Loop through all controls of a Form, even those in GroupBoxes

The Controls collection of Forms and container controls contains only the immediate children. In order to get all the controls, you need to traverse the controls tree and to apply this operation recursively

private void AddTextChangedHandler(Control parent)
{
    foreach (Control c in parent.Controls)
    {
        if (c.GetType() == typeof(TextBox)) {
            c.TextChanged += new EventHandler(C_TextChanged);
        } else {
            AddTextChangedHandler(c);
        }
    }
}

Note: The form derives (indirectly) from Control as well and all controls have a Controls collection. So you can call the method like this in your form:

AddTextChangedHandler(this);

A more general solution would be to create an extension method that applies an action recursively to all controls. In a static class (e.g. WinFormsExtensions) add this method:

public static void ForAllControls(this Control parent, Action<Control> action)
{
    foreach (Control c in parent.Controls) {
        action(c);
        ForAllControls(c, action);
    }
}

The static classes namespace must be “visible”, i.e., add an appropriate using declaration if it is in another namespace.

Then you can call it like this, where this is the form; you can also replace this by a form or control variable whose nested controls have to be affected:

this.ForAllControls(c =>
{
    if (c.GetType() == typeof(TextBox)) {
        c.TextChanged += C_TextChanged;
    }
});