Question

我看到以下错误：

对象引用未设置为对象的实例！

在调用方法之前检查以确定对象是否为空！

我是C＃的新手，我为Sorted Linked Lists制作了一个程序。这是错误发生的代码！

    public void Insert(double data)
    {
        Link newLink = new Link(data);
        Link current = first;
        Link previous = null;

        if (first == null)
        {
            first = newLink;
        }

        else
        {
            while (data > current.DData && current != null)
            {
                previous = current;
                current = current.Next;
            }

            previous.Next = newLink;
            newLink.Next = current;
        }
    }

它表示当前引用为空while (data > current.DData && current != null)，但我分配了它：current = first;

其余的是本程序的完整代码！

class Link
{
    double dData;
    Link next=null;

    public Link Next
    {
        get { return next; }
        set { next = value; }
    }

    public double DData
    {
        get { return dData; }
        set { dData = value; }
    }

    public Link(double dData)
    {
        this.dData = dData;
    }


    public void DisplayLink()
    { 
    Console.WriteLine("Link : "+ dData);
    }

}

class SortedList
{
    Link first;

    public SortedList()
    { 
        first = null;
    }

    public bool IsEmpty()
    {
        return (this.first == null);
    }

    public void Insert(double data)
    {
        Link newLink = new Link(data);
        Link current = first;
        Link previous = null;

        if (first == null)
        {
            first = newLink;
        }

        else
        {
            while (data > current.DData && current != null)
            {
                previous = current;
                current = current.Next;
            }

            previous.Next = newLink;
            newLink.Next = current;
        }
    }

    public Link Remove()
    {
        Link temp = first;
        first = first.Next;
        return temp;
    }

    public void DisplayList()
    {
        Link current;
        current = first;

        Console.WriteLine("Display the List!");

        while (current != null)
        {
            current.DisplayLink();
            current = current.Next;
        }
    }
}

class SortedListApp
{
    public void TestSortedList()
    {
        SortedList newList = new SortedList();
        newList.Insert(20);
        newList.Insert(22);
        newList.Insert(100);
        newList.Insert(1000);
        newList.Insert(15);
        newList.Insert(11);

        newList.DisplayList();
        newList.Remove();
        newList.DisplayList();

    }
}

Answer 1

你可能假设while循环在第一次迭代时破坏了，而不是它在while循环中的赋值最终会破坏它。

最终根据您的代码，当前为NULL，您甚至测试它 - 将其更改为此，它应该没问题：

while (current != null && data > current.DData)
{
    previous = current;
    current = current.Next;
}

Answer 2

同意你已经完成了

current = first;

但是你的班级首先是空的

class SortedList
{
    Link first;

请为first分配一些内容，否则它将为空

对象引用未设置为对象链接列表示例的实例

2 个答案: