为什么设置 comboBox.SelectedIndex 无法更改 ComboBox 的可编辑部分中显示的内容？答案

【问题标题】：Why does setting comboBox.SelectedIndex fail to change what appears in the editable portion of my ComboBox?为什么设置 comboBox.SelectedIndex 无法更改 ComboBox 的可编辑部分中显示的内容？
【发布时间】：2018-06-05 13:38:52
【问题描述】：

我有一个 C# Winform 应用程序。它有一个ComboBox。我的目标是在输入delete 键时，让ComboBox 下拉列表中选择的项目出现在ComboBox 的可编辑部分中。例如，如果ComboBox 有项目A，B。和C，然后在加载Form 时显示项目A。如果我单击下拉菜单，将鼠标悬停在项目C 上，然后键入delete 键，我希望下拉列表被关闭，C 出现在ComboBox 的可编辑部分。

事实上，我已验证我正在获取所选项目的文本，但代码行 comboBox.SelectedIndex = comboBox.FindStringExact(selectedItemText); 不会更改 ComboBox 的可编辑部分中显示的内容

MCVE：

注意：表单有一个名为 combobox 的组合框和一个名为 textbox 的文本框

using System.Collections;
using System.Collections.Specialized;
using System.Windows.Forms;

namespace Winforms_Scratch
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            //using string collection because I need to simulate what is returned from an Application Settings list
            StringCollection computerList = new StringCollection { "C", "B", "A" };

            ArrayList.Adapter(computerList).Sort();

            comboBox.DataSource = computerList;

            comboBox.KeyDown += ComboBox_KeyDown;

            computerList = null;

        }

        private void ComboBox_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Delete && comboBox.DroppedDown)
            {
                ComboBox comboBox = (ComboBox)sender;

                //get the text of the item in the dropdown that was selected when the Delete key was pressed
                string selectedItemText = comboBox.GetItemText(comboBox.SelectedItem);

                //take focus away from the combobox to force it to dismiss the dropdown
                this.Focus();

                //load selectedItemText into the textbox just so we can verify what it is
                textBox.Text = selectedItemText;

                //set the comboBox SelectedIndex to the index of the item that matches the  
                //text of the item in the dropdown that was selected when the Delete key was pressed
                comboBox.SelectedIndex = comboBox.FindStringExact(selectedItemText);
                comboBox.Refresh();

                //Stop all further processing
                e.Handled = true;

            }
            else if (e.KeyCode == Keys.Down && !comboBox.DroppedDown)
            {
                //If the down arrow is pressed show the dropdown list from the combobox
                comboBox.DroppedDown = true;
            }
        }


    }
}

【问题讨论】：

您是否已验证 comboBox.FindStringExact(selectedItemText); 会返回您所期望的结果？
@JuanR: As it is, I've verified that I am getting the selected item text
这不是我问的。我问FindStringExact的返回值是多少。
@JuanR：我很抱歉。是的，comboBox.FindStringExact(selectedItemText); 返回与selectedItemText 对应的索引
设置comboBox.DroppedDown = false;而不是聚焦窗口。然后设置comboBox.SelectedItem = selectedItemText;

标签： c# winforms combobox keydown

【解决方案1】：

我的猜测是组合框失去焦点后的行为会有所不同。无论如何，根据我对您的要求的理解，我进行了以下更改并且它有效。如果有单独的要求将焦点返回到窗口，您可以在设置所选项目后调用this.Focus()。您的 SelectedIndex/FindStringExact 方法与将 SelectedItem 设置为字符串相同。

我去掉了文本框，因为据我了解，这只是为了调试目的。

private void ComboBox_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Delete && comboBox.DroppedDown)
    {
        ComboBox comboBox = (ComboBox)sender;

        //  Get the text of the item in the dropdown that was selected when the 
        //  Delete key was pressed
        string selectedItemText = comboBox.GetItemText(comboBox.SelectedItem);

        comboBox.DroppedDown = false;

        comboBox.SelectedItem = selectedItemText;

        //Stop all further processing
        e.Handled = true;

    }
    else if (e.KeyCode == Keys.Down && !comboBox.DroppedDown)
    {
        //  If the down arrow is pressed show the dropdown list from the combobox
        comboBox.DroppedDown = true;
    }
}

【讨论】：