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static extern int OleGetClipboard([MarshalAs(UnmanagedType.IUnknown)]out object ppDataObj);

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static extern int OleGetClipboard(out IDataObject ppDataObj);

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static extern object OleGetClipboard();

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    static extern int OleGetClipboard([MarshalAs(UnmanagedType.IUnknown)]out object ppDataObj);

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       int    result = OleGetClipboard(out unk);

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    static extern object OleGetClipboard();

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       object     unk = OleGetClipboard();

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Documentation

[OleGetClipboard] on MSDN