试图在我的代码中找到错误的来源 [英] Trying to find the source of an error in my code

问题描述

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ClassLibrary3
{
    public class Destination
    {

        public string DestinationName; // Nom de l'opérateur

        public List<int> PrefixCall; // Les codes de l'opérateurs [phone]

        public int MinDigit; // Nombre minimum des chiffres

        public int MaxDigit;

        public string DestinationType; // Termination, VAS, MSRN Roaming

        public  List<Trunk> Trunks;


        public bool ExistPrefixCall(int code)
        {
            if (PrefixCall.Exists(code))//i have problem here
                return true;
            return false;
        }


        public bool addPrefixCall(int code)
        {
            if (PrefixCall.Exists(code))//i have problem here
                return false;
            else
            {

                PrefixCall.Add(code);
                return true;
                    
            }
        }






    }
}

我尝试过：



前缀是一个整数列表我不知道为什么当我使用exists属性时它会给出错误

What I have tried:

prefixcall is a list of integer i don't know why it gives error when i use exists property

推荐答案

你的列表需要初始化;例如

Your "list" needs to be "initialized"; e.g.

public List<int> PrefixCall = new List<int>(); // Les codes de l'opérateurs [phone]

...否则只会是空。

... otherwise it will just be "null".

当您定义自己的谓词以检查元素是否存在时，使用Exists方法。因此，该方法希望您提供一个谓词，而不仅仅是一个数字。



根据您发布的代码，您似乎只想知道一个数字是否是在列表中与否。为此，请尝试使用列表< t> .Contains（T）方法 [ ^ ]

Exists method is used when you define your own predicate to check whether an element exists or not. So the method expects you to provide a predicate, not just a number.

Based on the code you posted it looks like you just want to find out if a number is in the list or not. For this purpose try to use List<t>.Contains(T) Method [^]

在Gerry Schmitz的解决方案上稍微扩展一下：



您需要做的是测试'PrefixCall为null，和/或测试'PrefixCall没有条目。



我喜欢利用可空的布尔来处理这样的任务：

To expand a little on the solution by Gerry Schmitz:

What you need to do is test 'PrefixCall for null, and/or test 'PrefixCall for having no entries.

I like to handle tasks like this by taking advantage of a nullable bool:

public bool? ValidatePrefixCall()
{
    if(PreFixCall == null) return null;

    return PreFixCall.Count > 0;
}

public bool ThrowOnNullList = true;

// use example
bool? pfcIsValid = ValidatePrefixCall();

if (pfcIsValid == true)
{
    // the list is initialized and has at least one item
}
else
{
    if (pfcIsValid == false)
    {
        // the list is initialized and has no items    
    }
    else
    {
        // the list is not initialized
        if (ThrowOnNullList)
        {
            throw new NullReferenceException("PreFixCall is null");
        }
    }
}

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