C#

using System;

class PriceCalc
{
    static void Main(string[] args)
    {
        float costPrice = 0.0f;
        float profit;
        float tax;
        float totalCost = 0.0f;
        float totalProfit = 0.0f;
        float totalTax = 0.0f;
        bool finished = false;
        float salePrice;

        string priceString;
        string nameString;
        float taxRate = 0.2f;

        while (!finished)
        {
            Console.Write("Enter the name of the product : ");

            nameString = Console.ReadLine();
            Console.WriteLine("Product is " + nameString);
            if (nameString.Length != 0)
            {
                finished = true;
            }
            else
            {
                bool dataOK = false;
                while (!dataOK)
                {
                    Console.Write("Enter price of the product : ");
                    priceString = Console.ReadLine();
                    try
                    {
                        costPrice = float.Parse(priceString);
                        dataOK = true;
                    }
                    catch
                    {
                        Console.WriteLine("That is not a valid number");
                        Console.WriteLine("Enter a number, e.g. 15.5");
                    }
                }

                if (costPrice < 20.0f)
                {
                    profit = costPrice * 0.2f;
                }
                else
                {
                    if (costPrice < 40.0f)
                    {
                        profit = costPrice * 0.15f;
                    }
                    else
                    {
                        profit = costPrice * 0.1f;

                    }
                }
                tax = (costPrice + profit) * taxRate;
                totalTax = totalTax + tax;
                totalCost = totalCost + costPrice;
                totalProfit = totalProfit + profit;
                salePrice = costPrice + profit + tax;
            }
        }

        Console.ReadKey();
    }
}

class PriceCalc
{
    static int readInteger (string prompt) 
    {
        Console.WriteLine(prompt);
        // read an int or float from the console, preferably using TryParse.
        priceString = Console.ReadLine();
        return priceString;
    }

    static void Main(string[] args)
    {
        float costPrice = 0.0f;
        float profit;
        ..and so on

// replace these in your main-method;
Console.Write("Enter price of the product : ");
priceString = Console.ReadLine();
// with this
  priceString = readInteger("Enter price of the product : ");

• The signature states an "int", not a float. Might seem a minor detail, but in the case of calculating money, there's always a devil there.
  
• I would go for Int.TryParse[^].
  
• There's a small bug in there that causes the app to exit after entering a product-name. I'd recommend replacing the "nameString.Lengt != 0" with
  
  C#

  if (string.IsNullOrEmpty(nameString))
  

  
  
• It would probably be helpfull to the end user if you also wrote the results to the screen before exiting
  

static int readInteger (string prompt) 

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]

static /*int*/ float readInteger (string prompt) // should be named readFloat !!!
{
    do
    {
       Console.Write(prompt);
       float value;
       if (float.TryParse(Console.ReadLine(), out value))
            return value;
       Console.WriteLine("That is not a valid number");
       Console.WriteLine("Enter a number, e.g. 15.5");
    } while (true);
}

// ...
if (nameString.Length != 0)
{
    finished = true;
}
else
{
    costPrice = readInteger("Enter price of the product : ");
    // It should be named readFloat as I noted above!
 
    if (costPrice < 20.0f)
// ...

Matt T Heffron wrote:

You have a method whose signature says it is returning int, but you
are returning a string!

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]

Matt T Heffron wrote:

You have a method whose signature says it is returning int, but you
are returning a string!

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]

Eddy Vluggen wrote:

I would still recommend using integers, not floats. You cannot pay half a cent AFAIK, and it would eliminate rounding errors

Matt T Heffron wrote:

Of course, this is exactly what the decimal data type is for!

Compared to floating-point types, the decimal type has more precision and a smaller range, which makes it appropriate for financial and monetary calculations.

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]

Eddy Vluggen wrote:

Show me 0.000783678 cents and I might change my mind

Matt T Heffron wrote:

349.9 cents/gallon

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]