Question

When trying a regular expression for no characters my winforms EnterValue is still being triggered after the if statement, how can I stop it from going any further after the trigger?

    private void EnterValue_Click(object sender, EventArgs e)
    {
        if (textBox1.Text != string.Empty && !Regex.IsMatch(textBox1.Text, @"^[0-9]+$"))
        {
            MessageBox.Show("Please only enter numbers");
            textBox1.Clear();
        }

        //convert input to double
        listDouble.Add(Convert.ToDouble(textBox1.Text)); // this line still throws exception
        textBox1.Clear();
        //clear existing items
        listBox1.Items.Clear();
        // clear any existing list items
        for (int i = 0; i < listDouble.Count; i++)
        {
            listBox1.Items.Add(listDouble[i]);
        }
        //for each value added, add this to our list 
    }

Answer 1

Return from the method:

if (textBox1.Text != string.Empty && !Regex.IsMatch(textBox1.Text, @"^[0-9]+$"))
{
    MessageBox.Show("Please only enter numbers");
    textBox1.Clear();
    return; // nothing after this will execute
}

This will execute only if the if predicate is true, and the method will return as soon as the return; statement has been hit, without any of the other code being run.