C#, pls help to change my foreach loop to for loop

Something along the line of:

for(int i = 0; i < controls.Controls.Count; i++){
   if (controls.Controls[i] is TextBox)
             {
                 ((TextBox)controls.Controls[i]).Clear();
             }
             if (controls.Controls[i].HasChildren)
             {
                 ClearAllTextBoxes(controls.Controls[i]);
             }
}

This is documented and is basic .Net knowledge.

This version uses the most modern form of C#.

public void ClearAllTextBoxes(Control controls)
{
    for (int n = 0; n < controls.Controls.Count; n++)
    {
         var control = control.Controls[n];
         if (control is TextBox textBox)
         {
             textBox.Clear();
         }
         if (control.HasChildren)
         {
             ClearAllTextBoxes(control);
         }
    }
}

I prefer to handle such recursive manipulations with extension methods.

Use examples:

this.ApplyToControls<TextBox>(tbx => tbx.Clear());

var tbxs = this.GetAllControlsOfType<TextBox>();

tbxs.ApplyToControlCollection<TextBox>(tbx => tbx.Clear());

tbxs.ApplyToControlCollection<TextBox>(tbx => tbx.Text = "default");

Code:

public static class ControlExtensions
{
    public static IEnumerable<T> GetAllControlsOfType<T>(this Control cntrl)
        where T : Control
    {
        var stack = new Stack<Control>(cntrl.Controls.Cast<Control>());

        while (stack.Any())
        {
            Control nextControl = stack.Pop();

            if (nextControl.HasChildren)
            {
                foreach (Control childControl in nextControl.Controls)
                {
                    stack.Push(childControl);
                }
            }

            if(nextControl is T) yield return nextControl as T;
        }
    }

    public static void ApplyToControlCollection<T>(this IEnumerable<T> instances, Action<T> action)
    {
        foreach (T instance in instances)
        {
            action(instance);
        }
    }

    public static void ApplyToControls<T>(this Control cntrl, Action<T> action)
    {
        foreach (T cinstance in cntrl.Controls.OfType<T>())
        {
            action(cinstance);
        }

        if (cntrl.HasChildren)
        {
            foreach (Control childcntrl in cntrl.Controls)
            {
                ApplyToControls<T>(childcntrl, action);
            }       
        }
    }
}

for(int i = 0; i < controls.Controls.Count; i++)
{
var control = control.Controls[i]
if (control is TextBox)
{
((TextBox)control).Clear();
}
if (control.HasChildren)
{
ClearAllTextBoxes(control);
}
}

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