singleton.cs

// Not thread safe
// https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/sealed
public sealed class Singleton {
  // https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/static
  private static Singleton instance = null;

  // hide constructor from public interface
  private Singleton() {}

  public static Singleton Instance {
    get {
      if (instance == null) {
        instance = new Singleton();
      }
      return instance;
    }
  }
}

// Thread safe
public sealed class Singleton {
  // https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly
  private static readonly Singleton InstanceHolder = new Singleton();

  // https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-constructors
  static Singleton() {}

  private Singleton() {}

  public static Singleton Instance => InstanceHolder;
}

// Thread safe & Lazy
// https://docs.microsoft.com/en-us/dotnet/api/system.lazy-1
public sealed class Singleton {
  private static readonly Lazy<Singleton> InstanceHolder = new Lazy<Singleton>(() => new Singleton());
  
  private Singleton() {}

  public static Singleton Instance => InstanceHolder.Value;
}

// Generic Singleton
// For the sake of example only since it defeats the singleton purpose (the is no control over T's implementation)
public sealed class Singleton<T> where T : new()
{
  private static readonly Lazy<T> InstanceHolder = new Lazy<T>(() => new T());
  private Singleton() {}
  public static T Instance => InstanceHolder.Value;
}