Linq to SQL OrderBy 基于 DropDownList 中的选定值

Question

我有 10 篇文章的列表，在页面加载事件中显示以下 Linq to SQL 查询：

if (!Page.IsPostBack)
        {
            MyDataClassesDataContext db = new MyDataClassesDataContext();

            var pressRelease = from pr in db.PressReleases
                orderby pr.DatePublished ascending 
                where pr.Published == true
                select pr;

            int i = 0;

            foreach (var release in pressRelease)
            {
                Literal literal = new Literal();
                literal.Text = "<div class='row no-gutters pb-5'><div class='col-12'><h2>" +
                               release.Title + "</h2>" + Server.HtmlDecode(release.Content) + "</h2>" +
                               "<a href='Press-Release.aspx?id=" + release.Id +
                               "'><b>Read article...</b></a></div></div>";
                pnlPressReleaases.Controls.Add(literal);
            }
        }

在我的网页上，我有一个包含以下值的下拉列表，并且自动回发设置为 true。

<asp:DropDownList ID="ddlPressSortBy" AutoPostBack="True" runat="server" OnSelectedIndexChanged="ddlPressSortBy_SelectedIndexChanged">
                <asp:ListItem Value="pr.DatePublished descending">Date Published Desc</asp:ListItem>
                <asp:ListItem Selected="True" Value="pr.DatePublished ascending">Date Published Asc</asp:ListItem>
                <asp:ListItem Value="pr.Title ascending">Alphabetical</asp:ListItem>
            </asp:DropDownList>

一旦我改变了下拉列表的值，返回的数据在对应的select语句上没有orderby。这是我的 OnSelectIndexChanged 事件代码：

protected void ddlPressSortBy_SelectedIndexChanged(object sender, EventArgs e)
    {
        MyDataClassesDataContext db = new MyDataClassesDataContext();

        var pressRelease = from pr in db.PressReleases
            orderby ddlPressSortBy.SelectedValue
            where pr.Published == true
            select pr;


        foreach (var release in pressRelease)
        {
            Literal literal = new Literal();
            literal.Text = "<div class='row no-gutters pb-5'><div class='col-12'><h2>" +
                           release.Title + "</h2>" + Server.HtmlDecode(release.Content) + "</h2>" + "<a href='Press-Release.aspx?id=" + release.Id + "'><b>Read article...</b></a></div></div>";
            pnlPressReleaases.Controls.Add(literal);
        }
    }

我有点不确定为什么它没有与 orderby 一起触发。

任何帮助将不胜感激。

谢谢 抢

Answer 1

你不能这样排序：

var pressRelease = from pr in db.PressReleases
        orderby ddlPressSortBy.SelectedValue
        where pr.Published == true
        select pr;

因为结果查询类似于（假设您选择了第一项）：

SELECT * FROM PressReleases
WHERE Published = 1
ORDER BY 'pr.DatePublished descending'

ORDER BY 子句将是一个固定值，导致默认排序。

Linq OrderBy 采用 Expression<Func<T, object>>，如果您使用 Fluent 语法，则类似于 pr => pr.DatePublishing。查询语法只是以漂亮的形式隐藏了它，但它是如何工作的。

根据this article，您可以为 IQueryable 创建一个扩展方法，允许您使用属性名称进行排序：

public static class extensionmethods
{
    public static IQueryable<T> OrderByField<T>(this IQueryable<T> q, string SortField, bool Ascending)
    {
        var param = Expression.Parameter(typeof(T), "p");
        var prop = Expression.Property(param, SortField);
        var exp = Expression.Lambda(prop, param);
        string method = Ascending ? "OrderBy" : "OrderByDescending";
        Type[] types = new Type[] { q.ElementType, exp.Body.Type };
        var mce = Expression.Call(typeof(Queryable), method, types, q.Expression, exp);
        return q.Provider.CreateQuery<T>(mce);
    }
}

然后像这样在你的代码中使用它：

var sortingInfo = ddlPressSortBy.SelectedValue.Split(' ');
var sortingProperty = sortingInfo[0];
var descending = sortingInfo[1] == "descending";

var pressRelease = from pr in db.PressReleases
        where pr.Published == true
        select pr;
pressRelease = pressRelease.OrderByField(sortingProperty, descending);
// rest of code