根据来自combox的选定项目，从数据库显示texbox中的文本 [英] Displaying text in texbox from databse based on selected items from combox

问题描述

我基本上有一个组合框和一个文本框。我该怎么办是组合框由主题代码组成，而文本框由主题名称组成，存储在数据库中。我是否知道有什么方法可以选择组合框中的项目时，根据选择的主题代码自动显示主题名称，该主题代码将来自数据库。 belwo是我的代码，但没有工作

 私人  void  FormAttendanceFaculty_Load（ object  sender，EventArgs e）
 con.Open（） ; 
 OleDbDataAdapter oda1 =  new  OleDbDataAdapter（  select来自主题的代码，其中代码如'％'，con）; 
 DataTable dt1 =  new  DataTable（）; 
 oda1.Fill（dt1）; 
 comboBoxCode.DataSource = dt1; 
 comboBoxCode.DisplayMember =   code; 
 comboBoxCode.SelectedIndex = -1; 
  string  myquery =   SELECT name FROM subject其中name =' + comboBoxName.Text +  '; 
 使用（ var  command =  new  OleDbCommand（myquery，con））
 {
 MyTextBox.Text = command.ExecuteScalar（）。ToString（）; 
} 
} 

我的尝试：



我试过基于通过在线和msdn编写的代码。但仍然没有工作

解决方案

可能最简单的方法是同时使用 DisplayMember 和 ValueMember ComboBox的属性例如

  private   void  FormAttendanceFaculty_Load（ object  sender，EventArgs e）
 {
 con.Open（）; 
 OleDbDataAdapter oda1 =  new  OleDbDataAdapter（  select代码，来自主题的名称，其中代码不为空，con）; 
 DataTable dt1 =  new  DataTable（）; 
 oda1.Fill（dt1）; 
 comboBoxCode.DisplayMember =   code; 
 comboBoxCode.ValueMember =   name; 
 comboBoxCode.DataSource = dt1; 
 comboBoxCode.SelectedIndex = -1; 
} 

请注意，我正在从主题表中检索代码和名称，并且我已将

代码更改为' ％' 

检查非空。我在设置显示和价值成员之后设置 DataSource 。



要在ComboBox更改时更改TextBox的内容，可以使用 SelectedIndexChanged 事件，但请注意它将是当你用数据加载ComboBox时调用，所以你需要处理它，例如

  private   void  comboBoxCode_SelectedIndexChanged（对象发​​件人，EventArgs e）
 {
 如果（comboBoxCode.SelectedValue！=  null ）
 MyTextBox.Text = comboBoxCode.SelectedValue.ToString（）; 
  else  
 MyTextBox.Text =  ; 
} 

更好的选择是使用 SelectionChangeCommitted 事件（reference [ ^ ]）as仅当用户选择ComboBox中的值时才会引发，例如

  private   void  comboBoxCode_SelectionChangeCommitted（ object  sender，EventArgs e）
 {
 MyTextBox.Text = comboBoxCode.SelectedValue.ToString（ ）;这个事件中的
} 

 comboBoxName_SelectedIndexChangedlike  this  
< pre lang =   C＃> private  void  comboBoxName_SelectedIndexChanged（ object  sender，EventArgs e）
 {
  string  myquery =& quot; SELECT name FROM subject 其中 name =&＃39;& quot; + comboBoxName.Text +& quot;&＃39;& quot ;; 
 
} < /   pre  >  

I have basically a combobox and and a text box. What should i do is the combobox consist of subject code whereas the textbox consist of subject name which is stored in database. Can i know is there any ways that when an item from combobox is chosen, it is automatically display subject name based on subject code chosen which will be from the database. belwo is my code but not working

private void FormAttendanceFaculty_Load(object sender, EventArgs e)
            con.Open();
            OleDbDataAdapter oda1 = new OleDbDataAdapter("select code from subject where code like '%'", con);
            DataTable dt1 = new DataTable();
            oda1.Fill(dt1);
            comboBoxCode.DataSource = dt1;
            comboBoxCode.DisplayMember = "code";
            comboBoxCode.SelectedIndex = -1;
string myquery = "SELECT name FROM subject where name = '"  +comboBoxName.Text + "'";
using (var command = new OleDbCommand(myquery, con))
{
  MyTextBox.Text = command.ExecuteScalar().ToString();
}
}

What I have tried:

I tried based on the code whic ilearn via online and msdn written. But still not working

解决方案

Probably the simplest way is to utilise both the DisplayMember and the ValueMember properties of the ComboBox E.g.

private void FormAttendanceFaculty_Load(object sender, EventArgs e)
{
    con.Open();
    OleDbDataAdapter oda1 = new OleDbDataAdapter("select code, name from subject where code is not null", con);
    DataTable dt1 = new DataTable();
    oda1.Fill(dt1);
    comboBoxCode.DisplayMember = "code";
    comboBoxCode.ValueMember = "name";
    comboBoxCode.DataSource = dt1;
    comboBoxCode.SelectedIndex = -1;
}

Note that I'm retrieving both code AND name from the subject table and I've changed your

code like '%'

to check for not null. Also I'm setting the DataSource after I've set up the Display and Value members.

To change the contents of the TextBox when the ComboBox changes you could use the SelectedIndexChanged event but note that it is going to be called when you are loading up the ComboBox with data, so you need to handle that e.g.

private void comboBoxCode_SelectedIndexChanged(object sender, EventArgs e)
{
    if (comboBoxCode.SelectedValue != null)
        MyTextBox.Text = comboBoxCode.SelectedValue.ToString();
    else
        MyTextBox.Text = "";
}

A better option is to use the SelectionChangeCommitted event (reference[^]) as that is only raised when the User selects the value in the ComboBox e.g.

private void comboBoxCode_SelectionChangeCommitted(object sender, EventArgs e)
{
    MyTextBox.Text = comboBoxCode.SelectedValue.ToString();
}

in this event

comboBoxName_SelectedIndexChangedlike this 
<pre lang="C#">private void comboBoxName_SelectedIndexChanged(object sender, EventArgs e)
       {
           string myquery = &quot;SELECT name FROM subject where name = &#39;&quot; + comboBoxName.Text + &quot;&#39;&quot;;

       }</pre>

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