VB.Net DLLImport - vb.net

Question

I got a PL/I Dll and I'm trying to import the DLL into my VB.Net Application.
The first try worked but the program did quit without comment after a few calls.
And it is not possible to call these function twice from two different threads.
I get the result from the DLL in paramone. Any Ideas why this doesn't work right and how to get it to work?
<DllImport(("PLIDLL.dll"), CallingConvention:=CallingConvention.StdCall)> _
Public Shared Sub MYFUNC(ByVal LogonString As String, _
<MarshalAs(UnmanagedType.VBByRefStr)> ByRef paramone As String, _
ByVal paramtwo As String)
End Sub
If anything is unclear please ask.
Greetings Lim
PS: I already tried to rerewrite the PL/I Code so that it returns a String instead of the ByRef value. Same issue.
The Sub in PLI:
MYFUNC: PROC(LOGONSTRING,PARAMONE,PARAMTWO) REORDER
OPTIONS(FROMALIEN NODESCRIPTOR
BYADDR LINKAGE(STDCALL));
DEFAULT RANGE(*) STATIC;
DCL LOGONSTRING CHAR(30);
DCL PARAMONE CHAR(2033);
DCL PARAMTWO CHAR (5500);

Answer 1

Found at least a working solution.
Module NativeMethods
<DllImport("kernel32.dll", SetLastError:=True)> _
Public Function LoadLibrary(ByVal lpFileName As String) As IntPtr
End Function
<DllImport("kernel32.dll", SetLastError:=True)> _
Public Function GetProcAddress(ByVal hModule As IntPtr, ByVal procedureName As String) As IntPtr
End Function
<DllImport("kernel32.dll", SetLastError:=True)> _
Public Function FreeLibrary(ByVal hModule As IntPtr) As IntPtr
End Function
End Module
<UnmanagedFunctionPointer(CallingConvention.StdCall)>
Private Delegate Sub MYFUNC(ByVal LogonString As String, <MarshalAs(UnmanagedType.VBByRefStr)> ByRef PARAMONE As String, ByVal PARAMTWO As String)
Public Shared Sub CallFunction(ByRef workingObject As PLIOBJECT)
Dim EntryPointer As IntPtr = NativeMethods.LoadLibrary("PLIDLL.dll")
Dim FunctionPointer As IntPtr = NativeMethods.GetProcAddress(EntryPointer, "MYFUNC")
Dim MyFUNC As MYFUNC= CType(Marshal.GetDelegateForFunctionPointer(FunctionPointer, GetType(MYFUNC)), MYFUNC)
MyFUNC(workingObject.Logonstring, workingObject.PARAMONE, workingObject.PARAMTWO)
Dim result As Boolean = NativeMethods.FreeLibrary(EntryPointer)
End Sub