/// <summary>
/// Initializes a new instance of the <see cref="IndexAttribute" /> class.
/// </summary>
/// <param name="propertyNames"> The properties which constitute the index, in order (there must be at least one). </param>
public IndexAttribute(params string[] propertyNames)
{
Check.NotEmpty(propertyNames, nameof(propertyNames));
Check.HasNoEmptyElements(propertyNames, nameof(propertyNames));
PropertyNames = propertyNames.ToList();
}