P/Invoke not returning expected result with StringBuilder parameter for call to GetUserDefaultLocaleName()

Question

I am trying to get the culture name for the locale that is currently set in the regional settings control panel using GetUserDefaultLocaleName. However, I am not getting the expected results when I pass a StringBuilder as the parameter.

static class NativeMethods
{
    [DllImport("kernel32.dll")]
    public static extern int GetUserDefaultLocaleName(StringBuilder buf, int bufferLength);
}

static void Main(string[] args)
{
    const int nChars = 256;
    var sb = new StringBuilder(nChars);
    int cultureNameLength = NativeMethods.GetUserDefaultLocaleName(sb, nChars);
    string cultureName = sb.ToString();
    Console.WriteLine(cultureName);
    Console.ReadLine();
}

If I have my current culture set to "en-US", when I run the code, cultureName gets set to "e" (only the first letter of current culture), but cultureNameLength is set to 6 (which is expected for the null terminated string "en-US\0")

Why is this only returning the first letter of the culture name (I also tested other cultures and "fr-FR" returns "f")? Is there a different data structure that I can pass instead of StringBuilder to get this to successfully return the culture name?

Answer 1

Symptoms: GetUserDefaultLocaleName() returns a value > 0 (successful), but the StringBuilder buffer appears to only contain 1 char.

• The DllImport attribute's Charset field default value is set to Charset.Ansi, thus platform invoke marshals strings from their managed format (Unicode) to ANSI format.
• Since the string is generated in Unicode format, the marshaled ANSI string is truncated at the first null (\0) char.

Use this field with a member of the CharSet enumeration to specify the marshaling behavior of string parameters and to specify which entry-point name to invoke (the exact name given or a name ending with "A" or "W"). The default enumeration member for C# and Visual Basic is CharSet.Ansi and the default enumeration member for C++ is CharSet.None, which is equivalent to CharSet.Ansi. In Visual Basic, you use the Declare statement to specify the CharSet field.

See also: Specifying a Character Set

To solve the problem, set explicitly CharSet = CharSet.Unicode.
CharSet = CharSet.Auto would also solve the problem, but better not trust it, the string is generated in Unicode format. Better not let the target platform determine a different string format, it may fail.

internal const int LOCALE_NAME_MAX_LENGTH = 85;

[DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
internal static extern int GetUserDefaultLocaleName(StringBuilder buf, int bufferLength);

// [...]

var sb = new StringBuilder(LOCALE_NAME_MAX_LENGTH);
int locale = GetUserDefaultLocaleName(sb, LOCALE_NAME_MAX_LENGTH);

Note: LOCALE_NAME_MAX_LENGTH should be imported, not hard-coded to 85, but that's what you can do in C# :)