MasterMemory.ValidatorExtension.cs

using System;
using System.Linq;
using System.Linq.Expressions;
using MasterMemory.Validation;

namespace Sandbox
{
    public static class ValidatorExtension
    {
        public static void Single<TElement>(this ValidatableSet<TElement> self, Expression<Func<TElement, bool>> predicate)
        {
            var f = predicate.Compile(true);
            var count = self.TableData.Count(a => f(a));
            if (count != 1)
            {
                throw new Exception($"Unique failed ({count} items): {predicate.ToSpaceBodyString()}");
            }
        }
    }
}