ComboBox 显示重复值

Question

我想添加 ComboBox 的项目。我已成功添加项目，但 ComboBox 有 重复项目 。为什么？

这是我的代码：

Public Sub load_consigneecode()
    Dim con As OleDbConnection = New OleDbConnection(constring)
    con.Open()
    Dim sql As String = "select con_code from TblConsignee order by con_code"
    Dim cmd As New OleDbCommand(sql, con)
    Dim myreader As OleDbDataReader = cmd.ExecuteReader()

    While myreader.Read()
        cmbCode.Items.Add(myreader(0).ToString)
    End While

    myreader.Close()
    con.Close()
End Sub

这是结果：

在我的table:

Answer 1

我要检查的第一件事是确保您没有添加两次

我建议的第二件事是：

cmbCode.Items.Clear()

在添加项目之前的函数开头

Answer 2

Public Sub load_consigneecode()
    Dim con As OleDbConnection = New OleDbConnection(constring)
    con.Open()
    Dim sql As String = "select con_code from TblConsignee order by con_code"
    Dim cmd As New OleDbCommand(sql, con)
    Dim myreader As OleDbDataReader = cmd.ExecuteReader()

    'add this in code
    cmbCode.Items.Clear();
    While myreader.Read()
        cmbCode.Items.Add(myreader(0).ToString)
    End While

    myreader.Close()
    con.Close()
End Sub

否则在您的 SQL 查询中使用 distinct

Answer 3

您可能没有在添加新项目之前清除 ComboBox 中的项目：

cmbCode.Items.Clear()

也就是说，您最好使用 DataTable 绑定到 ComboBox 的 .DataSource 并设置 .DisplayMember 和 .ValueMember 属性。

我也会考虑实施 Using:

Sometimes your code requires an unmanaged resource, such as a file handle, a COM wrapper, or a SQL connection. A Using block guarantees the disposal of one or more such resources when your code is finished with them. This makes them available for other code to use.

经过更改，您的代码将如下所示：

Dim dt As New DataTable
Using con As OleDbConnection New OleDbConnection(constring),
      cmd As New OleDbCommand("SELECT [con_code] FROM [TblConsignee] ORDER BY [con_code]", con)
    con.Open()

    dt.Load(cmd.ExecuteReader())
End Using

cmbCode.DataSource = dt
cmbCode.DisplayMember = "con_code"
cmbCode.ValueMember = "con_code"

ComboBox 显示重复值

ComboBox displays duplicate values

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combobox

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