private delegate float CalculateDelegate(float x, float y);
private CalculateDelegate CalculateFunct;
private CalculateDelegate CalculateAdd = delegate(float x, float y) { return x + y; };
private CalculateDelegate CalculateMult = delegate(float x, float y) { return x * y; };

// Called by CheckChanged of "Add" RadioButton
private void SetAddMode(object sender, EventArgs e)
{
    if (sender is RadioButton && ((RadioButton)sender).Checked)
	CalculateFunct = CalculateAdd;
}

// Called by CheckChanged of "Add" RadioButton
private void SetMultMode(object sender, EventArgs e)
{
    if (sender is RadioButton && ((RadioButton)sender).Checked)
	CalculateFunct = CalculateMult;
}

private void Calculate(object sender, EventArgs e)
{
    float a,b;
    if (!TryParseFloatInTextBox(aTextBox, out a))
	return;
    if (!TryParseFloatInTextBox(bTextBox, out b))
	return;
    resultsLabel.Text = CalculateFunct(a, b).ToString();
}

private bool TryParseFloatInTextBox(TextBox textBox, out float f)
{
    if (!float.TryParse(aTextBox.Text, out f))
    {
	Console.Beep();
	aTextBox.Focus();
	return false;
    }
    return true;
}