NullReferenceException 和事件

Question

我是 C# 的新手，我从以下行收到 NullReferenceException：

CommandEntered(readedLine);

其余的相关代码应该是这样的：

using System;

namespace Eksamensprojekt
{
    class StregSystemCLI : IStregSystemUI
    {
        IStregSystem stregSystem = new StregSystem();
        public event StregSystemEvent CommandEntered;

        public StregSystemCLI(IStregSystem stregSystem)
        {
            this.stregSystem = stregSystem;
        }         
        public void Start()
        {
            Console.WriteLine(stregSystem.activeProducts().ToString());
            Console.WriteLine("Enter your username and then a product ID and hit enter to buy a product. Example: 'username 49340'");
            Console.WriteLine("Alternatively, to buy multiple instances of the same product you can use: 'username amount ID' instead. Example: 'username 10 49340'");

            string readedLine = Console.ReadLine();
            Console.WriteLine("{0}", readedLine);

            if(readedLine != null)
            {
                CommandEntered(readedLine);
            }
        }
    }
}


namespace Eksamensprojekt
{
    class StregSystemCommandParser
    {
        IStregSystem stregSystem;
        IStregSystemUI stregSystemUI;

        public StregSystemCommandParser(IStregSystem stregSystem, IStregSystemUI stregSystemUI)
        {
            stregSystem = new StregSystem( );
            stregSystemUI = new StregSystemCLI(stregSystem);
            stregSystemUI.CommandEntered += ParseCommand;

        }

        void ParseCommand(string command)
        {
            ...lots of code here, that I don't think is relevant to the solution
        }
    }
}



namespace Eksamensprojekt { 
    public class Program 
    {

        static void Main(string[] args)
        {
            IStregSystem stregSystem = new StregSystem();
            IStregSystemUI ui = new StregSystemCLI(stregSystem);
            StregSystemCommandParser sc = new StregSystemCommandParser(stregSystem, ui);

            ui.Start();
        }
    }
}

namespace Eksamensprojekt
{
    public delegate void StregSystemEvent(string Command);
}

我希望上面的内容在没有完整代码的情况下也可以阅读。我想要发生的是，如果在 readedLine 中输入了某些内容并且字符串不为空，我希望它触发一个事件，该事件导致方法 ParseCommand() 在 class StregSystemCommandParser 中成为运行 . 如何制定修复 NullReferenceException 的解决方案？

Answer 1

您的 CommandEntered 事件没有附加监听器。

call an event 的正确 C# 习语是：

CommandEntered?.Invoke(readedLine);

NullReferenceException 和事件

NullReferenceException and events

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events

nullreferenceexception