Validation validator I use:

public class TaskValidator : AbstractValidator<TaskViewModel>{
   public TaskValidator{

   }
}

How can I make sure that at least one DocIdentifier object in the list Documents has its Selected property value True?

You have to use predicate validator Must, in which you could specify custom condition based on LINQ extensions:

public class TaskValidator : AbstractValidator<TaskViewModel>{
   public TaskValidator()
   {
       RuleFor(task => task.Documents)
           .Must(coll => coll.Any(item => item.Selected)) // you can secify custom condition in predicate validator
           .WithMessage("At least one of {0} documents should be selected",
               (model, coll) => coll.Count); // error message can use validated collection as well as source model
   }
}