Why do we have to create unique uuid for every interface in a idl?

Why do we have to create unique uuid for each interface in a idl ?

For example in this interface there's a unique uuid for every interface in TradingLib.

import "oaidl.idl";

[uuid(7C948DDC-8C22-46CF-91AD-1E043A0E1A10), object]
interface IInventory : IUnknown
{
    HRESULT GetStock([out, retval] long* pStock);
    HRESULT Buy([in] long quantity, [in] float rate);
    HRESULT Sell([in] long quantity, [in] float rate);
};

[uuid(F7CF450D-C4BE-4943-A384-AA5BB4A89202), object]
interface IAccount : IUnknown
{
    HRESULT GetBalance([out, retval] double* pBalance);
    HRESULT Credit([in] double amount);
    HRESULT Debit([in] double amount);
};

[uuid(9791C352-4665-403C-8A37-3EC8485A87D5), version(1.0), helpstring("XYZ Trading Library")]
library TradingLib
{
    importlib("stdole32.tlb");

    [uuid(03698856-A173-417F-93CF-AEBC27EB042A)]
    coclass Trader
    {
        [default] interface IInventory;
        interfa开发者_运维百科ce IAccount;
    };

    [uuid(E596BE02-0DCE-4B7C-B8D4-4F621F675BF0)]
    enum TradingErrors
    {
        TRADER_E_OUTOFSTOCK = 0x80040101,
        TRADER_E_INSUFFICIENTFUNDS = 0x80040102
    };
};

Why can't we just use a single uuid for the TradingLib library itself? Why is it necessary to create uuid for every interface?

Thanks.

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Each COM interface must have a unique id associated with it. Each time you introduce a new interface you have to assign it a new unique id.

That's one of COM conventions - if you know an interface id you know what the interface is (all it's method with exact signatures). So in your example the answer is - you can't reuse an id because each distinct interface must have its own unique id.

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when you ask for an instance of a com object, you usually do it using a method called CreateInstance, and then call QueryInterface on that instace to get a reference to the specific interface on that object that you need. QueryInterface takes a uuid as a parameter. so you cant use the same guid for two distict interfaces.

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In Windows the UUID is used in the registry as the node name where the interface is registered as existing. There might be more reasons but this is one place where it is used.