关于ado.net [英] about ado.net

问题描述

如何检查用户是否存在于数据库中

how to check whether the user exists in the database

推荐答案

为选择用户编写查询，如果返回空则数据库中没有用户

write query for select user, if it return empty there is no user else have user in database

公共表1_string作为字符串= String.Empty
将公共Table1_command作为OleDb.OleDbCommand
将公共Table1_dataadapter作为OleDb.OleDbDataAdapter
将公共Table1_commandbuilder作为OleDb.OleDbCommandBuilder
公用Table1_table作为新数据表
将公共Table1_bindingsource作为新的BindingSource
以OleDb.OleDbDataReader的形式公开Table1_data_reader
将公共db18_connection作为新的OleDb.OleDbConnection("Provider = Microsoft.Jet.OLEDB.4.0; Data Source = C:\ 1 \ db18.mdb; Persist Security Info = True")

私有子Button1_Click(ByVal发送者为System.Object，ByVal e为System.EventArgs)处理Button1.Click
昏暗的UserName_variable为String = String.Empty

Table1_string ="SELECT * FROM Table1 WHERE UserName = @UserName"
Table1_command =新的OleDb.OleDbCommand(Table1_string，db18_connection)
使用Table1_command
Table1_command.Parameters.Add(New OleDb.OleDbParameter("@ UserName"，OleDb.OleDbType.Char，50)).Value = Me.UserNameTextBox.Text
结尾为
如果Table1_command.Connection.State = ConnectionState.Closed然后
Table1_command.Connection.Open()
如果结束
Table1_data_reader = Table1_command.ExecuteReader()
如果是Table1_data_reader.Read然后
如果不是TypeOf Table1_data_reader("UserName")是DBNull，则
UserName_variable = CStr(Table1_data_reader("UserName"))
如果结束

Form3.Show()
其他
MsgBox(密码错误")
如果结束
Table1_data_reader.Close()
Table1_command.Connection.Close()
结束Sub< big></big>

Public Table1_string As String = String.Empty
Public Table1_command As OleDb.OleDbCommand
Public Table1_dataadapter As OleDb.OleDbDataAdapter
Public Table1_commandbuilder As OleDb.OleDbCommandBuilder
Public Table1_table As New DataTable
Public Table1_bindingsource As New BindingSource
Public Table1_data_reader As OleDb.OleDbDataReader
Public db18_connection As New OleDb.OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\1\db18.mdb;Persist Security Info=True")

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim UserName_variable As String = String.Empty

Table1_string = "SELECT * FROM Table1 WHERE UserName = @UserName"
Table1_command = New OleDb.OleDbCommand(Table1_string, db18_connection)
With Table1_command
Table1_command.Parameters.Add(New OleDb.OleDbParameter("@UserName", OleDb.OleDbType.Char, 50)).Value = Me.UserNameTextBox.Text
End With
If Table1_command.Connection.State = ConnectionState.Closed Then
Table1_command.Connection.Open()
End If
Table1_data_reader = Table1_command.ExecuteReader()
If Table1_data_reader.Read Then
If Not TypeOf Table1_data_reader("UserName") Is DBNull Then
UserName_variable = CStr(Table1_data_reader("UserName"))
End If

Form3.Show()
Else
MsgBox("Incorrect Password")
End If
Table1_data_reader.Close()
Table1_command.Connection.Close()
End Sub<big></big>

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