c#中单例类与静态类的区别以及使用场景

public class SingletonA
 {
  //私有成员，使用时分配内存
  private static SingletonA _instance = null;
  //私有构造，杜绝直接new类
  private SingletonA() { }

  //获取实例
  public static SingletonA GetInstance ()
  { 

   if (_instance == null)
   {
    _instance = new SingletonA();
   }
   return _instance;
  }
 } 

 public class SingletonB
 {
  //私有静态成员，声明类实例时，分配
  private static readonly SingletonB _instance = new SingletonB();
  //私有构造，杜绝直接new类
  private SingletonB() { }
  public static SingletonB GetInstance()
  {   
   return _instance;
  }
 }

public class SingletonD
 {
  private static SingletonD _instance = null;
  private static readonly object _lockObject = new object();
  private SingletonD() { }
  public static SingletonD GetInstance()
  {
   if (_instance == null)
   {
    lock (_lockObject)
    {
     if (_instance == null)
      _instance = new SingletonD();
    }
   }
   return _instance;
  }
 }

public sealed class SingletonC
 {
  private SingletonC() { }
  public static readonly SingletonC Instance = new SingletonC();
 }

//FileLogger只需要定义成一般的类即可，无需按照单例模式进行实现，当然也不能是静态类。

public void ConfigureServices(IServiceCollection services)
{
  services.AddControllersWithViews();
  services.AddSingleton<ILogger, FileLogger>();
}

public static class StaticExample
 {
  private static readonly object lockObj = new object();
  public static void Log(string message)
  {
   //Write code here to log data.
  }
 }