Why string is functioning differently

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.

Answer 2

new char() returns an empty character (contains two zero bytes), so ptr is '\0'.

When you use s = new System.String(ptr, 0);, the constructor concatenates the '\0' character zero times, resulting in an empty string (""). Adding the empty string to another string has no effect, so s+"Something" is equal to "Something"+s which is "Something".

In another way, using s = new System.String(ptr, 1); creates a string containing a single '\0' character. Then adding this string before "Something" results in "\0Something" where adding after it results in "Something\0".

In C# strings are not manipulated as NULL-Terminated Strings, so when you create "\0Something", you really have it (you can make sure by taking the Length of the string which is 10), however MessageBox.Show is converted to Win32 MessageBox function of the windows user32 library. And that function requires a null-terminated string (it stops when encounters a zero byte or two zero bytes for unicode strings).