ComboBox的事件有疑问。因为当我使用这个时：
Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
Dim connstring As String = "server = localhost; user id = root; database = db; password = root"
Dim Sql As String = "SELECT * FROM candidate WHERE cid= '" & ComboBox1.SelectedItem & "'"
Dim conn As New MySqlConnection(connstring)
Dim cmd As New MySqlCommand(Sql, conn)
Dim dr As MySqlDataReader = Nothing
conn.Open()
dr = cmd.ExecuteReader()
dr.Read()
If IsDBNull(dr("photo")) = False Then
Dim imagebytes As Byte() = CType(dr("photo"), Byte())
Using ms As New IO.MemoryStream(imagebytes)
PictureBox1.Image = Image.FromStream(ms)
PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage
End Using
End If
conn.Close()
End Sub
发生此错误：Operator '&' is not defined for string "SELECT * FROM candidate WHERE ci" and type 'DataRowView'.
但如果我使用这个：
Private Sub ComboBox1_SelectionChangeCommitted(ByVal sender As Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectionChangeCommitted
Dim connstring As String = "server = localhost; user id = root; database = db; password = root"
Dim Sql As String = "SELECT * FROM candidate WHERE cid= '" & ComboBox1.SelectedIndex & "'"
Dim conn As New MySqlConnection(connstring)
Dim cmd As New MySqlCommand(Sql, conn)
Dim dr As MySqlDataReader = Nothing
conn.Open()
dr = cmd.ExecuteReader()
dr.Read()
If IsDBNull(dr("photo")) = False Then
Dim imagebytes As Byte() = CType(dr("photo"), Byte())
Using ms As New IO.MemoryStream(imagebytes)
PictureBox1.Image = Image.FromStream(ms)
PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage
End Using
End If
conn.Close()
End Sub
首次选择正在运行，但是当我在ComboBox中选择另一个项目时，表单挂起（无响应）。我的代码或笔记本电脑有什么问题吗？
这就是我填充ComboBox es的方式：
Private Sub Form4_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
HookKeyboard()
Me.TopMost = True
con.ConnectionString = ("server=localhost;user id=root;database=db;password=root")
Try
con.Open()
With cmd
.Connection = con
.CommandText = "SELECT case when SCQ='0' or cfname='Select a Candidate' then cfname else CONCAT_WS(' ', idn, cfname, cmname, clname,'from', cparty,'party') end names, " & _
"cpos, cid from candidate WHERE cpos='President' AND candidacy='Filed' OR " & _
"cpos='Vice President' AND candidacy='Filed' OR " & _
"cpos='Secretary' OR " & _
"cpos='Treasurer' OR " & _
"cpos='Auditor' OR " & _
"cpos='Public Information Officer' OR " & _
"cpos='Peace Officer' OR " & _
"cpos='Representative (Grade 7)' order by SCQ desc "
End With
Dim dt As New DataTable
da.SelectCommand = cmd
da.Fill(dt)
With ComboBox1
Dim dv = New DataView(dt, "cpos='President'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv
End With
With ComboBox2
Dim dv1 = New DataView(dt, "cpos='Vice President'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv1
End With
With ComboBox3
Dim dv2 = New DataView(dt, "cpos='Secretary'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv2
End With
With ComboBox4
Dim dv3 = New DataView(dt, "cpos='Treasurer'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv3
End With
With ComboBox5
Dim dv4 = New DataView(dt, "cpos='Auditor'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv4
End With
With ComboBox6
Dim dv5 = New DataView(dt, "cpos='Public Information Officer'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv5
End With
With ComboBox7
Dim dv6 = New DataView(dt, "cpos='Peace Officer'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv6
End With
With ComboBox8
Dim dv7 = New DataView(dt, "cpos='Representative (Grade 7)'", "", DataViewRowState.CurrentRows)
.DisplayMember = "names"
.ValueMember = "names"
.DataSource = dv7
End With
Catch ex As Exception
MsgBox(ex.Message)
End Try
con.Close()
End Sub
答案 0 :(得分：0)
在大多数情况下，当您不关心这些碎片或为用户创造新的东西时，您会使用CONCAT字段。例如，将FirstName和LastName连接成FullName。这不是真的，你做想得到一些信息。您可以使用过程简化查询，以获取每个位置的数据（即具有7或8个表的数据集 - 每个位置一个）。
Public Class FormX
Private myDS As DataSet ' our data set for the results
Private Form_Load(....
myDS = New DataSet
FillPositionInfo("President")
FillPositionInfo("Vice-President")
...etc
' bind to CBOs:
With ComboBox1
.DisplayMember = "FullName" ' what to show
.ValueMember = "cid" ' value to return to code
.DataSource = myDS.Tables("President")
End With
' add a table to the DS with the candidate info for the requested position
Private Sub FillPositionInfo(position As String)
' maybe CONCAT to create a full name field
Dim sql As String = "SELECT cid, ... FROM Candidate WHERE cpos = @cpos"
' youd use MySql Objects - I only have SQL Server and Access available
Using con As OleDbConnection = GetConnection()
con.Open()
Using cmd As New OleDbCommand(sql, con),
da As New OleDbDataAdapter(sql.Value, con)
cmd.Parameters.AddWithValue("@cpos", position)
da.SelectCommand = cmd
' add a new Tbl named "President" or whatever is requested
myDS.Tables.Add(position)
da.Fill(myDS, position) ' fill the table
' debug - look in the output window
For n As Integer = 0 To myDS.Tables(position).Rows.Count - 1
Console.WriteLine(myDS.Tables(position).Rows(n).Item("cname")
Next
End Using ' dispose cmd and da
con.Close ' return connection to pool
End Using ' dispose con
End Sub
注意：
我不熟悉您的应用和数据库的所有复杂性和细节，因此可能需要修复和修改。这是所涉及的原则，正在进行说明。
由于职位的文本现在至关重要，您可能希望使用常量或名称数组来防止拼写错误。 E.g：
Private posTitles As String() = {"President", "Vice President"...}
Private Enum TitleIndex
President = 0
VP = 1
...
End Enum
...
myDS.Tables.Add(posTitles(TitleIndex.VP)) ' add named tbl using array text
CBO.DataSource = myDS.Tables(TitleIndex.VP) ' reference by position
关键是你要确保对DS表，CBO，SQL等使用相同的确切文本，而不是在这里使用“副总裁”，在其他地方使用“副总统”。
通过不将所有内容粘在一起，您的原始问题可能会被废弃。如果用于创建表的SQL是最小的（“SELECT Fname，LName，CID From ...”），那么您将需要再次点击DB以获得更多信息，如照片：
' assumes cid is the PK and mapped as the ValueMember
Sql = "SELECT * FROM candidate WHERE cid = '" & ComboBox1.SelectedValue & "'"
（如果cid是数字，则您不需要或不需要刻度 - 可能是数据类型转换。）
如果你使用“SELECT *”，那么每个DataTable已经包含了所有信息，无需查看任何内容。所选候选项的字节数组如下：
myDS.Tables(X).Rows(Cbo.SelectedIndex).Item("photo")`
您仍然需要将其转换为实际图片，但无需找到它。有几种方法可以解决这个问题 - 所以你可能想修补它，如果这就是你的方法。
最后，请注意，使用SQL参数是一种很多更好的做法，如FillPositionInfo所示。字符串格式中的错误，数据类型不匹配以及对SQL注入攻击的保护要少得多。