Question What is the easiest way to determine the string type of the DLL

[aaaron]

If I use Declare Auto and the OS is UNICODE and the DLL is ANSI the runtime will not convert the UNICODE to ANSI.

Is that correct?

If so, why the word AUTO?

If I must tell the runtime what conversion to use what is the easiest way to determine the string type of the DLL?

 

[JohnH]

Historic reasons probably. It also controls name mangling, for example you could import MessageBox auto to resolve OS default MessageBoxA or MessageBoxW with automatic right charset. Some resources:

DllImportAttribute.CharSet Field (System.Runtime.InteropServices)

Indicates how to marshal string parameters to the method and controls name mangling.

learn.microsoft.com

Specifying a Character Set - .NET Framework

Learn how to specify a character set that uses narrow (ANSI) or wide (Unicode) encoding. You can alternatively specify automatic runtime selection.

learn.microsoft.com

Charsets and marshalling - .NET

Learn how different values of CharSet can change how .NET marshals your data to native code.

learn.microsoft.com

One of those also says:

Make sure you know what representation your native representation expects when picking your charset.

How to find/figure out? There should be documentation (or maybe just header files), here for example for MessageBox:

MessageBoxA function (winuser.h) - Win32 apps

Displays a modal dialog box that contains a system icon, a set of buttons, and a brief application-specific message, such as status or error information. The message box returns an integer value that indicates which button the user clicked. (MessageBoxA)

learn.microsoft.com

MessageBoxW function (winuser.h) - Win32 apps

The MessageBoxW (Unicode) function displays a modal dialog box that contains a system icon, a set of buttons, and a brief application-specific message.

learn.microsoft.com

Conventions for Function Prototypes - Win32 apps

The Windows SDK provides function prototypes in generic, Windows code page, and Unicode versions.

learn.microsoft.com

The W version uses "wide" unicode strings LPCWSTR.
This is sometimes useful: Windows Data Types (BaseTsd.h) - Win32 apps

Also, if it is a common function you could find declarations on the web.
For Windows declarations this is something I wished existed many years ago, although it has been around a few years now: GitHub - dahall/Vanara: A set of .NET libraries for Windows implementing PInvoke calls to many native Windows APIs with supporting wrappers.