如何使用if-else语句中陈述的值？

Question

I have recently started studying C# and I wanted to make a length converter with different units. However, it seems that I cannot use values stated inside the if-else statement. Can anyone please help me?

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace converter
{
    public partial class MainForm : Form
    {
    public MainForm()
    {
        InitializeComponent();
    }
    void ClearBClick(object sender, EventArgs e)
    {
        convIn.Clear();
        convOut.Clear();
        cmbConv.Refresh();
    }
    void ExitBClick(object sender, EventArgs e)
    {
        this.Close();
    }
    void ConvBClick(object sender, EventArgs e)
    { double exchangeRate;

        if(cmbConv.SelectedItem.ToString() == "Miles to Kilometers")
        {
            exchangeRate = 1.60934;
        }
        else if(cmbConv.SelectedItem.ToString() == "Kilometers to Miles")
        {
            exchangeRate = 0.621371;
        }
        else if(cmbConv.SelectedItem.ToString() == "Inches to Centimeters")
        {
            exchangeRate = 0.393701;
        }
        else if(cmbConv.SelectedItem.ToString() == "Centimeters to Inches")
        {
            exchangeRate = 2.54;
        }

        double conv = Convert.ToDouble(convIn.Text);
        var conversion = conv * exchangeRate;

        var result = Convert.ToString(conversion);

        convOut.Text = result;
        }

    }
}

Before I used this, I used a different structure which also didn´t work, in which I separated the if-else statement in different methods.

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace converter
{
    public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();
    }
    void ClearBClick(object sender, EventArgs e)
    {
        convIn.Clear();
        convOut.Clear();
        cmbConv.Refresh();
    }
    void ExitBClick(object sender, EventArgs e)
    {
        this.Close();
    }
    void ConvBClick(object sender, EventArgs e)
    {           double conv = Convert.ToDouble(convIn.Text);
        var conversion = conv * exchangeRate;

        var result = Convert.ToString(conversion);

        convOut.Text = result;
    }
    void CmbConvSelectedIndexChanged(object sender, EventArgs e)
    {
         double exchangeRate;

        if(cmbConv.SelectedItem.ToString() == "Miles to Kilometers")
        {
            exchangeRate = 1.60934;
        }
        else if(cmbConv.SelectedItem.ToString() == "Kilometers to Miles")
        {
            exchangeRate = 0.621371;
        }
        else if(cmbConv.SelectedItem.ToString() == "Inches to Centimeters")
        {
            exchangeRate = 0.393701;
        }
        else if(cmbConv.SelectedItem.ToString() == "Centimeters to Inches")
        {
            exchangeRate = 2.54;
        }

     }
 }

Answer 1

You're trying to use a variable called exchangeRate in your ConvBClick method, but that variable doesn't exist in that method. You created a similarly named variable in your CmbConvSelectedIndexChanged method, but then never did anything with it.

More to the overall structure of the code however, your CmbConvSelectedIndexChanged method doesn't really do anything. It executes any time the selected index changes on that control, but it doesn't affect anything else. Instead of responding to that SelectedIndexChanged event at all, just write a method which returns the value you want based on whatever the current selected index is:

double GetExchangeRate()
{
    double exchangeRate = 0.0;

    if(cmbConv.SelectedItem.ToString() == "Miles to Kilometers")
    {
        exchangeRate = 1.60934;
    }
    // etc., the rest of your conditions

    return exchangeRate;
}

Then in your button click you can just call that method to get the current exchange rate:

double conv = Convert.ToDouble(convIn.Text);
var exchangeRate = GetExchangeRate();
var conversion = conv * exchangeRate;

var result = Convert.ToString(conversion);

convOut.Text = result;