c# stream writer: how to write numbers on new lines?

i want to make stream writer function where i can write numbers several times and at the end of the program show me sum of those numbers. how i can code this thing?

public static void bought(float a)
    {
        StreamWriter SW = new StreamWriter(@"C:\Users\ETN\source\repos\Apple-store\Apple-store\buy.txt");
        SW.Write(a);
        SW.Close();

    }

Solution

There are a couple of things you would want to change in your code. Speficially:

You are opening a stream writer, write one value and close it. Unless you already have a list of values you want to write, you generally open and close the stream writer once and call it multiple times.
When you want to add a new line after your written value, you use WriteLine instead of Write.
When you write numeric values to a text file, they will be converted to text dependent on the culture. Note that the default is the culture of your system. If you read the file from a different computer with a different culture, the file might be unreadable. Therefore you should always provide a specific culture. To do that, check the Convert.ToString method.
You should enclose your code that writes to the stream writer in a try/finally block, with the StreamWriter.Close() method in the finally. Otherwise your file is not guaranteed to be closed in the event of an error.
It is not recommended to store monetary information (such as prices or account balances) as a float. Use decimal instead, which is optimized for this purpose (as opposed to float which is for scientific calculations).

This code should give you a head start at the issue. It's up to you to complete it and organize it into methods, classes etc., depending on your specific requirements:

StreamWriter writer = new StreamWriter(@"C:\Users\ETN\source\repos\Apple-store\Apple-store\buy.txt");
try {
    while (true) {
        decimal price:
        //Your code that determines the price goes here
        string priceText = Convert.ToString(price, CultureInfo.InvariantCulture);
        writer.WriteLine(priceText);

        bool shouldContinue;
        //Your code that determines whether there are more values to be written goes here
        if (!shouldContinue) {
            break;
        }
    }
    writer.Flush();
}
finally {
    writer.Close();
}