在C＃中的哪里声明变量

Question

I have created a simple calculator using VS 2013 Pro... and here is the segment of the codes:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

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

    }


    string input = string.Empty;
    double numb1, numb2, result;


    private void button1_Click(object sender, EventArgs e)
    {
        double.TryParse(textBox1.Text, out numb1);
        double.TryParse(textBox2.Text, out numb2);
        result = numb1 + numb2;
        textBox3.Text = result.ToString();
    }

    private void button4_Click(object sender, EventArgs e)
    {
        double.TryParse(textBox1.Text, out numb1);
        double.TryParse(textBox2.Text, out numb2);
        result = numb1 - numb2;
        textBox3.Text = result.ToString();
    }
}
}

now the problem I'm facing is, I've got two more buttons for multiplying and dividing, which in turn forces me to copy paste double.TryParse(textBox1.Text, out numb1); double.TryParse(textBox2.Text, out numb2); double.TryParse(textBox1.Text, out numb1); double.TryParse(textBox2.Text, out numb2);

for each button. I tried to put the codes with the other variables(double numb1, numb2, result) but getting an error... Here is the screenshots

Pretty new to Visual Studio and C#. Help is appreciated! :)

Answer 1

The declaration of the variables is fine at the class level. However, in order to reduce the code duplication you can extract that specific functionality into its own method. Something like this perhaps:

private void CaptureValues()
{
    double.TryParse(textBox1.Text, out numb1);
    double.TryParse(textBox2.Text, out numb2);
}

Then in the handlers:

private void button1_Click(object sender, EventArgs e)
{
    CaptureValues();
    result = numb1 + numb2;
    textBox3.Text = result.ToString();
}

This gives you a convenient place to put additional code. Checking the inputs and displaying a message, for example:

private void CaptureValues()
{
    if (!double.TryParse(textBox1.Text, out numb1))
        // textBox1 couldn't be parsed, show an error message
    if (!double.TryParse(textBox2.Text, out numb2))
        // textBox2 couldn't be parsed, show an error message
}

You could even go a step further and put the values into class-level properties. Something like this:

private double Value1
{
    get
    {
        double result;
        if (!double.TryParse(textBox1.Text, out result))
            throw new Exception("Couldn't parse the first text box!");
        return result;
    }
}

private double Value2
{
    get
    {
        double result;
        if (!double.TryParse(textBox2.Text, out result))
            throw new Exception("Couldn't parse the second text box!");
        return result;
    }
}

With those, you don't need your numb1 or numb2 variables at all, just reference the properties directly:

textBox3.Text = (Value1 + Value2).ToString();

Currently the properties can throw exceptions, so you might want to handle that:

try
{
    textBox3.Text = (Value1 + Value2).ToString();
}
catch (Exception ex)
{
    // examine what happened with ex and show an error
}

You can throw a more specific Exception type of course, even a custom one. Or you can respond to the error in the properties instead of in the handlers and not use exceptions at all. (There's an argument to be made, and I agree with it, never to use exceptions for normal logic, and this is potentially one of those edge cases. If it's normal logic, don't use exceptions. If it's an exceptional case and the value should never be un-parseable, go ahead and use them. I'd prefer not to in this case, but was just adding it as a possibility.)

There are a lot of options.