Setting focus in a user control

Here is how to set focus in a user control. I used it in an update panel.

ScriptManager.GetCurrent(this.Page).SetFocus(txtTarget);

Web User Control Library error

When you create a User Control from a class and try to drag and drop the control into the design page, you may get an Invalid FORMATETC structure (Exception from HRESULT: 0x80040064 (DV_E_FORMATETC)) error. Talk about confusing. My solution was to make my (user control) class public.

FindContol Solution to add Javascript to a control from a user control

Here is a great solution to finding a control using FindControl method from a user control. TargetcontrolID is the public property for TextBoxFooExtender (User Control). _txt is the targeted TextBox assuming it is found.

    protected override void CreateChildControls()
    {
        txt = (TextBox)this.Parent.FindControl(TargetControlID);
        if (txt != null) {
            _txt.Attributes.Add("ondblclick", "alert('foo')");
        }
        base.CreateChildControls();
    }

Understanding Default Values for User Web Controls

Understanding default values for User Web Controls can be confusing.
First, there is the DefaultValue for the code behind, which does not act as a default value, but as a default property value for the properties window. Here is the Default Property Value for the properties window set in the ascx.cs page:

    [Browsable(true)]
    [DefaultValue("")]
    public string Text
    {
        get { return Label.Text; }
        set {  Label.Text = value;
        }
    }

Now, what you really want is the default value for the control to be set in the ascx page as follows:

<asp:Label ID="Label" runat="server" Text=""></asp:Label>

You can then manipulate the value in the DataBind, CreateChildControls, or Property section. Be aware that the property section doesn't necessarily fire, hence the need to use the ascx properties.

Creating a ListItemCollection Template User Control

Creating a ListItemCollection Template User Control. Here are the 3 files necessary. A link to the reference link is below.

First: The user control code-behind:

[DefaultEvent("SelectedIndexChanged"),
Designer("System.Web.UI.Design.WebControls.ListControlDesigner, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"),
ParseChildren(true, "Items"), ControlValueProperty("SelectedValue"),
DataBindingHandler("System.Web.UI.Design.WebControls.ListControlDataBindingHandler, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
public partial class ComboBoxControl : System.Web.UI.UserControl
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    private ListItemCollection m_Items;

  [DefaultValue((string)null), MergableProperty(false), 
  PersistenceMode(PersistenceMode.InnerDefaultProperty), 
  Editor("System.Web.UI.Design.WebControls.ListItemsCollectionEditor,System.Design, Version=2.0.0.0,Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a",
  typeof(UITypeEditor))]     
    public virtual ListItemCollection Items
    {
        get
        {
            if (this.m_Items == null)
            {
                this.m_Items = new ListItemCollection();
            }
            return this.m_Items;
        }
    }
    protected override void CreateChildControls()
    {
        if (this.m_Items == null)
        {
            this.m_Items = new ListItemCollection();
        }
        
        if (m_Items.Count>0) {
            foreach (ListItem item in m_Items)
            {
                _ComboBoxControl.Items.Add(item);
            }
        }
    }
    public override void DataBind()
    {
        CreateChildControls();
        ChildControlsCreated = true;
        _ComboBoxControl.DataBind();
        base.DataBind();
    }
}

Next, the User control

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="ComboBoxControl.ascx.cs" Inherits="ComboBoxControl" %>
<asp:DropDownList ID="_ComboBoxControl" runat="server">
</asp:DropDownList>

And finally the aspx:

        <uc1:ComboBoxControl ID="ComboBoxControl1" runat="server">
        <Items>
            <asp:ListItem Value="Value1" Text="Text1" />
            <asp:ListItem Value="Value2" Text="Text2" />
            <asp:ListItem Value="Value3" Text="Text3" />
            <asp:ListItem Value="Value4" Text="Text4" />
        </Items>
        </uc1:ComboBoxControl>

Credit to: http://developers.de/blogs/damir_dobric/archive/2007/03/30/Implementing-Custom-UserControl-with-ListItem-collection.aspx

Web Control IDs INamingContainer to avoid duplicate IDs

The ID property of the WebUserControl for controls that may be used mulitiple times in an aspx page should not be repeated. Simple implment the INamingContainer to prevent duplicate IDs as follows:

public partial class UCRepeater : System.Web.UI.UserControl, INamingContainer

Marker Interface - An interface that does not require explicit implementation.

Web User Control shortcut

I just discovered a Web User Control shortcut for using your controls in apsx pages.
As per the reference link's example, add your implementation of this example:

        <add tagPrefix="scottgu" src="~/Controls/Footer.ascx" tagName="footer"/>
        <add tagPrefix="ControlVendor" assembly="ControlVendorAssembly"/>

All in the Web.config file in the elements.

Then you can reference it in aspx as

Reference: Link

Creating a Web User Control

Creating a Custom Web Control.
Within Visual Studio:
To keep organized, I create a company/website/user folder. Within the folder I create another folder named Controls. For example I'll have this directory structure: FooInc/Controls.

Within that folder, rt. click and Add New Item. Select the WebUserControl option. I'll name this one WebUserControl_DatePanel.ascx.

Switch to design view. Add a Panel (named Panel1 by default). Add a Label (named Label1 by default) within Panel1. Set Panel1's background to cyan for a custom feel.

In the load Event of WebUserControl_DatePanel.ascx, add the following:
Label1.Text = DateTime.Now.ToString();

Now save and close those files.

In your project, create a new Web Form named WebUser.aspx. Switch to design view.

From the Solution Explorer, drag the WebUserControl_DatePanel.ascx onto the page (much like adding a CSS file). Switch to Source View.

You will notice the following header:

<%@ Register Src="FooInc/Controls/WebUserControl_DatePanel.ascx" TagName="WebUserControl_DatePanel"
    TagPrefix="uc1" %>

You will also notice the following web user control in the body:

        <uc1:WebUserControl_DatePanel ID="WebUserControl_DatePanel1" runat="server" />

There it is. You can add custom functions at the web user control source if you want additional functionality.