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.

string is immutable. Try creating a StringBuilder and pass that instead. — Matthew Watson
– Matthew Watson, Commented Mar 13, 2019 at 9:01
Try to set charset for DllImport or setup custom marshalling for your string, like here learn.microsoft.com/en-us/dotnet/framework/interop/… — Pavel Anikhouski
– Pavel Anikhouski, Commented Mar 13, 2019 at 9:02
StringBuilder instead of String? be creaful with encoding: CharSet = CharSet.Unicode or CharSet = CharSet.Ansi — Dmitrii Bychenko
– Dmitrii Bychenko, Commented Mar 13, 2019 at 9:03
The information you provide is not sufficient to answer. What is Rectangle both on C# and C side? What is the C function doing with the parameters? How do you call the C# method? — Simon Mourier
– Simon Mourier, Commented Mar 13, 2019 at 9:10
Don't take this the wrong way, but a some research would have led to hundreds of topics on StringBuilder. — David Heffernan
– David Heffernan, Commented Mar 13, 2019 at 9:36

Dmitrii Bychenko · Accepted Answer · 2019-03-13 09:19:32Z

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;
}

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