如何合并两个 FlowDocument对象为一个？

Question

 private FlowDocument FlowDocumentJoint(FlowDocument doc1, FlowDocument doc2)
        {
            FlowDocument FlowDocument = new FlowDocument();
            foreach (Block item in doc1.Blocks)   //这个有问题，报错
            {
                FlowDocument.Blocks.Add(item);
            }
            foreach (Block item in doc2.Blocks)
            {
                FlowDocument.Blocks.Add(item);
            }
            return FlowDocument;
        }