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c#cpointersmarshallingvoid-pointers
akshay dhule
akshay dhule

Reputation: 167

C# - Marshall & Pass void pointer to C function calling from C#

I am implementing c#.net wrapper for C-based Library. currently, working on function with passing A void pointer, which will set value.

NOTE : void pointer can take different data type depending on Enum.

Objective: to pass void pointer and set value

I checked many solutions to implement and finally came to following state :

The C-dll function:

int function_call(enum a, void * var);

The C function call code:

char name[255];
name = "asd";
function_call(enum a, &name);

The C# wrapper code:

[DllImport("mylibrary.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int function_call(enum a, IntPtr var);

The C# function call code:

IntPtr name;
string val = "asd";
name = Marshal.StringToHGolbalUni(val);
int ret = function_call(Enum_type, name);

By this method it only sets First character a from the input asd.

I have already checked "Duplicate" questions but doesnt solve the issue:

c++ - What is void* in C#

How to declare void pointer in C#

void* in C#

Upvotes: 0

Views: 2505

Answers (2)

akshay dhule
akshay dhule

Reputation: 167

The Issue was resolved with :

[DllImport("mylibrary.dll", CharSet = CharSet.Ansi,  CallingConvention = CallingConvention.Cdecl)]

and 

name = Marshal.StringToHGolbalAnsi(val);

On client side :

IntPtr nameptr;
string name = "asd";
nameptr= Marshal.StringToHGlobalAnsi(name);

Upvotes: 0

Grantly
Grantly

Reputation: 2556

I would treat your void * as a char * and use 

[MarshalAs(UnmanagedType.LPStr)] StringBuilder var

instead of IntPtr

You should find it works as expected, but take care with size of the char array that your C function expects. Perhaps an explicit size in your declaration of your StringBuilder variable before you call the C function like this:

StringBuilder newvar= new StringBuilder(512);

Good link explaining the circumstances where StringBuilder is used instead of String or IntPtr is explained here - Thanks Amy for the link

Upvotes: 3

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