Money in Floats in C#

A computer stores decimal numbers, floats (in C#, double), in binary. In binary, 0.1 has no exact value, just as 1/3 has none in decimal (0.333...). So in C# 0.1 + 0.2 is 0.30000000000000004, a tiny bit more than 0.3. For money, that's a bug. Store amounts as whole numbers of the smallest unit, cents, instead: 19.99 euros is 1999 cents, and 1999 + 1 is exactly 2000.

An AI assistant wrote the cart total for the shop's stationery corner, with the prices in euros, as doubles. A customer who bought one sticker and one pencil was told to pay 0.30000000000000004:

var PRICES = new Dictionary<string, double> { ["sticker"] = 0.10, ["pencil"] = 0.20, ["eraser"] = 0.45, ["postcard"] = 1.15 };

app.MapPost("/cart/total", (JsonElement data) => {
    var items = data.GetProperty("items");
    var total = 0.0;
    foreach (var item in items.EnumerateArray()) {
        total = total + PRICES[item.GetProperty("product").GetString()] * item.GetProperty("quantity").GetInt32();
    }
    return Results.Json(new { total = total });
});

For 3 postcards it said 3.4499999999999997, a little less than the real price. Some totals came out right, like 3 postcards and 2 erasers (4.35), and that's how the bug got past a quick try. When AI code works with money, look for floats.

The client sends the cart as JSON, like {"items": [{"product": "sticker", "quantity": 3}]}. Your version must answer whole cents:

POST /cart/total  {"items": [{"product": "sticker", "quantity": 1}, {"product": "pencil", "quantity": 1}]}  →  200  {"total_cents": 30}
POST /cart/total  {"items": [{"product": "sticker", "quantity": 3}]}  →  200  {"total_cents": 30}
POST /cart/total  {"items": [{"product": "postcard", "quantity": 3}]}  →  200  {"total_cents": 345}
POST /cart/total  {"items": [{"product": "postcard", "quantity": 3}, {"product": "eraser", "quantity": 2}]}  →  200  {"total_cents": 435}
POST /cart/total  {"items": [{"product": "sticker", "quantity": 7}]}  →  200  {"total_cents": 70}
POST /cart/total  {"items": []}  →  200  {"total_cents": 0}

Your task: write the price list PRICES as a Dictionary<string, int> in cents (sticker 10, pencil 20, eraser 45, postcard 115) and rewrite the MapPost function: add up each item's price times its quantity, and answer {"total_cents": ...}.

Assignment
Follow the Coding Tutorial to rewrite the price list and the MapPost function so every amount is a whole number of cents.

Hint
Keep every number whole, from the price list to the total: start the total at 0, not 0.0. Dividing by 100 is for whoever shows the price to a person, not for this function. (C# also has an exact decimal type, but whole cents work in every language, database and payment service.)