In this article I will explain with an example, how to use Custom ASP.Net Server control DropDownCheckBoxes which is a Multiple Selection DropDownList consisting of a CheckBoxList control within it.
This article makes use of DropDownCheckBoxes library for implementing ASP.Net DropDownList CheckBoxList Server control.
 
 

Installing DropDownCheckBoxes library from Nuget

You will need to install the DropDownCheckBoxes library from Nuget Package Manager Console using the following command.
Install-Package DropDownCheckBoxes –Version 1.1.0.31568
 
DropDownCheckBoxes: ASP.Net DropDownList CheckBoxList Server control
 
DropDownCheckBoxes: ASP.Net DropDownList CheckBoxList Server control
 
Note: For more details on how to use Nuget Package Manager Console, please refer the article Install Nuget package in Visual Studio 2012, 2013, 2015.
 
 

Registering the DropDownCheckBoxes DLL

After successful installation, you will need to register the control on the page using the Register directive as shown below.
<%@ Register TagPrefix="asp" Namespace="Saplin.Controls" Assembly="DropDownCheckBoxes" %>
 
 

HTML Markup

The following HTML Markup consists of:
DropDownCheckBoxes – For display an additional SelectAll CheckBox when set to true.
The DropDownCheckBoxes will be validated using ExtendedRequiredFieldValidator.
 
ExtendedRequiredFieldValidator has been specified with following properties:
ControlToValidate - Specifies the ID of the control to be validated.
Error Message - It has been specified with a string value, which will be displayed to the user when the validation fails.
Button – For submit the value.
The Button has been assigned with an OnClick event handler.
<asp:DropDownCheckBoxes ID="DropDownCheckBoxes1" runat="server" Width="180px" UseSelectAllNode="false">
    <Style SelectBoxWidth="195" DropDownBoxBoxWidth="160" DropDownBoxBoxHeight="90" />
    <Items>
        <asp:ListItem Text="Mango" Value="1"></asp:ListItem>
        <asp:ListItem Text="Apple" Value="2"></asp:ListItem>
        <asp:ListItem Text="Banana" Value="3"></asp:ListItem>
    </Items>
</asp:DropDownCheckBoxes>
&nbsp;
<asp:ExtendedRequiredFieldValidator ID="ExtendedRequiredFieldValidator1" runat="server"
ControlToValidate="DropDownCheckBoxes1" ErrorMessage="Required" ForeColor="Red"></asp:ExtendedRequiredFieldValidator>
<br />
<br />
<asp:Button Text="Submit" runat="server" OnClick="Submit" />
 
 

Fetching the selected values on Server Side on Button click

When the Submit Button is clicked, a loop is executed over the Items of the DropDownCheckBoxes control and the selected Items are displayed using JavaScript Alert message box.
C#
protected void Submit(object sender, EventArgs e)
{
    string message = string.Empty;
    foreach (ListItem item in DropDownCheckBoxes1.Items)
    {
        if (item.Selected)
        {
            message += item.Text + " " + item.Value + "\\n";
        }
    }
    ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "alert('" + message + "');", true);
}
 
VB.Net
Protected Sub Submit(sender As Object, e As EventArgs)
    Dim message As String = String.Empty
    For Each item As ListItem In DropDownCheckBoxes1.Items
        If item.Selected Then
            message += item.Text + " " + item.Value + "\n"
        End If
    Next
    ClientScript.RegisterClientScriptBlock(Me.GetType(), "alert", "alert('" & message & "');", True)
End Sub
 
 

Screenshot

DropDownCheckBoxes: ASP.Net DropDownList CheckBoxList Server control
 
 

Demo

View Demo
 
 

Downloads

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