C# Method 1 - via registry
C# Method 2 - via running process
VB.NET
Java

import java.util.regex.Matcher;
import java.util.regex.Pattern;
import com.sun.jna.platform.win32.Advapi32Util;
import com.sun.jna.platform.win32.Win32Exception;
import com.sun.jna.platform.win32.WinReg;

String getOsuInstallationDirectory() {
	if (!System.getProperty("os.name").startsWith("Win")) return null;
	String value;
	try {
		value = Advapi32Util.registryGetStringValue(WinReg.HKEY_CLASSES_ROOT, "osu\\DefaultIcon", null);
	} catch (Win32Exception e) { return null; }
	Matcher m = Pattern.compile("\"(.+)\\\\[^\\/]+\\.exe\"").matcher(value);
	return (m.find()) ? m.group(1) : null;
}

private string GetOsuRunningDirectory()
{
    var processes = Process.GetProcessesByName("osu!");
    if (processes.Length > 0)
    {
        string dir = processes[0].Modules[0].FileName;
        dir = dir.Remove(dir.LastIndexOf('\\'));
        return dir;
    }
    return string.Empty;
}

String GetSongsDirectory()
        {
            const String keyName1 = @"HKEY_CLASSES_ROOT\osu\shell\open\command";
            const String keyName2 = @"HKEY_CLASSES_ROOT\osu!\shell\open\command";
            String path = String.Empty, songsPath = String.Empty;
            try
            {
                path = Microsoft.Win32.Registry.GetValue(keyName1, String.Empty, String.Empty).ToString();
                if (path == String.Empty)
                    path = Microsoft.Win32.Registry.GetValue(keyName2, String.Empty, String.Empty).ToString();
                if (path != String.Empty)
                {
                    path = path.Remove(0, 1);
                    path = path.Split('\"')[0];
                    path = System.IO.Path.GetDirectoryName(path);

                    String iniPath = System.IO.Path.Combine(path, "osu!." + Environment.UserName + ".cfg");
                    StreamReader s = File.OpenText(iniPath);

                    while (!s.EndOfStream && !(songsPath != String.Empty))
                    {
                        string l = s.ReadLine();
                        if (l.StartsWith("#"))
                            continue;

                        string[] separator = { " = " };
                        string[] kv = l.Split(separator, StringSplitOptions.None);
                        if (kv[0] == "BeatmapDirectory")
                            songsPath = kv[1];
                    }

                    if(songsPath == string.Empty)
                        songsPath = System.IO.Path.Combine(path, "Songs");
                }
            }
            catch
            {
                //probaly security or io exception or unexpected value from registry
                //Beatmap directory undetected
                return "";
            }
            if(System.IO.Path.IsPathRooted(songsPath))
                return songsPath;
            else
                return System.IO.Path.Combine(path,songsPath);
        }