C# Pinvoke string

Question

I am trying to bind this C function using PInvoke.

bool GuiTextBox(Rectangle bounds, char *text, int textSize, bool editMode); 

Here is the PInvoke signature.

[DllImport(nativeLibName,CallingConvention = CallingConvention.Cdecl)]
public static extern bool GuiTextBox(Rectangle bounds, 
                                     string text, 
                                     int textSize, 
                                     bool freeEdit);

When I try to use it the string does not get modified. I tried passing it as ref but it crashes with attempted to read or write protected memory when I try to use it.

Answer 1

I expect it should be something like this:

// private : do not expose inner details; 
// we have to manipulate with StringBuilder
[DllImport(nativeLibName,
           CallingConvention = CallingConvention.Cdecl,
           EntryPoint = "GuiTextBox",
           CharSet = CharSet.Unicode)] //TODO: Provide the right encoding here
private static extern bool CoreGuiTextBox(Rectangle bounds, 
                                          StringBuilder text, // We allow editing it
                                          int textSize, 
                                          bool freeEdit);

// Here (in the public method) we hide some low level details
// memory allocation, string manipulations etc.
public static bool CoreGuiTextBox(Rectangle bounds, 
                                  ref string text, 
                                  int textSize, 
                                  bool freeEdit) {
  if (null == text)
    return false; // or throw exception; or assign "" to text

  StringBuilder sb = new StringBuilder(text);  

  // If we allow editing we should allocate enough size (Length) within StringBuilder
  if (textSize > sb.Length)
    sb.Length = textSize;

  bool result = CoreGuiTextBox(bounds, sb, sb.Length, freeEdit);   

  // Back to string (StringBuilder can have been edited)
  // You may want to add some logic here; e.g. trim trailing '\0'  
  text = sb.ToString();

  return result;
}