How to use const in C# class constructor

Question

Let's say I have a class like this:

public class SampleClass
{
    const string UnChangableUserName { get; private set; }
    string Password { get; private set; }

    public SampleClass(string UnChangableUserName, string Password)
    {
        this.Password = Password;
        this.UnChangableUserName = UnChangableUserName;
    }
}

I want the constructor to assign a value to the const, and not for the value to be set when the const is defined. How can I do it?

I'm aware I can simply not use set but using const is more resource efficent and more elegant as well as clearer to other devs to understand, I don't want a set so they will not add it.

Avner Shahar-Kashtan · Accepted Answer · 2019-07-08T21:25:04.340

You can't. By definition, consts in C# are defined at compile-time, not at runtime. They can't be changed by any code, even constructor code, because they're not even there at runtime - they're replaced by their literal values at every point of usage.

What you're looking for is either readonly fields or read only properties.

Readonly fields are marked by the readonly modifier:

private readonly string _username;

Readonly fields are only assignable during construction, either directly assigned in the definition itself (readonly string _username = "blah") or in the constructor. The compiler will enforce this limitation, and will not allow you to set a value anywhere else.
Readonly properties are simply properties that don't expose a setter at all. Until C# 6, you could use them to return the value of a readonly field (as above), but you couldn't assign to them. As of C# 6, though, there's syntax supporting read-only auto-properties that can be assigned: even though there's no explicit setter, there's an implicit setter that can, again, only be called from the constructor, or from a field initializer:

public string Username { get } = "Username";

It may look strange that you're setting to a property with no setter, but that's simply an implicit part of the language - there's an implicit backing field that's readonly, and can be set at initialization.

Readonly properties supported from C# 1... Newer versions bring different syntax to express that so... — Alexei Levenkov, Jul 08 '19 at 21:15
Technically, pre-C# 6 properties can be read-only, but not *assignable* - you can't have assignable read-only auto-properties. You can have properties with getters that return a read-only field, that's true, but you'll have to do the assignment separately. — Avner Shahar-Kashtan, Jul 08 '19 at 21:23

score 1 · Answer 2 · answered Jul 08 '19 at 21:00

1

a const is a CONSTANT at compilation. You need a readonly property.

https://stackoverflow.com/a/3917886/910741

answered Jul 08 '19 at 21:00

Bestter

877
1
12
29

How to use const in C# class constructor

2 Answers2