How could this give me an Invalid Cast Exception?

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My app is crashing with "Invalid Cast Exception" on running the following code:

C#

private void ClearControls()
{
    foreach (TextBox txtbx in this.Controls)
    {
        if (txtbx != null)
        {
            txtbx.Text = string.Empty;
        }
    }
    foreach (ComboBox cmbx in this.Controls)
    {
        if (cmbx != null)
        {
            cmbx.SelectedIndex = -1;
        }
    }
}

How could that be? Only TextBoxes are treated as TextBoxes, and only ComboBoxes are treated as ComboBoxes, so what's the problem?

Posted 20-Nov-14 12:21pm

B. Clay Shannon

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2 solutions

Solution 3

When your code iterates over the collection of Controls on the Form, the fact you specify a specific Control Type in your loops means that when the iterator comes to a Control of another Type you will get an "invalid cast" error.

Here's another, simpler way:

C#

foreach (TextBox tb in this.Controls.OfType<textbox>())
{
    tb.Text = string.Empty;
}

foreach (ComboBox cbx in this.Controls.OfType<combobox>())
{
    cbx.Text = string.Empty; // probably unnecessary
    cbx.SelectedIndex = -1;
}

Posted 21-Nov-14 7:11am

BillWoodruff

Updated 21-Nov-14 7:12am

v2

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PIEBALDconsult · Accepted Answer · 2014-11-20T12:44:00

Solution 2

foreach doesn't filter. And why iterate the collection twice?

C#

private void ClearControls()
{
    foreach ( Control c in this.Controls )
    {
        if ( c is ComboBox )
        {
            ((ComboBox) c).SelectedIndex = -1;
        }
        else if (c is TextBox )
        {
            ((TextBox) c).Text = string.Empty;
        }
    }
}

Posted 20-Nov-14 12:44pm

PIEBALDconsult

Comments

B. Clay Shannon 20-Nov-14 18:47pm

Odd that "(TextBox)" is grayed out, as if it's redundant, but "(ComboBox)" is not.

BillWoodruff 21-Nov-14 13:01pm

+5 This is a good answer, does not deserve a down-vote.