Retrieving privilege name from LUID

Question

I am writing a method for finding out the privilege name associated with a given LUID. The following is the code for the same :

private string PrivilegeName(LUID luid)
{
    StringBuilder sbuilder = new StringBuilder();
    int nameLength = 0;
    IntPtr ptrToLuid = Marshal.AllocHGlobal(Marshal.SizeOf(luid));
    Marshal.StructureToPtr(luid, ptrToLuid, true);
    if(!InvokeAPI.LookupPrivilegeName(null, ptrToLuid, null, ref nameLength))
    {
        Console.WriteLine("Unable to lookup value.");
        Console.WriteLine(Marshal.GetLastWin32Error());
        Marshal.FreeHGlobal(ptrToLuid);
        return null;
    }
    sbuilder.EnsureCapacity(nameLength + 1);

    InvokeAPI.LookupPrivilegeName(null, ptrToLuid, sbuilder, ref nameLength);
    Marshal.FreeHGlobal(ptrToLuid);

    return sbuilder.ToString();
}

For reference my LUID structure looks like this

[StructLayout(LayoutKind.Sequential)]
public struct LUID
{
    public UInt32 LowPart;
    public Int32 HighPart;
}

While executing this code I get a ERROR_INSUFFICIENT_BUFFER (122) instead of the privilege name associated with the LUID.

Could someone help me out on what I am missing in the above code that prevents me from retrieving the privilege name ?

Answer 1

Written "better" pinvoke, written inside a comment an explanation of the error. Written inside comments the funny handling of lengths of LookupPrivilegeName.

// https://www.pinvoke.net/default.aspx/advapi32.lookupprivilegename
[DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Auto)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool LookupPrivilegeName(
    string lpSystemName,
    ref LUID lpLuid,
    StringBuilder lpName,
    ref int cchName);

private static string PrivilegeName(LUID luid)
{
    StringBuilder sbuilder = new StringBuilder();
    int nameLength = 0;

    LookupPrivilegeName(null, ref luid, sbuilder, ref nameLength);

    // Will always fail with nameLength == 0. We simply check that nameLength != 0
    if (nameLength == 0)
    {
        Console.WriteLine("Unable to lookup value.");
        Console.WriteLine(Marshal.GetLastWin32Error());
        return null;
    }

    // On "failure" (that is really a success, only we get 
    // just the length), nameLength is the length required 
    // for the buffer, including the \0
    sbuilder.EnsureCapacity(nameLength);

    // On a success of this second call, nameLength is the 
    // "real" length of the name, excluding the \0
    // so on a success, now nameLength = oldNameLength - 1
    // Don't ask, it is absurd... Classical case where the 
    // Windows API are quite random in their handling of 
    // buffer lengths
    if (!LookupPrivilegeName(null, ref luid, sbuilder, ref nameLength))
    {
        Console.WriteLine("Unable to lookup value.");
        Console.WriteLine(Marshal.GetLastWin32Error());
        return null;
    }

    return sbuilder.ToString();
}