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Compiler Error C3121

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cannot change GUID for class 'class_name'

You attempted to change the class ID with __declspec(uuid).

For example, the following code generates C3121:

// C3121.cpp
[emitidl];
[module(name="MyLibrary")];

[coclass, uuid="00000000-0000-0000-0000-111111111111"]
class __declspec(uuid("00000000-0000-0000-0000-111111111112")) A   // C3121
{
};
int main()
{
}