Practical generic type Singleton class

Singleton mode is one of the most common design patterns, and we often write such code. Because the basic principle is to save a static object instance, we can use generics to write out a generic singleton class.

The code is simple:

public class Singleton<T>
　　...{
　　　　static readonly T _t;
　　　　static Singleton()
　　　　...{
　　　　　　_t = Construct();
　　　　}
　　　　public static T GetInstance()
　　　　...{
　　　　　　return _t;
　　　　}
　　　　private static T Construct()
　　　　...{
　　　　　　Type type = typeof(T);
　　　　　　ConstructorInfo ctor;
　　　　　　ctor = type.GetConstructor(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic,
　　　　　　　　　　　　　　　　　　　　null, new Type[0], new ParameterModifier[0]);
　　　　　　System.Diagnostics.Debug.Assert(ctor != null, "Error in ENLS.Basic.Singleton.Construct().");
　　　　　　return (T)ctor.Invoke(new object[0]);
　　　　}
　　}

Because the special generic classes are different types,singleton<tuple<int>> and singleton<tuple<int,long>> are different, so the _ in these two classes T is a different static instance. Then the new instance is created by invoking a common or private default constructor parameter in the construct method. Why is there a public constructor? This is to use the Nullobject mode. For example, for the sake of convenience, I want to add a method to the tuple base class to the Null object

public static Tuple GetNullInstance<_Tuple>() where _Tuple: Tuple
{
　　 return Singleton<_Tuple>.GetInstance();
}With this empty object, I can simply use the function that iterates through the control described in the previous article.

public IEnumerable<Control> Iterator<_Tuple>(Control baseCtl) where _Tuple : Tuple
　　　　...{
　　　　　　Tuple tuple = Tuple.GetNullInstance<_Tuple>();
　　　　　　foreach(Control c in baseCtl.Controls)
　　　　　　...{
　　　　　　　　if (!tuple.HasType(c))
　　　　　　　　...{
　　　　　　　　　　foreach (Control c1 in Iterator<_Tuple>(c))
　　　　　　　　　　　　yield return c1;
　　　　　　　　}
　　　　　　　　else
　　　　　　　　　　yield return c;
　　　　　　}
　　　　}

This makes it easy to call the

foreach (Control c in this.Iterator<Tuple<TextBox, TreeView, CheckBox>>(this))
　　　　　　　　MessageBox.Show(c.Name);