What is the proper way to handle exceptions in DLL

Important: as a general rule, you should write your application in a such way that it will behave correctly without EurekaLog on board. This means that if your or 3rd party code throws an exception in a DLL - it must be handled in a way that you expect. For example, by showing an error message, canceling action, retrying, etc. Once your application behaves like it is supposed to do - then you add EurekaLog for it. EurekaLog will auto-handle all unhandled exceptions automatically (by hooking few well-known places), and for everything else - you can call EurekaLog from your own handlers when needed. In other words:

•	Incorrect: your application does not show messages for exceptions (or do other things you want) from DLL and you are adding EurekaLog in hopes to fix this behavior;

•	Correct: your application correctly handles DLL exceptions and you are adding EurekaLog to receive reports about such exceptions.

As you should already understood by now: rule #1 when working with exceptions in DLLs is "never let exception escape DLL". That because caller may not know how to work with exception object generated by different programming language. For example, a Delphi .exe file have no idea to to read exception message from Microsoft C++ exception; it doesn't know how to properly release exception object after exception is handled. Therefore, all exceptions in DLL functions must be captured and handled by translating them to error code or other error signature as required by DLL API.

How this should be done? That highly depends on what your DLL API is. This also depends on what framework you do use. There are 3 possible cases:

1.	You develop DLL by using a framework. For example: you write a control panel applet by using VCL. Or you write ISAPI module by using IntraWeb.

2.	You develop DLL for already established API without using a ready framework. For example: you write a plugin for 3rd party application (like Total Commander). Or you write a global system hook (which requires DLL).

3.	You develop both DLL and API specification. For example: you write your own DLL to be used by different applications.

Note: if you develop DLL that will only be used in executable compiled in exactly the same version of the compiler, and you want to pass exceptions between modules for simplicity - see this article.

See also:

•	Creating an API (contract) for your own DLL (don't create your own DLLs without reading this article)

•	Creating bug reports for DLL exceptions

•	Configuring call stack

•	List of supported debug information formats

•	Sharing memory with EurekaLog

•	Sharing memory with Delphi

•	Sharing exceptions between executable modules

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Build date: 2024-09-30
Last edited: 2024-07-17

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