常规写法

public class SingletonSample6
{
    //为了一个单例模式写很多的代码不合适，有一个懒人模式
    private static readonly Lazy<SingletonSample6> _instance = new Lazy<SingletonSample6>(() => new SingletonSample6());
    private int _counter = 0;

    //private外边不能被创建
    private SingletonSample6() { }

    //不能new 不能设值，开个只读的属性
    public static SingletonSample6 Instance
    {
        get
        {
            return _instance.Value;
        }
    }

    public int IncreaseCount()
    {
        return ++_counter;
    }
}

主窗体调用：

int count1 = SingletonSample6.Instance.IncreaseCount();
int count2 = SingletonSample6.Instance.IncreaseCount();

Console.WriteLine($"count1={count1},count2={count2}");
Console.ReadKey();

***out
count1=1,count2=2
***

接口写法

//写成一个基类，这样就不会每个单例都要写一堆代码
public class SingletonSampleBase<TSingleton> where TSingleton : class
{
    private static readonly Lazy<TSingleton> _instance = new Lazy<TSingleton>(() => (TSingleton)Activator.CreateInstance(typeof(TSingleton), true));

    protected SingletonSampleBase() { }

    public static TSingleton Instance
    {
        get
        {
            return _instance.Value;
        }
    }
}

使用：

 public class SingletonSample7 : SingletonSampleBase<SingletonSample7>
 {
     private int _counter = 0;

     private SingletonSample7() { }

     public int IncreaseCount()
     {
         return ++_counter;
     }
 }

调用：

int count1 = SingletonSample7.Instance.IncreaseCount();
int count2 = SingletonSample7.Instance.IncreaseCount();

Console.WriteLine($"count1={count1},count2={count2}");
Console.ReadKey();
***out
count1=1,count2=2
***