Constants and Readonly Values

Some values should not move once they are set: a VAT rate, the date an invoice was raised, a reference number. C# gives you two ways to say so.

They look interchangeable and are not. The difference is when the value is decided, and it shows up in a way that can leave two running services disagreeing about the same number.

decimal net = 1240.50m;
decimal vat = net * TaxRules.StandardVatRate;

Console.WriteLine($"VAT at {TaxRules.StandardVatRate:P0}: {vat}");

public static class TaxRules
{
    public const decimal StandardVatRate = 0.20m;
    public const int MaxInvoiceLineItems = 250;
    public const string CurrencyCode = "GBP";
}

• A const can only hold something the compiler can evaluate itself: a number, bool, char, string, enum member or null.
• A const is automatically static — it belongs to the type, not to an instance — and you do not write static yourself.
• In a file using top-level statements, the statements come first and type declarations follow. The compiler requires that order.
• The :P0 in the output is a format string: show it as a percentage with no decimal places.

var first = new Invoice("INV-2041");
var second = new Invoice("INV-2042");

Console.WriteLine($"{first.Reference} raised at {first.CreatedUtc:HH:mm:ss.fff}");
Console.WriteLine($"{second.Reference} raised at {second.CreatedUtc:HH:mm:ss.fff}");
Console.WriteLine($"Late fee for both: {Invoice.LateFee}");

public class Invoice
{
    public static readonly decimal LateFee = 25m;

    public readonly string Reference;
    public readonly DateTime CreatedUtc;

    public Invoice(string reference)
    {
        Reference = reference;
        CreatedUtc = DateTime.UtcNow;
    }
}

• A readonly field can be assigned in exactly two places: where it is declared, or inside a constructor. Anywhere else is a build error.
• Because the assignment happens while the program runs, the value can be anything — a DateTime, a list, the result of a calculation.
• static readonly means one shared value for the whole type, worked out once. DateTime.UtcNow could never be a const, because the compiler cannot know it.
• The two invoices have different CreatedUtc values, which is the point: readonly is per object unless you mark it static.

var order = new Order();

order.Lines.Add("Desk lamp");     // allowed
order.Lines.Add("Cable tidy");    // also allowed
// order.Lines = new List<string>();   // build error: cannot assign to a readonly field

Console.WriteLine(order.Lines.Count);   // 2

public class Order
{
    public readonly List<string> Lines = new List<string>();
}

• readonly stops the field pointing at a different list. It says nothing about the contents of that list.
• If the contents must not change either, expose the field as IReadOnlyList<string> or hand back a copy. The keyword alone does not make anything immutable.