/// <summary>
/// Executes the delegate in a context where every read returns the value as
/// it was at transaction opening. Writes still work, even though their
/// effects cannot be seen in this context. And please note that
/// <see cref="Shielded<T>.Modify"/> will not be affected and will expose
/// the last written value.
/// </summary>
public static T ReadOldState <T>(Func <T> act)
{
T retVal = default(T);
Shield.ReadOldState(() => { retVal = act(); });
return(retVal);
}