/// <summary>
///
/// </summary>
/// <param name="exp"></param>
public override void Execute(IResultValue res)
{
// set value to 'global' Expression variable
if (_isNull)
{
res.SetResultValue(null, StorageAttribute.NONE);
}
else
{
if (_val == null)
{
_val = "";
}
res.SetResultValue(_val, _attr);
}
}