c# - List.Except 方法返回错误结果

Question

我试图比较 2 个结果类型的列表，它不断地返回整个结果列表，它似乎没有过滤掉任何内容。

这是代码:

  List<Results> Veranderingen = resultaten2.Except(resultaten).ToList();

        foreach(Results x in Veranderingen)
        {
            MessageBox.Show("Nieuwe Data gevonden: " + x.titel + "Van de website" + x.url + "");
        }

填充列表的代码是这样的(不太重要):

 private void Lijst2invullen()
    {

        OleDbConnection connection = new OleDbConnection();
        connection.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\martijn\Dropbox\Proftaak Periode 2 Identity\Database11.accdb;
        Persist Security Info=False;";
        connection.Open();

        OleDbCommand cmd2 = new OleDbCommand();
        cmd2.Connection = connection;
        cmd2.CommandText = "SELECT ZoekcriteriaID from Zoekcriteria WHERE ZoekCriteria = '" + Convert.ToString(cbzoektermselecteren.Text) + "';";
        OleDbDataReader reader2 = cmd2.ExecuteReader();

        if (reader2.Read())
        {
            refreshid2 = Convert.ToInt32(reader2["ZoekcriteriaID"]);
        }




        OleDbCommand command5 = new OleDbCommand();
        command5.Connection = connection;
        command5.CommandText = "SELECT Titel, Webadress from Resultaat WHERE ZoekcriteriaID = " + refreshid2 + ";";
        OleDbDataReader reader3 = command5.ExecuteReader();

        while (reader3.Read())
        {
            Results result = new Results();
            result.url = Convert.ToString(reader3["Webadress"]);
            result.titel = Convert.ToString(reader3["Titel"]);

            resultaten2.Add(result);
        }
        reader3.Close();
        label1.Text = "Ziet er goed uit!";
    }

    private void Lijst1invullen()
    {
        OleDbConnection connection = new OleDbConnection();
        connection.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\martijn\Dropbox\Proftaak Periode 2 Identity\Database11.accdb;
        Persist Security Info=False;";
        connection.Open();


        OleDbCommand cmd1 = new OleDbCommand();
        cmd1.Connection = connection;
        cmd1.CommandText = "SELECT ZoekcriteriaID from Zoekcriteria WHERE ZoekCriteria = '" + Convert.ToString(cbzoektermselecteren.Text) + "';";
        OleDbDataReader reader1 = cmd1.ExecuteReader();

        if (reader1.Read())
        {
            refreshid = Convert.ToInt32(reader1["ZoekcriteriaID"]);
        }


        OleDbCommand command = new OleDbCommand();
        command.Connection = connection;
        command.CommandText = "SELECT Titel, Webadress from Resultaat WHERE ZoekcriteriaID = " + refreshid + ";";
        OleDbDataReader reader = command.ExecuteReader();



        while (reader.Read())
        {
            Results result = new Results();
            result.url = Convert.ToString(reader["Webadress"]);
            result.titel = Convert.ToString(reader["Titel"]);

            resultaten.Add(result);
        }
        reader.Close();
        reader1.Close();

        OleDbCommand command2 = new OleDbCommand();
        command2.Connection = connection;
        command2.CommandText = "DELETE * FROM Resultaat WHERE ZoekcriteriaID = " + refreshid + ";";
        command2.ExecuteNonQuery();

        OleDbCommand command3 = new OleDbCommand();
        command3.Connection = connection;
        command3.CommandText = "DELETE * FROM Zoekcriteria WHERE ZoekCriteriaID = " + refreshid + ";";
        command3.ExecuteNonQuery();

        search.zoekterm = cbzoektermselecteren.Text;
        search.InsertZoekcriteria();
        search.searchding();

    }

我认为我在 except 方法的语法中做错了什么，有人可以帮助我吗？

Answer 1

有两种方法可以比较(是否相等)两个 Results 对象(以及一般所有引用类型对象):

• 第一种方法是比较两个 Results 对象的属性值。

• 第二种方法是比较引用文献本身。我的意思是，如果两个 Results 对象实际上是单个对象，但您有两个对其的引用，那么它们是相等的。例如，您可以创建一个 Results 对象并将其放入两个列表中。

两个列表中的对象显然是不同的对象，所以我猜测您想使用第一种比较方式。

默认情况下，C# 中引用类型对象的相等性检查是第二种类型。但是，如果您愿意，您可以覆盖此行为。

重写此行为的一种方法是重写类中的 Equals 和 GetHashCode 方法。这是一个例子:

public class Results
{
    public string url { get; set; }
    public string title { get; set; }

    public override bool Equals(object obj)
    {
        Results other = obj as Results;

        if (other == null)
            return false;

        return other.url == this.url && other.title == this.title;
    }

    public override int GetHashCode()
    {
        return new {url, title}.GetHashCode();
    }
}

这样，我们就告诉系统应该如何测试这种类型的对象的相等性。