Is there a way to use a "C" API's in C#? , Calling / using "C" API's from C#?

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I'm attempting to finish up a project in C#, but I'm having problems with the C#'s calling conventions that calls "C" functions. I'm also dealing with char and integer,
and bools datatypes.

Within my C# program, I've been using the following declaration(1):
1) [DllImport("mydllfile.dll")] static extern
int ACeeFunction(StringBuilder aBuffer, ref int iostatz, int nSize);
but I've also seen two others:
2) [DllImport("mydllfile.dll")] static extern
int ACeeFunction(String aBuffer, ref int iostatz, int nSize);
and
3) [DllImport("mydllfile.dll")] static extern
int ACeeFunction(char aBuffer, ref int iostatz, int nSize);

I'll be using these "C" function declarations(below), as an example for my next few questions:

int ACeeFunctOne( int nsize, char * charbuff, int * iostatz ) {
}
int ACeeFunctTwo( char * newbuff, int * iostatz, int * nsize, char * charbuff) {
}
int ACeeFunction( char * charbuff, int * iostatz, int nsize) {
}

In all my DLL "C" functions, I use a variety of these " char * charbuff " declarations in all my function parameter lists and also in the .def file. Like above.

Which of the three "DllImport" statements, using the first paramter of each; match the "ACeeFunction"'s first parameter??
Does it match correctly in form of size, data type, length, and so forth?

As of right now:
1) While in debug mode, these two programs work together PERFECTLY with no
exceptions.
2) I can passed data back and forth, between C# and the DLL's C routines.
3) But, when both are changed from debug to release x86 mode, things get
a little nick-picky and errors/exceptions start popping up, and sometimes
the C# program will unknowly abend.
4) I can pass numerical data by reference or by value in c#.
I can also pass bool values.

So, which of the three C# "DllImport" statements is the right one to use?

I hope this all makes sense, cause this is the best way I could put it all down.

Oh OriginalGriff, RSVP
I need help with this problem, It's like writing a new language parser. Remember.

Thanks, If you have questions, email me.

What I have tried:

I've written a complete c# program, along with a windows dll file. They communicate quite will. I have written a big C# program, along with a Windows DLL, filled with many "C" routines.
1) While in debug mode, these two programs work together PERFECTLY with no exceptions.
2) I can passed data back and forth, between my C# program and the DLL's C routines.
3) But when both are changed from debug to release x86 mode, things get a little nick-picky and errors/exceptions
start popping up, and sometimes the C# program will unknowly abend.
4) ) I can pass numerical data by reference or by value in c#. I can also pass bool values back and forth.

Posted 6-Oct-21 17:47pm

Member 15028314

Updated 12-Oct-21 8:11am

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Richard Deeming · Answer 1 · 2021-10-06T22:46:00

Native interoperability best practices - .NET | Microsoft Docs[^]

If your C function is not modifying the charbuff, then declaring the parameter as string is the right thing to do. If you can make it use Unicode (wchar_t*), and mark the P/Invoke function argument with [MarshalAs(UnmanagedType.LPWSTR)], that will give you the best performance, since the string won't need to be copied at all.

If your C function is modifying the charbuff, then passing a string is definitely the wrong thing to do. You would need to mark it as ref or out to see the changes from C#, and that would destabilize the runtime.

The recommendation is also to avoid using a StringBuilder. It will work, but it will involve a lot of copying between the C# and C memory. Instead, Microsoft recommend renting a shared char[] array of sufficient size from an array pool, and passing that in. Unfortunately, I couldn't find an example of that in the documentation - the links for "more information" only show string and StringBuilder examples, and include examples which explicitly violate their "best practices" recommendations.

int parameters are a lot simpler: a C int needs to be declared as int in C#, whereas a C int* needs to be either ref int or out int in C#.

JudyL_MD · Answer 2 · 2021-10-12T08:11:00

If your "int *" is simply a pointer to a single int that is going to change, use "ref int" in your C# DllImport statement and "int *" in your C function.

Passing arrays back and forth between C# and C requires explicit marshalling. An example for an array of int32_t (when marshalling, it is always advisable to use explicity-sized things):

in C#:

[[DllImport ("dll.dll", Calling Convention = CallingConvention.Cdecl)]
private static extern void A_Function (IntPtr arrayPtr, int arraySize);

IntPtr unmanArray = Marshal.AllocHGlobal (300 *sizeof (Int32));
// loop and use Marshal.WriteInt32 or unsafe pointers to fill your array
A_Function (unmanArray, 300);

Int32[] manArray = new Int32[300]; // or reuse the managed array that was the data source if it's big enough
Marshal.Copy (unmanArray, manArray, 0, 300);

Marshal.FreeHGlobal (unmanArray);

// now use the managed array manArray in your C# code

The corresponding C code is the expected

void A_Function (int32_t * arrayInts, int arraySize);

I always pass an array size when going between C and C# -- it avoids buffer overruns to have an explicit size in the C code.

The same principle applies to an array of anything other base type (float, double, byte).

please note - if you have any more questions, be warned that my responses will be slow in
coming but i'll try to help.

Is there a way to use a "C" API's in C#? , Calling / using "C" API's from C#?

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