VB.NET中的Search ComboBox Project

介绍

VB.NET是一个面向对象的多语言、多平台的免费开源.NET开发环境，它是VB语言的升级版，拥有更多新的特性和更加强大的功能。Search ComboBox是VB.NET中一个很重要的控件，在一些需要搜索的程序中很常用。

本文主要介绍搜索组合框（Search ComboBox）的使用方法和相关问题解决方案。

Search ComboBox基本语法

Search ComboBox的基本语法如下：

Private Sub ComboBox1_TextChanged(sender As Object, e As EventArgs) Handles ComboBox1.TextChanged
    ComboBox1.Items.Clear()
    If Not ComboBox1.Text.Trim.Equals("") Then
        For i As Integer = 1 To 100
            ComboBox1.Items.Add(ComboBox1.Text & i)
        Next
    End If
End Sub

搜索ComboBox数据库

通过编写相关的代码，我们可以在ComboBox中实现数据库搜索。以下是代码示例：

Dim con As New OleDb.OleDbConnection
Dim cmd As New OleDb.OleDbCommand
Dim dr As OleDb.OleDbDataReader
Dim query As String

con.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\MyDatabase.accdb"
con.Open()

query = "SELECT ItemName FROM ItemsTable WHERE ItemName LIKE '%" & ComboBox1.Text & "%'"
cmd = New OleDb.OleDbCommand(query, con)
dr = cmd.ExecuteReader()

ComboBox1.Items.Clear()
While dr.Read()
    ComboBox1.Items.Add(dr("ItemName"))
End While

If ComboBox1.Items.Count > 0 Then
    ComboBox1.DroppedDown = True
    ComboBox1.SelectionStart = ComboBox1.Text.Length
    ComboBox1.SelectionLength = ComboBox1.Items(0).ToString.Length
End If

dr.Close()
con.Close()

关键问题

如何防止ComboBox死循环？

一些开发者在使用ComboBox时，在算法和搜索时会遇到ComboBox死循环的问题。此时，我们应该禁用ComboBox的TextChanged事件，并在DropDown和AutoCompleteCombo事件中开启ComboBox。

Private Sub ComboBox1_TextChanged(sender As Object, e As EventArgs) Handles ComboBox1.TextChanged
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = False
    updating = False
End Sub

Private Sub ComboBox1_DropDown(sender As Object, e As EventArgs) Handles ComboBox1.DropDown
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = True
    updating = False
End Sub

Private Sub ComboBox1_MouseClick(sender As Object, e As MouseEventArgs) Handles ComboBox1.MouseClick
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = True
    updating = False
End Sub

Private Sub ComboBox1_KeyUp(sender As Object, e As KeyEventArgs) Handles ComboBox1.KeyUp
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = True
    updating = False
End Sub

Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = False
    updating = False
End Sub

Private Sub ComboBox1_LostFocus(sender As Object, e As EventArgs) Handles ComboBox1.LostFocus
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = False
    updating = False
End Sub

Private Sub ComboBox1_GotFocus(sender As Object, e As EventArgs) Handles ComboBox1.GotFocus
    If updating Then
        Return
    End If
    updating = True
    ComboBox1.DroppedDown = True
    updating = False
End Sub

如何在ComboBox中显示匹配项？

当我们在ComboBox中输入一个字符后，我们需要在下拉列表中显示所有匹配项。以下是实现此一需求的方法：

Private Sub ComboBox1_TextChanged(sender As Object, e As EventArgs) Handles ComboBox1.TextChanged
    Dim matches As String() = Array.FindAll(_comboBoxItems, Function(s) s.ToLower().StartsWith(ComboBox1.Text.ToLower()))
    ComboBox1.Items.Clear()
    ComboBox1.Items.AddRange(matches)
    ComboBox1.SelectionStart = ComboBox1.Text.Length
    ComboBox1.DroppedDown = True
End Sub

如何在ComboBox中实现模糊查询？

在某些情况下，我们需要实现模糊查询。以下代码演示了如何实现模糊查询：

Private Sub ComboBox1_TextChanged(sender As Object, e As EventArgs) Handles ComboBox1.TextChanged
    ComboBox1.Items.Clear()

    If ComboBox1.Text.Length = 0 Then Exit Sub

    Dim sqlSelect As String = "SELECT * FROM Customers WHERE CustomerName like '%{0}%'"
    sqlSelect = String.Format(sqlSelect, ComboBox1.Text.Trim())

    Dim connection As New SqlConnection(My.Settings.ConnectionString)
    connection.Open()

    Dim command As New SqlCommand(sqlSelect, connection)
    Dim dataReader As SqlDataReader = command.ExecuteReader()

    While dataReader.Read()
        ComboBox1.Items.Add(dataReader("CustomerName").ToString())
    End While

    dataReader.Close()
    connection.Close()
    ComboBox1.DroppedDown = True
    ComboBox1.SelectionStart = ComboBox1.Text.Length
End Sub

总结

本文介绍了VB.NET中Search ComboBox的使用方法和常见问题的解决方案。通过本文的学习，您可以更好地实现ComboBox的各类功能并提高开发效率。