Question

我正在尝试为我正在制作的小型数据库编写自定义查询生成器，但应该出现在字符串的所有条目之间的逗号不会只出现在最后是。

private void BTN_advancedSearch_Click(object sender, EventArgs e)
    {
        // Creates the variable part of the custom query
        string customwhereclause = "";

        if (CHK_enableGameName.Checked == true)
        {
            Connectqry(customwhereclause);
            customwhereclause += "Game.GameName LIKE '%" + TXT_addGame.Text + "%'";
        }

        if (CHK_enableGenreName.Checked == true)
        {
            Connectqry(customwhereclause);
            customwhereclause += "Genre.GenreID =" + genreID + "";
        }

        if (CHK_enableConsoleName.Checked == true)
        {
            Connectqry(customwhereclause);
            customwhereclause += "Console.ConsoleID =" + consoleID + "";
        }

        if (CHK_enablePlayers.Checked == true)
        {
            Connectqry(customwhereclause);
            customwhereclause += "Game.Players >=" + NUD_players.Value + "";
        }
        if (CHK_enableDisc.Checked == true)
        {
            if (CHK_discOwned.Checked == true)
            {
                Connectqry(customwhereclause);
                customwhereclause += "Game.Disc ='" + "yes" + "'";
            }
            else
            {
                Connectqry(customwhereclause);
                customwhereclause += "Game.Disc ='" + "no" + "'";
            }
         }
         if (CHK_enableCompleted.Checked == true)
         {
            if (CHK_completed.Checked == true)
            {
                Connectqry(customwhereclause);
                customwhereclause += "Game.Completed ='" + "yes" + "'";
            }
            else
            {
                Connectqry(customwhereclause);
                customwhereclause += "Game.Completed ='" + "no" + "'";
            }
        }

        //varible query code being passed back to search form.
         frm_search.Cstmqry = customwhereclause;

        //close the form and reopen the other one.
         this.Close();
         frm_search.Show();
    }

    private void Connectqry(string s)
    {
        if (s == "")
        {
            Console.WriteLine("the query is blank");
        }
        else
        {
            s = s + " , ";
            Console.WriteLine(s);
        }
    }

输出目前是：

the query is blank

Game.GameName LIKE '%name%' ,

Game.GameName LIKE '%name%'Genre.GenreID =0 ,

Game.GameName LIKE '%name%'Genre.GenreID =0Console.ConsoleID =0 , 

Game.GameName LIKE '%name%'Genre.GenreID =0Console.ConsoleID =0Game.Players >=1 ,

Game.GameName LIKE '%name%'Genre.GenreID =0Console.ConsoleID =0Game.Players >=1Game.Disc ='no' ,

我不确定为什么删除字符串之间的逗号。

Answer 1

您应该添加代码：

if (!string.IsNullOrEmpty(customwhereclause))
{
    customwhereclause += " AND ";
}
customwhereclause += // Your condition

在所有条件下。它会在任何需要的地方添加AND运算符。

更好：

private static string computeCondition(string current, string newCondition)
{
    if (!string.IsNullOrEmpty(current))
    {
        current += " AND ";
    }
    return current + newCondition;
}

private void BTN_advancedSearch_Click(object sender, EventArgs e)
{
    // Creates the variable part of the custom query
    string customwhereclause = "";

    if (CHK_enableGameName.Checked == true)
    {
        Connectqry(customwhereclause);

        customwhereclause = computeCondition(customwhereclause, "Game.GameName LIKE '%" + TXT_addGame.Text + "%'");
    }
    ...

避免代码重复太多

甚至更好：

private void BTN_advancedSearch_Click(object sender, EventArgs e)
{
    // Creates the variable part of the custom query
    List<string> whereClausesList = new List<string>();

    if (CHK_enableGameName.Checked == true)
    {
        Connectqry(customwhereclause);

        whereClausesList.Add("Game.GameName LIKE '%" + TXT_addGame.Text + "%'");
    }
    ...
    string.Join(" AND ", whereClausesList);

根据Rob

的建议

Answer 2

您的代码无效，因为string无法使用。当您执行s = s + " , ";之类的字符串连接时，这不会更新s引用的对象。它正在创建新的string并将引用分配给s。因为您没有将s作为ref传递，所以您只更新引用的本地副本而不是原始副本。解决这个问题的正确方法是返回新的string并分配它。

private string Connectqry(string s)
{
    if (s == "")
    {
        Console.WriteLine("the query is blank");
    }
    else
    {
        s = s + " , ";
        Console.WriteLine(s);
    }

    return s;
}

并像

一样使用它

customwhereclause = Connectqry(customwhereclause);

正如其他人所提到的，你可能想要使用＆＃34; AND＆＃34;而不是使用逗号，并且使用string.Join或StringBuilder可能会更有效，但string是不可变的，并且字符串连接创建新字符串是您当前代码无法执行的操作你期待。

从字符串中缺少逗号

2 个答案: