C# calling C DLL routines (YET ANOTHER ONE)

I am totally baffled on the examples given by sites on how to call C routines in DLL via C#.
Not only are many of them repeating one same example after another, you soon realize they are not very helpful when you want to do something that is common like passing a structure with all kinds of types inside.

So here is one example that serve as an answer to MANY of the pinvoke/calling C apis problem :- ( i wanted to say 90% but i can't justify or prove it ;-))

I got this example structure that has :

struct ToughCallDef
{
char * ptrName; // this will contain some return string values
int ptrNameLen; // normal integer
MEMBLOCK somestruct; // a structure
ANOTERBLOCK* structpointer; // structure pointer
int* ptrtoMyInt; // integer pointer
char * ptrID // this must be supplied
};

Just translate it this way :-

[StructLayout(LayoutKind.Sequential)]
public class ToughCallDef
{
IntPtr ptrName;
MEMBLOCK somestruct; // you need to define MEMBLOCK also
IntPtr structPointer;
IntPtr ptrtoMyInt;
IntPtr ptrID;
}

And if the call requires a reference to this "ToughCall" :

[DllImport("anotherc.dll")]
public static extern int Call_C_API(ref ptrToughCall);

define it this way instead :

[DllImport("anotherc.dll")]
public static extern int Call_C_API(IntPtr ptrToughCall);

Basically anything to do with Pointers, regardless of whether its integer *, char *, structure reference etc, just use IntPtr.


1 . If the member is a "char *" that you expect return values you point the IntPtr this way :

szReason = new StringBuilder(100);
zReason.Append(' ', 100); // empty string with spaces
ToughCall.PtrName = Marshal.StringToHGlobalAnsi(m_szReason.ToString());

2. If the member is a "char *" that you need to assign a value before passing in :

ToughCall.ptrID = Marshal.StringToHGlobalAnsi(StringVar);


3. If the member is a structure pointer, you assign it this way :

ANOTHERBLOCK o = new ANOTHERBLOCK();
ToughCall.structPointer = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(ANOTHERBLOCK)));
Marshal.StructureToPtr(o, ToughCall.structPointer, true);


4. After calling the C routine you will need to do the following to get the values :-

1. If its a string, use :
   string s = Marshal.PtrToStringAnsi(ToughCall.PtrName)
2. If its a structure use :
   Marshal.PtrToStructure(ToughCall.structPointer, o);
3. Free any allocated strings/struct pointers via :
   Marshal.FreeHGlobal

That's it.