Can I have a method returning IEnumerator<T> and use it in a foreach loop?

Question

I need to set the height of every textbox on my form, some of which are nested within other controls. I thought I could do something like this:

private static IEnumerator<TextBox> FindTextBoxes(Control rootControl)
{
    foreach (Control control in rootControl.Controls)
    {
        if (control.Controls.Count > 0)
        {
            // Recursively search for any TextBoxes within each child control
            foreach (TextBox textBox in FindTextBoxes(control))
            {
                yield return textBox;
            }
        }

        TextBox textBox2 = control as TextBox;
        if (textBox2 != null)
        {
            yield return textBox2;
        }
    }
}

Using it like this:

foreach(TextBox textBox in FindTextBoxes(this))
{
    textBox.Height = height;
}

But of course the compiler spits its dummy, because foreach expects an IEnumerable rather than an IEnumerator.

Is there a way to do this without having to create a separate class with a GetEnumerator() method?

Answer 1

As the compiler is telling you, you need to change your return type to IEnumerable. That is how the yield return syntax works.

Answer 2

Just to clarify

private static IEnumerator<TextBox> FindTextBoxes(Control rootControl)

Changes to

private static IEnumerable<TextBox> FindTextBoxes(Control rootControl)

That should be all :-)

Answer 3

If you return IEnumerator, it will be a different enumerator object each time call that method (acting as though you reset the enumerator on each iteration). If you return IEnumerable then a foreach can enumerate based on the method with the yield statement.

Answer 4

// Generic function that gets all child controls of a certain type, 
// returned in a List collection
private static List<T> GetChildTextBoxes<T>(Control ctrl) where T : Control{
    List<T> tbs = new List<T>();
    foreach (Control c in ctrl.Controls) {
        // If c is of type T, add it to the collection
        if (c is T) { 
            tbs.Add((T)c);
        }
    }
    return tbs;
}

private static void SetChildTextBoxesHeight(Control ctrl, int height) {
    foreach (TextBox t in GetChildTextBoxes<TextBox>(ctrl)) {
        t.Height = height;
    }
}

Answer 5

If you are given an enumerator, and need to use it in a for-each loop, you could use the following to wrap it:

static public class enumerationHelper
{
    public class enumeratorHolder<T>
    {
        private T theEnumerator;
        public T GetEnumerator() { return theEnumerator; }
        public enumeratorHolder(T newEnumerator) { theEnumerator = newEnumerator;}
    }
    static enumeratorHolder<T> toEnumerable<T>(T theEnumerator) { return new enumeratorHolder<T>(theEnumerator); }
    private class IEnumeratorHolder<T>:IEnumerable<T>
    {
        private IEnumerator<T> theEnumerator;
        public IEnumerator<T> GetEnumerator() { return theEnumerator; }
        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { return theEnumerator; }
        public IEnumeratorHolder(IEnumerator<T> newEnumerator) { theEnumerator = newEnumerator; }
    }
    static IEnumerable<T> toEnumerable<T>(IEnumerator<T> theEnumerator) { return new IEnumeratorHolder<T>(theEnumerator); }
}

The toEnumerable method will accept anything that c# or vb would regard an acceptable return type from GetEnumerator, and return something that can be used in foreach. If the parameter is an IEnumerator<> the response will be an IEnumerable<T>, though calling GetEnumerator on it once will likely yield bad results.