I have a Form named A.
A contains lots of different controls, including a main GroupBox. This GroupBox contains lots of tables and others GroupBoxes. I want to find a control which has e.g. tab index 9 in form A, but I don't know which GroupBox contains this control.
How can I do this?
With recursion...
public static IEnumerable<T> Descendants<T>( this Control control ) where T : class { foreach (Control child in control.Controls) { T childOfT = child as T; if (childOfT != null) { yield return (T)childOfT; } if (child.HasChildren) { foreach (T descendant in Descendants<T>(child)) { yield return descendant; } } } } You can use the above function like:
var checkBox = (from c in myForm.Descendants<CheckBox>() where c.TabIndex == 9 select c).FirstOrDefault(); That will get the first CheckBox anywhere within the form that has a TabIndex of 9. You can obviously use whatever criteria you want.
If you aren't a fan of LINQ query syntax, the above could be re-written as:
var checkBox = myForm.Descendants<CheckBox>() .FirstOrDefault(x=>x.TabIndex==9); 
IsChecked) as well! @Ed Swangren what if you don't want to check on TabIndex anymore? A query (or predicate) is not a bad idea.
Recursively search through your form's Controls collection.
void FindAndSayHi(Control control) { foreach (Control c in control.Controls) { Find(c.Controls); if (c.TabIndex == 9) { MessageBox.Show("Hi"); } } } 
void iterateControls(Control ctrl) { foreach(Control c in ctrl.Controls) { iterateControls(c); } } You can make a method like this:
public static Control GetControl(Control.ControlCollection controlCollection, Predicate<Control> match) { foreach (Control control in controlCollection) { if (match(control)) { return control; } if (control.Controls.Count > 0) { Control result = GetControl(control.Controls, match); if (result != null) { return result; } } } return null; } ...that is used like this:
Control control = GetControl(this.Controls, ctl => ctl.TabIndex == 9); Note however that TabIndex is a tricky case, since it starts at 0 within each container, so there may be several controls in the same form having the same TabIndex value.
Either way, the method above can be used for checking pretty much any property of the controls:
Control control = GetControl(this.Controls, ctl => ctl.Text == "Some text"); I hate recursion, so I always use a stack for this sort of thing. This assigns a common event handler to the CheckedChanged event of every RadioButton control in the current control hierarchy:
Stack<Control> controlStack = new Stack<Control>(); foreach (Control c in this.Controls) { controlStack.Push(c); } Control ctl; while (controlStack.Count > 0 && (ctl = controlStack.Pop()) != null) { if (ctl is RadioButton) { (ctl as RadioButton).CheckedChanged += new EventHandler(rb_CheckedChanged); } foreach (Control child in ctl.Controls) { controlStack.Push(child); } } You could easily retrofit Josh Einstein's extension method to work this way.
