public delegate void TextEventHandler(string strText);

public TextEventHandler TextHandler;

private void btnOk_Click(object sender, EventArgs e)
{
    if (null != TextHandler)
    {
        TextHandler.Invoke(txtString.Text);
        DialogResult = DialogResult.OK;
    }
}

private void btnCancel_Click(object sender, EventArgs e)
{
    DialogResult = DialogResult.Cancel;
}

private void txtString_KeyPress(object sender, KeyPressEventArgs e)
{
    if (Keys.Enter == (Keys)e.KeyChar)
    {
        if (null != TextHandler)
        {
            TextHandler.Invoke(txtString.Text);
            DialogResult = DialogResult.OK;
        }
    }
}

public static class InputDialog
{
    public static DialogResult Show(out string strText)
    {
        string strTemp = string.Empty;

        FrmInputDialog inputDialog = new FrmInputDialog();
        inputDialog.TextHandler = (str) => { strTemp = str; };

        DialogResult result = inputDialog.ShowDialog();
        strText = strTemp;

        return result;
    }
}

string strText = string.Empty;
InputDialog.Show(out strText);