xml schema 中如何定义类似Map的结构

利用xs：unique关键字。在xs:element里添加unique节点，任意命名，然后用xs:selector来选择需要唯一的域， xs:field 里指定特定的字段。

例如：定义所有Item里的Key为唯一， 

                 <xs:complexType>

                                                 <xs:sequence minOccurs ="0" maxOccurs="unbounded ">

                                                                 <xs:element name ="Item" type="IdentifiersWithSourceItemType">

                                                                 </xs:element>

                                                 </xs:sequence>

                                 </xs:complexType>

                                 <xs:unique name ="KeyUnique">

                                                 <xs:selector xpath =".//Key"/>

                                                 <xs:field xpath =".//NamespaceID"/>

                                 </xs:unique>

                                 <xs:unique name ="KeyUnique2">

                                                 <xs:selector xpath =".//Key"/>

                                                 <xs:field xpath =".//UniversalIDType"/>

                                                 <xs:field xpath =".//Universal/UniversalID"/>

                                 </xs:unique>