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IEnumerable and Recursion using yield return

Asked 2010-01-13T10:26:20.997
349

I have an IEnumerable<T> method that I'm using to find controls in a WebForms page.

The method is recursive and I'm having some problems returning the type I want when the yield return is returnig the value of the recursive call.

My code looks as follows:

public static IEnumerable<Control> GetDeepControlsByType<T>(this Control control)
{
    foreach(Control c in control.Controls)
    {
        if (c is T)
        {
            yield return c;
        }

        if(c.Controls.Count > 0)
        {
            yield return c.GetDeepControlsByType<T>();
        }
    }
}

This currently throws a "Cannot convert expression type" error. If however this method returns type IEnumerable<Object>, the code builds, but the wrong type is returned in the output.

Is there a way of using yield return whilst also using recursion?

c# generics ienumerable yield
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1 Answer

15

You need to return the items from the enumerator, not the enumerator itself, in your second yield return

public static IEnumerable<Control> GetDeepControlsByType<T>(this Control control)
{
    foreach (Control c in control.Controls)
    {
        if (c is T)
        {
            yield return c;
        }

        if (c.Controls.Count > 0)
        {
            foreach (Control ctrl in c.GetDeepControlsByType<T>())
            {
                yield return ctrl;
            }
        }
    }
}
answered 2010-01-13T10:30:39.483

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