These restrictions are necessary because any calls to a partial method that was not implemented are simply ignored.
因为任何针对没有被实现的分部方法的调用都会简单地被忽略，所以说这些限制是非常有必要的。
A partial method is usually defined in a partial class file and is optionally implemented in the regular class file.
分部方法通常被定义在一个分部类中，在常规的类文件中也可实现。
A partial method has its signature defined in one part of a partial type, and its implementation defined in another part of the type.
分部方法在分部类型的一个部分中定义它的签名，并在该类型的另外一个部分中定义它的实现。
A partial method has its signature defined in one part of a partial type, and its implementation defined in another part of the type.
分部方法在分部类型的一个部分中定义它的签名，并在该类型的另外一个部分中定义它的实现。
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