Question

我在GetForumsByForumGroupID方法中得到了这个错误，它在我的Linq语句中选择了Forum对象。我之前没有遇到过这个错误所以我不知道为什么我有这个错误。我甚至不需要Forum对象我只需要Title和ForumID属性但我不知道如何返回匿名对象，例如

var forums = (from x in db.Forums select new {ForumID = x.ForumID, Title = x.Title}).ToList();

这里有错误。获取由下拉列表传递的选定论坛

    public List<Forum> GetForumsByForumGroupID(int ForumGroupID)
    {
        using (CMSEntities db = new CMSEntities())
        {
            List<Forum> forums = (from x in db.Forums where x.ParentID == ForumGroupID select 
                              new Forum(x.ForumID, "",DateTime.Now, x.Title, "", 0, x.Importance, false, "",
                                  "", "", DateTime.Now, true, "", 0, "", DateTime.Now, "", 0, 0, "", 0)).ToList<Forum>();
            return forums;
        }
    }

标记：

<asp:DropDownList ID="DropDownList1" runat="server" AppendDataBoundItems="true"   AutoPostBack="true" 
    DataSourceID="ObjectDataSource1" DataTextField="Key" 
    DataValueField="Value">

</asp:DropDownList>
<asp:ObjectDataSource ID="ObjectDataSource1" runat="server" 
    SelectMethod="GetDisplayForumGroups" TypeName="CMS.Framework.Forums">
</asp:ObjectDataSource>


<asp:GridView ID="GridView2" runat="server" AutoGenerateColumns="False" 
    DataSourceID="ObjectDataSource2">
    <Columns>
        <asp:BoundField DataField="ForumID" HeaderText="ForumID" 
            SortExpression="ForumID" />
        <asp:BoundField DataField="AddedBy" HeaderText="AddedBy" 
            SortExpression="AddedBy" />
        <asp:BoundField DataField="AddedDate" HeaderText="AddedDate" 
            SortExpression="AddedDate" />
        <asp:BoundField DataField="Title" HeaderText="Title" SortExpression="Title" />
        <asp:BoundField DataField="Description" HeaderText="Description" 
            SortExpression="Description" />
        <asp:BoundField DataField="ParentID" HeaderText="ParentID" 
            SortExpression="ParentID" />
        <asp:BoundField DataField="Importance" HeaderText="Importance" 
            SortExpression="Importance" />
        <asp:CheckBoxField DataField="Moderated" HeaderText="Moderated" 
            SortExpression="Moderated" />
        <asp:BoundField DataField="Url" HeaderText="Url" SortExpression="Url" />
        <asp:BoundField DataField="ImageUrl" HeaderText="ImageUrl" 
            SortExpression="ImageUrl" />
        <asp:BoundField DataField="UpdatedBy" HeaderText="UpdatedBy" 
            SortExpression="UpdatedBy" />
        <asp:BoundField DataField="UpdatedDate" HeaderText="UpdatedDate" 
            SortExpression="UpdatedDate" />
        <asp:CheckBoxField DataField="Active" HeaderText="Active" 
            SortExpression="Active" />
        <asp:BoundField DataField="ForumGroup" HeaderText="ForumGroup" 
            SortExpression="ForumGroup" />
        <asp:BoundField DataField="ThreadCount" HeaderText="ThreadCount" 
            SortExpression="ThreadCount" />
        <asp:BoundField DataField="LastPostBy" HeaderText="LastPostBy" 
            SortExpression="LastPostBy" />
        <asp:BoundField DataField="LastPostDate" HeaderText="LastPostDate" 
            SortExpression="LastPostDate" />
        <asp:BoundField DataField="LastPostTitle" HeaderText="LastPostTitle" 
            SortExpression="LastPostTitle" />
        <asp:BoundField DataField="LastPostID" HeaderText="LastPostID" 
            SortExpression="LastPostID" />
        <asp:BoundField DataField="ThreadID" HeaderText="ThreadID" 
            SortExpression="ThreadID" />
        <asp:BoundField DataField="ThreadTitle" HeaderText="ThreadTitle" 
            SortExpression="ThreadTitle" />
        <asp:BoundField DataField="ReplyCount" HeaderText="ReplyCount" 
            SortExpression="ReplyCount" />
    </Columns>
</asp:GridView>

<asp:ObjectDataSource ID="ObjectDataSource2" runat="server" 
    SelectMethod="GetForumsByForumGroupID" TypeName="CMS.Framework.Forums">
    <SelectParameters>
        <asp:ControlParameter ControlID="DropDownList1" PropertyName="SelectedValue" 
        Type="Int32" Name="ForumGroupID" /> 
    </SelectParameters>
</asp:ObjectDataSource>

更新：

    public   GetForumsByForumGroupID(int ForumGroupID)
    {
        using (CMSEntities db = new CMSEntities())
        {
            var forums = (from x in db.Forums where x.ParentID == ForumGroupID select 
                              x).ToList();
            return forums;
        }
    }

不能这样做

    public List<Forum>  GetForumsByForumGroupID(int ForumGroupID)
    {
        using (CMSEntities db = new CMSEntities())
        {
            List<Forum> formus = (from x in db.Forums where x.ParentID == ForumGroupID select 
                              x).ToList<Forum>();
            return forums;
        }
    }

Answer 1

为什么要首先创建新的Forum个对象？除非有充分理由不这样做，否则只需select x。

LINQ to Entities中仅支持无参数构造函数和初始值设定项

1 个答案: