Why does C# disallow readonly local variables?

Question

Having a friendly debate with a co-worker about this. We have some thoughts about this, but wondering what the SO crowd thinks about this?

Answer 1

One reason is there is no CLR support for a readonly local. Readonly is translated into the CLR/CLI initonly opcode. This flag can only be applied to fields and has no meaning for a local. In fact, applying it to a local will likely produce unverifiable code.

This doesn't mean that C# couldn't do this. But it would give two different meanings to the same language construct. The version for locals would have no CLR equivalent mapping.

Answer 2

I think it's a poor judgement on part of C# architects. readonly modifier on local variables helps maintain program correctness (just like asserts) and can potentially help the compiler optimize code (at least in the case of other languages). The fact that it's disallowed in C# right now, is another argument that some of the "features" of C# are merely an enforcement of personal coding style of its creators.

Answer 3

Addressing Jared's answer, it would probably just have to be a compile-time feature - the compiler would prohibit you from writing to the variable after the initial declaration (which would have to include an assignment).

Can I see value in this? Potentially - but not a lot, to be honest. If you can't easily tell whether or not a variable is going to be assigned elsewhere in the method, then your method is too long.

For what it's worth, Java has this feature (using the final modifier) and I've very rarely seen it used other than in cases where it has to be used to allow the variable to be captured by an anonymous inner class - and where it is used, it gives me an impression of clutter rather than useful information.

Answer 4

A proposal readonly locals and parameters for was briefly discussed by the C# 7 design team. From C# Design Meeting Notes for Jan 21, 2015:

Parameters and locals can be captured by lambdas and thereby accessed concurrently, but there's no way to protect them from shared-mutual-state issues: they can't be readonly.

In general, most parameters and many locals are never intended to be assigned to after they get their initial value. Allowing readonly on them would express that intent clearly.

One problem is that this feature might be an "attractive nuisance". Whereas the "right thing" to do would nearly always be to make parameters and locals readonly, it would clutter the code significantly to do so.

An idea to partly alleviate this is to allow the combination readonly var on a local variable to be contracted to val or something short like that. More generally we could try to simply think of a shorter keyword than the established readonly to express the readonly-ness.

Discussion continues in the C# Language Design repo. Vote to show your support. https://github.com/dotnet/csharplang/issues/188

Answer 5

I was that coworker and it wasn't friendly! (just kidding)

I would not eliminate the feature because it's better to write short methods. It's a bit like saying you shouldn't use threads because they're hard. Give me the knife and let me be responsible for not cutting myself.

Personally, I wanted another "var" type keyword like "inv" (invarient) or "rvar" to avoid clutter. I've been studying F# as of late and find the immutable thing appealing.

Never knew Java had this.

Answer 6

I would like local readonly variables in the same manner as I like local const variables. But it has less priority than other topics.
Maybe its priority is the same reason for C# designers to not (yet!) implement this feature. But it should be easy (and backward compatible) to support local readonly variables in future versions.

Answer 7

It is an oversight for c# language designer. F# has val keyword and it is based on CLR. There is no reason C# can't have the same language feature.

Answer 8

Readonly means the only place the instance variable can be set is in the constructor. When declaring a variable locally it doesn't have an instance (it's just in scope), and it can't be touched by the constructor.

Answer 9

I know, this doesn't answer the why to your question. Anyway, those reading this question might appreciate the code below nonetheless.

If you are really concerned with shooting your self in the foot when overriding a local variable that should only be set once, and you don't want to make it a more globally accessible variable, you could do something like this.

    public class ReadOnly<T>
    {
        public T Value { get; private set; }

        public ReadOnly(T pValue)
        {
            Value = pValue;
        }

        public static bool operator ==(ReadOnly<T> pReadOnlyT, T pT)
        {
            if (object.ReferenceEquals(pReadOnlyT, null))
            {
                return object.ReferenceEquals(pT, null);
            }
            return (pReadOnlyT.Value.Equals(pT));
        }

        public static bool operator !=(ReadOnly<T> pReadOnlyT, T pT)
        {
            return !(pReadOnlyT == pT);
        }
    }

Example usage:

        var rInt = new ReadOnly<int>(5);
        if (rInt == 5)
        {
            //Int is 5 indeed
        }
        var copyValueOfInt = rInt.Value;
        //rInt.Value = 6; //Doesn't compile, setter is private

Maybe not as less code as rvar rInt = 5 but it works.

Answer 10

You can declare readonly local variables in C#, if you're using the C# interactive compiler csi:

>"C:\Program Files (x86)\MSBuild\14.0\Bin\csi.exe"
Microsoft (R) Visual C# Interactive Compiler version 1.3.1.60616
Copyright (C) Microsoft Corporation. All rights reserved.

Type "#help" for more information.
> readonly var message = "hello";
> message = "goodbye";
(1,1): error CS0191: A readonly field cannot be assigned to (except in a constructor or a variable initializer)

You can also declare readonly local variables in the .csx script format.

Answer 11

c# already has a readonly var, albeit in a somewhat different syntax:

Consider the following lines:

var mutable = myImmutableCalculationMethod();
readonly var immutable = mutable; // not allowed in C# 8 and prior versions
return immutable;

Compare with:

var mutable = myImmutableCalculationMethod();
string immutable() => mutable; // allowed in C# 7
return immutable();

Admittedly, the first solution could possibly be less code to write. But the 2nd snippet will make the readonly explicit, when referencing the variable.

Answer 12

I think that's because a function that has a readonly variable may never be called, and there's probably something about it going out of scope, and when would you need to?

Answer 13

use const keyword to make read only variable.

reference: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/const

public class SealedTest
{
    static void Main()
    {
        const int c = 707;
        Console.WriteLine("My local constant = {0}", c);
    }
}