[转] LINQ Method cannot be translated into a store expression.

LINQ到实体框架中的转换问题

最新推荐文章于 2025-09-06 09:33:29 发布

转载最新推荐文章于 2025-09-06 09:33:29 发布 · 194 阅读

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原文链接：http://www.cnblogs.com/RuiLei/archive/2011/07/13/2105077.html

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#数据库

本文探讨了LINQ to Entities中无法识别特定方法的问题，并提供了如何绕过这一限制的有效解决方案。通过一个示例展示了如何在不抛出异常的情况下使用LINQ to Entities进行查询。

LINQ to Entities does not recognize the method 'Int32 ToInt32(System.String)' method, and this method cannot be translated into a store expression.

This is a shortcomming of LINQ to Entities, and indeed the ADO.NET Entity Framework at the current release! The reason for a similar exception message is that in queries on the Entity Framework, custom methods and extensions methods cannot be translated into store expressions.

There are workarounds to still enjoy LINQ to Entities and as an example please read the following:

My example uses the AdventureWorksDB database which can be downloaded from http://www.codeplex.com. I’ve generated the Entity Data Model from the database (in case you wish to replicate this behavior). The scenario is the following: I have a TextBox and a Button on a form and I want to print the name of the Contact by entering the ContactID into the TextBox.

    1.
    <div>
   
    2.
        <asp:Label ID="lblContactID" runat="server"
   
    3.
                   AssociatedControlID="txtContactID">ContactID:</asp:Label>
   
    4.
        <asp:TextBox ID="txtContactID" runat="server"></asp:TextBox>
   
    5.
        <asp:Button ID="btn" runat="server" Text="Search" OnClick="btn_Click" />
   
    6.
         
    7.
        <asp:Label ID="lblContactName" runat="server"></asp:Label>
   
    8.
    </div>

The obvious thing to do is to apply a Where clause on an EntityCollection and filtering everything on the ContactID given in the txtContactID TextBox. If you use Convert.ToInt32(“ID”) in the Where clause the upper Exception is thrown.

    01.
    protected void btn_Click(object sender, EventArgs e)
   
    02.
    {
   
    03.
        AdventureWorksEntities entities = new AdventureWorksEntities();        
   
    04.
        Contact contact = entities.Contact
   
    05.
                .Where(c => c.ContactID == Convert.ToInt32(txtContactID.Text)).FirstOrDefault();
   
    06.
        if (contact != null)
   
    07.
            lblContactName.Text = contact.FirstName + " " + contact.LastName;
   
    08.
        else lblContactName.Text = "Contact not found!";
   
    09.
    }

The workaround to this is to declare a variable of type int, which contains the converted value from the TextBox and use this variable in the Where query.

    01.
    protected void btn_Click(object sender, EventArgs e)
   
    02.
    {
   
    03.
        AdventureWorksEntities entities = new AdventureWorksEntities();
   
    04.
        int ContactID = Convert.ToInt32(txtContactID.Text);
   
    05.
        Contact contact = entities.Contact
   
    06.
                         .Where(c => c.ContactID == ContactID).FirstOrDefault();
   
    07.
        if (contact != null)
   
    08.
            lblContactName.Text = contact.FirstName + " " + contact.LastName;
   
    09.
        else lblContactName.Text = "Contact not found!";
   
    10.
    }

Because this feature is available in normal LINQ to SQL I hope the ADO.NET team will do some improvement on this in the next major release of the Entity Framework.

转载于:https://www.cnblogs.com/RuiLei/archive/2011/07/13/2105077.html