List.Except 方法返回错误的结果

Question

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

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 + "");
        }

列表被填充的代码是这样的（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任何人可以帮助我吗？

Answer 1

有两种方法可以比较（是否相等）两个Results 对象（以及所有引用类型的对象）：

• 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本身。我的意思是两个Results 对象如果它们实际上是一个对象但你有两个引用它是相等的。例如，您可以创建一个 Results 对象并将其放在两个列表中。

两个列表中的对象显然是不同的对象，所以我猜你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检查属于第二种类型。但是，您可以根据需要覆盖此行为。

覆盖此行为的一种方法是覆盖类中的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，我们告诉系统它应该如何测试这种类型的对象的相等性。

Answer 2

我确信 Yacoub 的答案会奏效，并且在大多数情况下可能是首选解决方案，但如果您无法修改 Results 类，这是另一种方法。为 Results 定义 IEqualityComparer 的实现，然后将其作为 except 方法的第二个参数传递。 编辑：

class Results {
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class ResultsEqualityComparer : IEqualityComparer<Results> {

    public bool Equals(Results res1, Results res2) {
        return (res1.FirstName == res2.FirstName && res1.LastName == res2.LastName);
    }

    public int GetHashCode(Results res) {
        return new { res.FirstName, res.LastName }.GetHashCode();
    }

}

// 这里是将ResultsComparer传递给Except的代码示例：

        var resultaten = new List<Results>() {
        new Results() {FirstName="Bob", LastName="Smith"},
        new Results() {FirstName="Ted", LastName="Wilson"},
        new Results() {FirstName="Alice", LastName="Wilson"},
        new Results() {FirstName="Carol", LastName="Smith"}
        };

        var resultaten2 = new List<Results>() {
        new Results() {FirstName="William", LastName="Smith"},
        new Results() {FirstName="Ted", LastName="Wilson"},
        new Results() {FirstName="Gerhardt", LastName="Wilson"},
        new Results() {FirstName="Carol", LastName="Smith"}
        };

        var comparer = new ResultsEqualityComparer();

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

        foreach (var x in Veranderingen) {
            Console.WriteLine(x.FirstName + " " + x.LastName);
        }

// 打印出来： 威廉史密斯 格哈特·威尔逊