对C#来说相当陌生。
我想从下拉式方块读取使用者输入，并使用选取的货币进行转换。输出不在主控台中，而是在Windows表单中。
这是我在组合框中输入的读取代码：

public void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    double first = comboBox1.SelectedIndex;
}

public void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
    double second = comboBox2.SelectedIndex;
}

这是我的代码，它使用选定的索引并转换它们：

internal class Currencies
{
    public double[] Convert(double[] currencies, int amount)
    {
        double[] convertedAmount = new double[currencies.Length];

        if (Converter.comboBox1.first == HRK && Converter.comboBox2.second == EURO)
        {
            int exchangeRate1 = 1;
            double exchangeRate2 = 7.4;

            for (int i = 0; i < currencies.Length; i++)
            {
                convertedAmount[i] = amount * exchangeRate1 / exchangeRate2;
            }
        }
        else if (Converter.comboBox1.first == EURO && Converter.comboBox2.seocnd == HRK)
        {
            int exchangeRate1 = 1;
            double exchangeRate2 = 7.4;

            for (int i = 0; i < currencies.Length; i++)
            {
                convertedAmount[i] = amount * exchangeRate1 / exchangeRate2;
            }
        }
        else
        {
            throw new NotSupportedException();
        }

        return convertedAmount;
    }
}

为什么这个方法不允许我使用变量“first”和“second”来转换货币呢？