如何在C#中将两个标签的数量相乘

Question

so i have 2 labels. one of them is a fixed number and doesn't change but the other one changes every 5 seconds. Now i want to multiply them automatically and show them in another label as Results.

what should i do? what am i doing wrong?

i tried this code but it says "operator * cannot be applied to string and string".

label1.Text = BTC_A.Text * BTCPrice_Label.Text;

then i tried

double txt1 = Convert.ToDouble(BTC_A.Text);
            double txt2 = Convert.ToDouble(BTCPrice_Label.Text);

            double sum = txt1 * txt2;

            label1.Text = sum.ToString();

but it says "Input string was not in a correct format"

Answer 1

Think it through step by step.

You have labels with a Text property. That property is of type string . C# is a strongly typed language: strings can not be multiplied like that. In the end, a label can be empty, or the user could input any random string. What would be the result of "foo" * "bar" ?

Also, when you do have a double as a result of some multiplication, you want to show it to the user in another label.Text . Here you have the inverse issue: C#/.Net does not convert the variable of type double implicitly to a string.

So you will have to

• check if the strings entered by the user is actually a valid double
• if they are, convert those strings to double and multiply them
• convert the result to a string, and assign it to the labels Text property
• if the strings are not valid numbers, leave the label empty, or show some other message

The logic to achieve this would be something like this:

var validPrice = int.TryParse(BTCPrice_Label.Text, out double price);
var validAmount = int.TryParse(BTCA_Label.Text, out double amount);
if (validPrice && validAmount)
{
    var result = price * amount;
    label1.Text = result.ToString();
}
else
{
    label1.Text = "something is wrong";
}

Answer 2

so the problem was a dollar sign ( $ ) that i put before the numbers.

i just deleted the sign and this it what the code looks like now:

double AA;
           if (!double.TryParse(BTC_A.Text, out AA))
           {
                MessageBox.Show($"Unable to convert the BTC_A \"{BTC_A.Text}\" to a floating point number");
                return;
           }
           double btcA;
           if (!double.TryParse(BTCPrice_Label.Text, out btcA))
           {
                MessageBox.Show($"Unable to convert the price \"{BTCPrice_Label.Text}\" to a floating point number");
                return;
           }
           label1.Text = (AA * btcA).ToString();