Question

I have a DLL from which I need to P/Invoke the following C method:

int DAStart(
    HANDLE hOpen,
    char* IPAddress,
    int IPPort,
    int threadPriority,
    char* name,
    char* password,
    char* userName)

Using the P/Invoke Assistant and my own research, I’ve come up with the following C# signature:

[DllImportAttribute("<libName>", EntryPoint="DAStart")]  
static extern  int DAStart(  
    IntPtr hOpen,  
    [MarshalAs(Unmanaged.LPStr)] StringBuilder IPAddress,
    int IPPort,
    int threadPriority, 
    [MarshalAs(Unmanaged.LPStr)] StringBuilder name, 
    [MarshalAs(Unmanaged.LPStr)] StringBuilder password,   
    [MarshalAs(Unmanaged.LPStr)] StringBuilder userName);

Now, I’m doing the call in the following way:

int port = 3000;
int threadPriority = 20;

DAStart(
    this.nativeDllHandle, // an IntPtr class field  
    new StringBuilder("10.0.10.1"),
    int port,
    int threadPriority,
    new StringBuilder("admin"),
    new StringBuilder("admin"),
    new StringBuilder("admin"));

Now, sometimes this works just fine, but something I get a Win32 Error 1008 - An attempt was made to reference a token that does not exist on following calls to this library.

Could it be that my StringBuilder objects get garbage collected so that the reference no longer exist if the native code tries to use it? Should I keep a reference for each one of them?

Would an IntPtr be better solution for passing my strings in that case?

** UPDATE **

This is all the API documentation I have for DAStart:

Inputs
HANDLE hInfo The handle returned by the DAOpen
char *IPAdress_in IP Address of the TMEE Server
int IPPort Console Port of the TMEE Server (default is port 3000)
int threadPriority The thread priority for the send file thread.
char *name Not used in Hardware DLL
char *password Not used in Hardware DLL
char *username Not used in Hardware DLL

Returns
ERROR_SUCCESS 0
ERROR_BAD_HANDLE -1
ERROR_BIND_FAILED -10

Comments
The DAStart API connects the client dll to the active TMEE Server service. The client thread is started with a priority set to the threadPriority parameter. The IP address parameter must be set to the address of the TMEE Server. The port parameter must be set to the port the TMEE Server listens on for Console connections (the default port the Consoles use is 3010). The Console thread is started with a priority set to the threadPriority parameter.

Answer 1

In the comments you indicate that the DLL takes a copy of the char* pointers (and not the contents of the strings) and then modifies the contents of the strings after DAStart returns.

In the face of this very unconventional interface design your only option is to take charge of the marshaling of the string parameters. You cannot use StringBuilder since the char* passed to the native code is valid only during the pinvoke call. It cannot be relied upon to be valid after the pinvoke call returns.

So your only solution is to pass the string parameters as IntPtr. Allocate the memory with Marshal.StringToHGlobalAnsi. Pass the resulting IntPtr to DAStart. When you are sure the DLL is done with the pointer, deallocate with Marshal.FreeHGlobal.