Singleton1.cs

//thread-safe

public class MySingleton
{
    private static readonly Lazy<MySingleton> _mySingleton = new Lazy<MySingleton>(() => new MySingleton());

    private MySingleton() { }

    public static MySingleton Instance
    {
        get
        {
            return _mySingleton.Value;
        }
    }
}

Singleton2.cs

//thread-safe

public class MySingleton {
    private static object myLock = new object();
    private static volatile MySingleton mySingleton = null;

    private MySingleton() {
    }

    public static MySingleton GetInstance() {
        if (mySingleton == null) {
            lock (myLock) {
                if (mySingleton == null) {
                    mySingleton = new MySingleton();
                }
            }
        }

        return mySingleton;
    }
}

Singleton3.cs

//non thread-safe

public class MySingleton {
    private static object myLock = new object();
    private static MySingleton mySingleton = null;

    private MySingleton() {
    }

    public static MySingleton GetInstance() {
        if (mySingleton == null) {
            lock (myLock) {
              mySingleton = new MySingleton();
            }
        }

        return mySingleton;
    }
}