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Target 32 Bit or 64 Bit native DLL depending on environment
DllImport - An attempt was made to load a program with an incorrect format [duplicate]
You cannot load a 32 bit DLL into a 64 bit process. To support this you will have to have two different EXE's, one compiled as 64 bit and one compiled as 32 bit.
If you run the 64 bit process and encounter a 32 bit dump, you'll have to launch the 32 bit version of the EXE to process the dump file. Once it is processed you can use some sort of IPC (Interprocess Communication) mechanism to send the results back to the 64 bit process.