哎呀！你如何正确地更新初始化组合框的SelectedValue的？

问题描述

考虑以下code。这是我在，我已经创建了一个用户控件执行过程的简化。

Consider the following code. This is a simplification of a process that I have implemented within a User Control that I've created.

//MyUserControl Constructor
public MyUserControl(field, value)
{
    InitializeComponents();
    string cType = resolveControlType(field);
    switch (cType)
    {
        ...
        case "ComboBox":  AddComboBox(field, value);
        ...
    }
}

AddComboBox(string fieldID, object value)
{
    ComboBox cbo = new ComboBox();
    cbo.DisplayMember = "DisplayMember";
    cbo.ValueMember = "ValueMember";

    //We set the DataSource to a DataTable
    cbo.DataSource = DBCaller.GetListAsDataTable(fieldID);
    this.Controls.Add(cbo);
    cbo.SelectedValue = value; //<-- Weird stuff happening here?!
                               //    If you don't break here, it
                               //    doesn't look like the correct
                               //    record is selected.
                               //    However, add a breakpoint,
                               //    scroll through cbo's properties
                               //    and this assignment will work
                               //    properly when you continue?!
}

我的问题是，当我的值赋给控制，组合框的文本从我的数据源表中显示的第一个项目。

My problem is that when I assign the value to the control, the text in the ComboBox displays the first item from my DataSource table.

但是，如果我把一个断点上的 cbo.SelectedValue =值; 行，并使用智能感知，滚动和我的组合框关联的属性，一些初始化的组合框修复此问题。有一次，我继续运行code，显示在我的组合框的形式加载与适当的值。

However, if I put a breakpoint on the cbo.SelectedValue = value; line and, using Intellisense, scroll through the properties associated with my ComboBox, something initializes on the ComboBox that fixes this problem. Once I continue running the code, my form loads with the proper value displayed on the ComboBox.

这是怎么回事，我该如何解决这个问题？

What is going on and how can I fix this?

推荐答案

我已经找到了如何解决你的问题，但解释为什么它是如此不容易。我发现了一些有趣的东西在这里。首先，我想说，我已经发现至少有2种方式来设置整治。下面是code对于那些2种方式：

I've found how to solve your problem but to explain why it's so is not easy. I've found some thing interesting here. First, I would like to say that I've found at least 2 ways to set the things in order. Here are the code for those 2 ways:

//Solution 1
//Simply you have to add the ComboBox to the parent control first
//before assigning its DataSource
this.Controls.Add(cbo);   //<---- This goes first
cbo.DataSource = DBCaller.GetListAsDataTable(fieldID); //<--- This goes after
cbo.SelectedValue = value;

//Solution 2
//This is very strange and interesting, you can also add your ComboBox to
//the parent control after assigning its DataSource (as in your code).
//But you have to ACCESS to the BindingContext property of your ComboBox
//I would like to emphasize the ACCESS, you can perform any kind of access (Read and Write).
//Here are some examples of such access:
cbo.DataSource = DBCaller.GetListAsDataTable(fieldID);
this.Controls.Add(cbo);  //<--- like in your code, this is placed here after the DataSource is assigned
//here you can ACCESS the BindingContext
var whatEver = cbo.BindingContext;//READ access
if(cbo.BindingContext == null) Text = "????"; //READ access and of course it's not null
cbo.BindingContext = new BindingContext();//WRITE access
cbo.SelectedValue = value; //<---- This should be placed here after all.

我找到了第二个解决方案是很奇怪的，不容易解释，虽然第一个解决方案，可以理解的（至少当时我还没有找到第二个）。

I found the second solution is very strange and not easy to explain, although the first solution can be understandable (at least at the time I had not found the second).

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