DllImportAttribute.PreserveSig -

• Mainly used in PInvoke Signatures
• By default, while using DllImportAttribute to p-invoke, the DllImportAttribute.PreserveSig is set to true.
• If false: Converts returned HRESULTS from unmanaged signature to managed exceptions if they are not S_OK.
• If true: Does not convert returned HRESULT to exceptions.

PreserveSigAttribute (pseudo custom attribute)–

• This is mainly used in COM interface definitions in managed code.
• By default, for COM interface methods, the CLR does HRESULT to exception transformations (opposite behavior to that of pinvoke).
• If attribute applied: Does not convert returned HRESULT to exceptions.
• If attribute not applied: Default behavior of HRESULT to exception transformations.

My short-term memory is bad and I tend to forget things as soon as I learn them. So I jotted this down while I was reading Adam Nathan's excellent manuscript on COM interop. Hope you find this tidbit useful as well.