How to: Retrieve all controls in a form, using generics (II) – Linq at rescue!

Hi again,

In my previous post, I created a recursive function to retrieve all controls inside a form and its containers. Today, my colleague and friend Eduard Tomàs (software architect @ RAONA) has posted another solution to the same topic based on Linq. It’s quite simply and really, really pretty:


A new approach:

Before starting, we need to solve this: Linq works over IEnumerable<T>, but Control.Controls property returns a ControlCollection type. In fact, nowadays, since we have Generics this class has no sense (like other 1.000 similar classes), but remember that Generics doesn’t appeared until .NET Framework 2.0. So, our first step will be retrieve an IEnumerable<Control> from a ControlCollection. With using an extensor method this will be really easy:

public static IEnumerable<Control> AsEnumerable
    (this Control.ControlCollection @this)
    foreach (var control in @this)
        yield return (Control)control;

Note that using this code we are able to transform CollectionControl to IEnumerable<Control>, and get full access to the power of Linq. Now, let’s create a method to retrieve all controls of a type, as follows:

public static IEnumerable<T>
    GetAllControls<T>(this Control @this) where T : Control
    return @this.Controls.AsEnumerable().Where(x => x.GetType() == typeof(T)).
        SelectMany(x => GetAllControls<T>(x)).

It looks cool, isn’t it? No loops, no ifs… only pure Linq power! :-)

There’s a small difference with my original solution and this new one. In my original solution I used an internal List<T> to copy all controls references. The new one only iterates over the original collection. There’s no internal lists. This is the power of Linq.

Another difference present in Linq solution is: We are returning an IEnumerable, so, we loose the ForEach method (because IEnumerable doesn’t implements this method). But building our own ForEach method is trivial:

public static void ForEach<T>
    (this IEnumerable<T> @this, Action<T> action)
    foreach (T t in @this)

HYEI, happy coding!

November 2010

How to: Retrieve all controls in a form, using generics (I)

Note: This is a very common question in MSDN forums. For this reason I decided to write this post, and use it for reference in future questions.


The typical question: How to clear the content of all the textboxes in a form?

Answer: Using generics it’s really easy… First of all, let’s create an extensor method, that will returns a collection of all the controls of a type in a Form (or container). And after, we will use this collection to do an action over each returned item.

The extensor method:

public static List<T> GetControls<T>(this Control container) where T : Control
    List<T> controls = new List<T>();
    foreach (Control c in container.Controls)
        if (c is T)
    return controls;

This method retrieves the collection of Controls of a control and then, it call itself recursively, retrieving the content of all his containers.

How to use it:

this.GetControls<TextBox>().ForEach(p => p.Text = string.Empty);

In this code, we will use the extensor method directly in a form (because Form class inherits from ContainerControl, that inherits from ScrollableControl, and ScrollableControl from Control). Or, in other words, our Form will implement our extensor method. So, at compile time, we should specify the type of the controls we want to retrieve (in the example we use TextBox, but you can use Button type instead), and then, use ForEach to apply an action on each one of the items returned.

Moreover, if we want to retrieve only the controls into a container (GroupBox, Panel, TabControl or another), we should call the method for this particular control.

this.GroupBox1.GetControls<TextBox>().ForEach(p => p.Text = "hello");
this.Panel1.GetControls<TextBox>().ForEach(p => p.Text = string.Empty);

Applying several actions on each control:

In most cases, we would apply several actions to each control (not only one). In this case, it’s quite easy: The only thing we have to do is create a method that receives a parameter of this type, and call it passing the control as an argument:

this.GetControls<TextBox>().ForEach(p => ApplyFormat(p));

private void ApplyFormat(TextBox text)
    text.BackColor = Color.LightGray;
    text.ForeColor = Color.Red;
    text.Text = "hello";
    text.TextAlign = HorizontalAlignment.Center;

Or maybe, using an ‘action delegate’, both options are correct:

this.GetControls<TextBox>().ForEach(p =>
    p.BackColor = Color.LightGray;
    p.ForeColor = Color.Red;
    p.Text = "hello";
    p.TextAlign = HorizontalAlignment.Center;

Happy coding! :-)

November 2010