编写vb.net代码，使得在form7中textbox输入resid，boxid和time后，先在sql server的reservations表里查询有无记录与输入的boxid，time两者同时重复且resid不同的，若有则显示“该时段包厢已有人预约！”，若无则修改信息成功

以下是示例代码，实现了上述功能：

Dim connStr As String = "Data Source=yourServerName;Initial Catalog=yourDatabaseName;Integrated Security=True;"
Dim conn As SqlConnection = New SqlConnection(connStr)

Try
    conn.Open()

    Dim sql As String = "SELECT COUNT(*) FROM reservations WHERE boxid=@boxid AND time=@time AND resid<>@resid"
    Dim cmd As SqlCommand = New SqlCommand(sql, conn)
    cmd.Parameters.AddWithValue("@boxid", TextBox1.Text)
    cmd.Parameters.AddWithValue("@time", TextBox2.Text)
    cmd.Parameters.AddWithValue("@resid", TextBox3.Text)
    Dim result As Integer = Convert.ToInt32(cmd.ExecuteScalar())

    If result > 0 Then
        MessageBox.Show("该时段包厢已有人预约！")
    Else
        sql = "UPDATE reservations SET boxid=@boxid, time=@time WHERE resid=@resid"
        cmd = New SqlCommand(sql, conn)
        cmd.Parameters.AddWithValue("@boxid", TextBox1.Text)
        cmd.Parameters.AddWithValue("@time", TextBox2.Text)
        cmd.Parameters.AddWithValue("@resid", TextBox3.Text)
        cmd.ExecuteNonQuery()
        MessageBox.Show("修改信息成功！")
    End If

Catch ex As Exception
    MessageBox.Show("发生错误：" & ex.Message)

Finally
    conn.Close()
End Try

注意替换示例代码中的连接字符串和SQL语句，以适应你自己的数据库表结构。同时，还需要在窗体7的“确定”按钮的Click事件中添加以上代码。