使用 linq 删除代码时出错 [重复]

Question

可能重复：
Errors with codes for deleting using linq

我遇到了关于使用组合框删除数据的问题。该错误提示我不知道如何解决它。有人可以帮我吗？

private void btnDel_Click(object sender, EventArgs e)
    {
        using (testEntities Setupctx = new testEntities())
        {
            var Lo = Convert.ToInt16(cbLocationData.SelectedValue);
            var DeleteLocation = (from delLocation in Setupctx.locations
                                  where delLocation.Location1 == Lo
                                  select delLocation).Single();
            Setupctx.DeleteObject(DeleteLocation);
            Setupctx.SaveChanges();
            this.Delete_Location_Load(null, EventArgs.Empty);
            MessageBox.Show("Selected Shift Timing Has Been Deleted.");
        }
    }

where delLocation.Location1 == Lo 部分显示错误

运算符“==”不能应用于“字符串”和“短”类型的操作数。”。

您的帮助将不胜感激。

以上问题的答案如下

 private void btnDel_Click(object sender, EventArgs e)
    {
        using (testEntities Setupctx = new testEntities())
        {
            string selectLo = cbLocationData.SelectedItem.ToString();

            var DeleteLocation = (from delLocation in Setupctx.locations
                                  where delLocation.Location1 == selectLo
                                  select delLocation).SingleOrDefault();
            if (DeleteLocation != null)
            {
                Setupctx.DeleteObject(DeleteLocation);
                Setupctx.SaveChanges();
                cbLocationData.SelectedIndex = -1;
                this.Delete_Location_Load(null, EventArgs.Empty);
                MessageBox.Show("Selected Shift Timing Has Been Deleted.");
            }
        }
    }

Answer 1

显然Location1 是string，不能直接与使用== 的short 进行比较。不要将Lo 转换为short，然后再转换回string，请尝试：

var Lo = (string)cbLocationData.SelectedValue;

Answer 2

这意味着您无法比较 delLocation.Location1 和 Lo 因为它们属于不同的数据类型。试试：

where delLocation.Location1.Equals(Lo.ToString())

Answer 3

错误表明您正在尝试将字符串与 Int16 进行比较。因为我们已经知道 Lo 是一个 Int16，所以 delLocation.Location1 必须是字符串。所以为了解决这个问题，你删除Convert.ToInt16()（因为下拉列表的SelectedValue是字符串）如下：

private void btnDel_Click(object sender, EventArgs e)
{
    using (testEntities Setupctx = new testEntities())
    {
        var Lo = Convert.ToString(cbLocationData.SelectedValue);
        var DeleteLocation = (from delLocation in Setupctx.locations
                              where delLocation.Location1 == Lo
                              select delLocation).Single();
        Setupctx.DeleteObject(DeleteLocation);
        Setupctx.SaveChanges();
        this.Delete_Location_Load(null, EventArgs.Empty);
        MessageBox.Show("Selected Shift Timing Has Been Deleted.");
    }
}

更新

如果您收到“序列不包含元素”错误，这意味着您的查询没有返回任何结果，您不能在空序列上执行Single()。您可以使用SingleOrDefault()，然后检查该值是否为空，如下所示：

private void btnDel_Click(object sender, EventArgs e)
{
    using (testEntities Setupctx = new testEntities())
    {
        var Lo = Convert.ToString(cbLocationData.SelectedValue);
        var DeleteLocation = (from delLocation in Setupctx.locations
                              where delLocation.Location1 == Lo
                              select delLocation).SingleOrDefault();
        if (DeleteLocation != null)
        {
            Setupctx.DeleteObject(DeleteLocation);
            Setupctx.SaveChanges();
            this.Delete_Location_Load(null, EventArgs.Empty);
            MessageBox.Show("Selected Shift Timing Has Been Deleted.");
        }
    }
}