Question

I have the following code

 char ptr=new char();


 int counter = 1;
 string s = new System.String(ptr, counter);

 // does not show something
 MessageBox.Show(s+"Something");

 //shows something
 MessageBox.Show("Something" + s);

The first Messagebox shows nothing

The Second Messagebox shows something

If the counter value is 0 then both messagebox shows same result but if counter is greater than 0 then the problem occurs.

I think the problem is with new string(ptr, counter) initilization

. But I want to know the internal mechnism why this is occured.

Answer 1

ptr is a null character ('\0') and s is a string with one copy of that character (i.e. "\0"). So, at runtime, your first call’s parameter evaluates to "\0Something" whereas your other one evaluates to "Something\0".

In C#, strings are allowed to have null characters; you can conceptualize them as just a char[] array (which has a known length); So null characters are OK. The issue comes about when you pass to a C API. C doesn’t have strings, so they’re immitated using null-terminated strings. As far as any C API is concerned, "\0Something" is an empty string (strlen would return 0). So, when you use MessageBox.Show, your string is passed on down to the Win32 API function, MessageBoxW which only understands null-terminated strings.

---

From the .NET source code for MessageBox.Show(string)

public static DialogResult Show(string text)
{
    return ShowCore(null, text, string.Empty, MessageBoxButtons.OK, MessageBoxIcon.None, MessageBoxDefaultButton.Button1, 0, false);
}

Towards the end of ShowCore, we see this line:

return (DialogResult)MessageBoxW(handle, text, caption, style);

And that MessageBoxW call is that Win32 API function in user32.dll.