当前缓存给出Null

Question

我正在尝试获取当前缓存，这使我无效。在下面的代码中，cc的值为null。谁能让我知道我在这里做错了什么？

public class SpCaseNewEntryExtension : PXGraphExtension<SPCaseNewEntry>
{
    protected PXGraph _Graph = null;

    public class CRCaseClassIDSelectorAttribute : PXCustomSelectorAttribute
    {


        public override void CacheAttached(PXCache sender)
        {
            _Graph = sender.Graph;
            base.CacheAttached(sender);
        }

        public CRCaseClassIDSelectorAttribute()
            : base(typeof(CRCase.caseClassID))
        {

            this.DescriptionField = typeof(CRCaseClass.description);
        }
        protected virtual IEnumerable GetRecords()
        {
            CRCase cc = ((CRCase)_Graph.Caches[typeof(CRCase)].Current);

            foreach (PXResult<CRCaseClass> pc in PXSelectJoinGroupBy<CRCaseClass, InnerJoin<CaseContract, On<CaseContract.caseClassID, Equal<CRCaseClass.caseClassID>>,
            InnerJoin<Contract, On<CaseContract.contractID, Equal<Contract.templateID>>,
            InnerJoin<CRCase, On<Contract.contractID, Equal<Current<CRCase.contractID>>>>>>,
                Where<CaseContract.active, Equal<True>>,
            Aggregate<GroupBy<CRCaseClass.caseClassID>>>.Select(this._Graph))
                {
                    yield return pc;
                }

        }