[Solved] Can’t add numbers using the void method

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To make IncreaseStudents work the way you want, you need to change two things: You need to return a value, meaning it can’t be void. And you need change the way you turn textBox3.Text into an integer. You don’t parse the whole expression, textBox3.Text + num; num is a number already. All you need (or want) to parse is textBox3.Text, because it’s a string. And we’ll use a different way of parsing, so that if the text doesn’t represent a number, it will fail politely instead of throwing an exception. 

public int IncreaseStudents(int num)
{
    int n;
    if (int.TryParse(textBox3.Text, out n))
    {
         return n + num;
    }
    else
    {
         MessageBox.Show("Not a number: " + textBox3.Text);
         return num;
    }
}

Then this line of code should work fine — unless s1._stdNumber is something weird that you can’t add to. 

listBox1.Items.Add(s1._stdNumber + IncreaseStudents(25));

But that’s not the assignment. The assignment says you have to use void. So, another try: 

public void IncreaseStudents(int num, out int result)
{
    int n;
    if (int.TryParse(textBox3.Text, out n))
    {
         result = n + num;
    }
    else
    {
         MessageBox.Show("Not a number: " + textBox3.Text);
         result = num;
    }
}

And call like this:

int result = 0;
IncreaseStudents(25, out result)

listBox1.Items.Add(s1._stdNumber + result);

Silly, but if your instructor wants that, that’s what he wants.

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solved Can’t add numbers using the void method