如何在c#中使字符串为空以转换文本框

Question

Here is the form i made

When I enter an empty value in the textbox I meet exception unhandled page. I want to show an error messagebox when user enter an empty value to the textbox how can I do that ?

namespace Random_çalışması
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Random rnd = new Random();
        int number;
        int answer;
        private void button1_Click(object sender, EventArgs e)
        {
            

            
            number = rnd.Next(1, 101);

            label1.Text = number.ToString();

            
        }

        private void button2_Click(object sender, EventArgs e)
        {
            answer = Convert.ToInt32(textBox1.Text);

            if(answer == number)
            {
                MessageBox.Show("Welcome to the page", "Welcome!", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            else if(String.IsNullOrEmpty(textBox1.Text))
            {
                MessageBox.Show("Icorrect entrance", "You can not acces to the page", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            else
            {
                MessageBox.Show("Icorrect entrance", "You can not acces to the page", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
    }

Answer 1

Just add the empty test at the beginning of the method:

private void button2_Click(object sender, EventArgs e)
{
  if ( textBox1.Text == "" )
  {
    MessageBox.Show(...);
    return;
  }
  answer = Convert.ToInt32(textBox1.Text);
  ...
}

You can also use int.TryParse() instead of Convert to better catch conversion errors:.

How the int.TryParse actually works

You can also simply set the button disable by default instead and add this handler on the TextChanged event:

private void textBox1_TextChanged(object sender, EventArgs e)
{ 
  button.Enable = textBox1.Text != "";
}

And also do the int.TryParse here instead too:

private void textBox1_TextChanged(object sender, EventArgs e)
{ 
  button.Enable = textBox1.Text != "" && int.TryParse(textBox1.Text, out _);
}

Hence the button is enabled only if not empty and convertible and you have a consistent UX.

Now the button click handler is:

private void button2_Click(object sender, EventArgs e)
{
  answer = Convert.ToInt32(textBox1.Text);
  if ( answer == number )
    MessageBox.Show("Welcome to the page", "Welcome!", MessageBoxButtons.OK, MessageBoxIcon.Information);
  else
    MessageBox.Show("Icorrect entrance", "You can not acces to the page", MessageBoxButtons.OK, MessageBoxIcon.Error);
}