Anicho Anicho - 4 months ago 25
ASP.NET (C#) Question

Get All Web Controls of a Specific Type on a Page

I have been pondering how I can get all controls on a page and then perform a task on them in this related question:

How to Search Through a C# DropDownList Programmatically

I need code that can scan the page, get all DropDownList Controls and return them in a list.

I'm currently having to edit each individual control, I would rather be able to dynamically loop over each control to preform my task.

Answer

Check my previous SO answer.

Basically, the idea is to wrap the recursion of iterating through the controls collection using :

private void GetControlList<T>(ControlCollection controlCollection, List<T> resultCollection)
where T : Control
{
    foreach (Control control in controlCollection)
    {
        //if (control.GetType() == typeof(T))
        if (control is T) // This is cleaner
            resultCollection.Add((T)control);

        if (control.HasControls())
            GetControlList(control.Controls, resultCollection);
    }
}

and to use it :

List<DropDownList> allControls = new List<DropDownList>();
GetControlList<DropDownList>(Page.Controls, allControls )
foreach (var childControl in allControls )
{
//     call for all controls of the page
}

[Edited 11/26/2013]: here is a more elegant way to reach this goal. I wrote two extensions methods that can walk the control tree in both directions. The methods are written in a more Linq way as it produces an enumerable:

/// <summary>
/// Provide utilities methods related to <see cref="Control"/> objects
/// </summary>
public static class ControlUtilities
{
    /// <summary>
    /// Find the first ancestor of the selected control in the control tree
    /// </summary>
    /// <typeparam name="TControl">Type of the ancestor to look for</typeparam>
    /// <param name="control">The control to look for its ancestors</param>
    /// <returns>The first ancestor of the specified type, or null if no ancestor is found.</returns>
    public static TControl FindAncestor<TControl>(this Control control) where TControl : Control
    {
        if (control == null) throw new ArgumentNullException("control");

        Control parent = control;
        do
        {
            parent = parent.Parent;
            var candidate = parent as TControl;
            if (candidate != null)
            {
                return candidate;
            }
        } while (parent != null);
        return null;
    }

    /// <summary>
    /// Finds all descendants of a certain type of the specified control.
    /// </summary>
    /// <typeparam name="TControl">The type of descendant controls to look for.</typeparam>
    /// <param name="parent">The parent control where to look into.</param>
    /// <returns>All corresponding descendants</returns>
    public static IEnumerable<TControl> FindDescendants<TControl>(this Control parent) where TControl : Control
    {
        if (parent == null) throw new ArgumentNullException("control");

        if (parent.HasControls())
        {
            foreach (Control childControl in parent.Controls)
            {
                var candidate = childControl as TControl;
                if (candidate != null) yield return candidate;

                foreach (var nextLevel in FindDescendants<TControl>(childControl))
                {
                    yield return nextLevel;
                }
            }
        }
    }
}

Thanks to the this keyword, these methods are extensions methods and can simplify the code.

For example, to find all DropDownList in the page, you can simply call:

var allDropDowns = this.Page.FindControl<DropDownList>();

Because of the use of the yield keyword, and because Linq is smart enough to defer execution of the enumeration, you can call (for example):

var allDropDowns = this.Page.FindControl<DropDownList>();
var firstDropDownWithCustomClass = allDropDowns.First(
    ddl=>ddl.CssClass == "customclass"
    );

The enumeration will stop as soon as the predicate in the First method is satisfied. The whole control tree won't be walked.