从匿名Linq查询填充WinForms DataGridView

本文介绍了从匿名Linq查询填充WinForms DataGridView的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

//从我的表单

BindingSource bs = new BindingSource();

private void fillStudentGrid()
{
     bs.DataSource = Admin.GetStudents();
     dgViewStudents.DataSource = bs;
}

//从管理类

public static List<Student> GetStudents()

{
    DojoDBDataContext conn = new DojoDBDataContext();

    var query =
        (from s in conn.Students
         select new Student
         {
             ID = s.ID,
             FirstName = s.FirstName,
             LastName = s.LastName,
             Belt = s.Belt
         }).ToList();

    return query;
}

我正在Winforms中填充一个datagridview控件，我只想一些价值观。代码编译，但会抛出运行时错误：

I'm trying to fill a datagridview control in Winforms, and I only want a few of the values. The code compiles, but throws a runtime error:

不允许在查询中显式实现实体类型DojoManagement.Student的构造。

有没有办法让它以这种方式工作？

Is there a way to get it working in this manner?

推荐答案

您已经有一个 IEnumerable< Student> 实例，因为）

You already have an IEnumerable<Student> instance and you cannot project entities from a query for the reasons described here)

你也不需要创建一个绑定到此数据源的列表 - 您可以通过将其更改为以下方式来大大简化您的方法：

You also don't need to create a list to bind to this data source - you can greatly simplify your method by changing it to this:

public static IEnumerable<Student> GetStudents()
{
    return new DojoDBDataContext().Students;
}

没有理由投射一个新的实例来只映射几个属性，通过执行查询，您返回所有的值无论如何，您的投影不会保存任何东西。如果你真的只想从这个查询返回一些值，以便隐藏信息，你可以这么做：

There is no reason to project a new instance to only map a few of the properties, by executing the query you are returning all the values anyhow and your projection doesn't save you anything. If you really want to only return a few values from this query for the purposes of information hiding you could do this:

public static IEnumerable<Object> GetStudents()
{
    DojoDBDataContext conn = new DojoDBDataContext();

    return conn.Students
               .Select(s => new {
                   ID = s.ID,
                   FirstName = s.FirstName,
                   LastName = s.LastName,
                   Belt = s.Belt
               });
}

编辑：如果您没有使用C＃ 4，你必须将 IEnumerable< T> 的内容显式转换为 Object 。只有C＃4支持 IEnumerable< T> 的协方差。所以如果你使用C＃3，你必须这样做：

If you aren't using C# 4 you will have to cast the contents of the IEnumerable<T> to Object explicitly. Only C# 4 supports covariance for IEnumerable<T>. So if you are using C# 3 you will have to do this:

public static IEnumerable<Object> GetStudents()
{
    DojoDBDataContext conn = new DojoDBDataContext();

    return conn.Students
               .Select(s => new {
                   ID = s.ID,
                   FirstName = s.FirstName,
                   LastName = s.LastName,
                   Belt = s.Belt
               }).Cast<Object>();
}

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VALUES