Skip to main content

Delegates in VB.NET

Delegates in .NET Framework are equivalent to Function pointers in C and C++ language. Delegates helps in multithreaded application where one thread is accessing a UI element created by another thread.

For e.g. the follwoing code will always return an error of type "Cross-thread operation not valid: Control 'TextBox1' accessed from a thread other than the thread it was created on."

Imports System.Threading
Imports System.Reflection

Public Class Form1
    Private trd As Thread
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        trd = New Thread(AddressOf DoTask)
        trd.IsBackground = True
        trd.Start()

    End Sub

    Private Sub DoTask()
        Dim i As Integer
        i = 1
        While i <= 1000
            System.Console.WriteLine("the value of i is:" + i.ToString())
            i = i + 1
            Thread.Sleep(1000)
            TextBox1.Text = i.ToString()
        End While
    End Sub

In the above example the thread is invoking the textbox1 which is running under MainForm Thread. That's why we receive this error. This can be removed with the help of Delegates. The following code shows how to remove the above error and run the code:

Imports System.Threading
Imports System.Reflection

Public Class Form1
    Private trd As Thread

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        trd = New Thread(AddressOf DoTask)
        trd.IsBackground = True
        trd.Start()
    End Sub

    Private Sub DoTask()
        Dim i As Integer
        i = 1
        While i <= 1000
            System.Console.WriteLine("the value of i is:" + i.ToString())
            i = i + 1
            Thread.Sleep(1000)
            SetControlPropertyValue(TextBox1, "Text", i)
       End While
    End Sub

    Private Delegate Sub SetControlValueCallback(ByVal oControl As Control, ByVal propName As String, ByVal propValue As Object)

    Private Sub SetControlPropertyValue(ByVal oControl As Control, ByVal propName As String, ByVal propValue As Object)
        If oControl.InvokeRequired Then
            Dim d As New SetControlValueCallback(AddressOf SetControlPropertyValue)
            oControl.Invoke(d, New Object() {oControl, propName, propValue})
        Else
            Dim t As Type = oControl.[GetType]()
            Dim props As PropertyInfo() = t.GetProperties()
            For Each p As PropertyInfo In props
                If p.Name.ToUpper() = propName.ToUpper() Then
                    p.SetValue(oControl, propValue.ToString(), Nothing)
                End If
            Next
        End If
    End Sub

End Class

Comments

Popular posts from this blog

SQLServer Error: 15404, Could not obtain information about Windows NT group/user Error code 0x5. [SQLSTATE 42000] (ConnIsLoginSysAdmin)

If we encounter this error "SQLServer Error: 15404, Could not obtain information about Windows NT group/user Error code 0x5. [SQLSTATE 42000] (ConnIsLoginSysAdmin)" in SQL Server in which the job fails because of the user account related problem, then we need to take the following steps to make it work: a. Go to SQL Server Agent. b. Then select the job which is giving error and not running successfully. c. Then choose properties and in the General Tab, change the owner to "SA" or any account that has the administrative privileges. The problem will be resolved.

Connecting Nodejs to SQL Server with Knex ORM - Part 1

Normally we connect Nodejs to the databases like SQLite, MongoDB, Postgres etc . In this multiple part post, I am going to show you, how we can connect Nodejs with Microsoft's SQL Server using Knex and run CRUD operations. 1. Installation of required packages In order to connect Nodejs to SQL Server with Knex, we will need following node packages installation: i. npm install knex ii. npm install mssql Once the packages are installed, we can write the following code to connect: //Code to KNEX connection settings. var knex = require('knex')({ client: 'mssql', connection: { user: 'sa', password: 'pwd', server: 'localhost', database: 'Test' } }); //Code to query the Table DEPT knex.select("*").from("dept") .then(function (depts){ depts.forEach((dept)=>{ //use of Arrow Function console.log({...dept}); }); }).catch(function(err) { ...

How to use QueryExtenderControl in ASP.NET with LINQ

Queryextender control is a control that helps in doing actions like filtration, searching, sorting etc. on the LINQ Data source with few steps. In fact we don’t have to write any code with this. Screenshot given below is just an example that can easily be achieved through this control. In the above screenshot, we can see that there is a GridView Control that has been binded to a LINQData Source Control. And we have a DropDownlistBox that is also binded to another LINQData Source Control and displaying only grouped jobs. The GridView display all the employees who are matching the Job Parameter. This is done through QueryExtender Control. Also we can search for any name in the employee name's through Query Extender control. The code in the HTML goes like: < form id ="form1" runat ="server">     < div >         Select Job: < asp : DropDownList ID ="Job" runat ="server" DataSourceID ="LinqDataSource2...