CODEWITHSUNDEEP
c#structunsafec#-7.2
Fit Dev
Fit Dev

Reputation: 3689

Why is it not possible to use both readonly and fixed-size buffers in structs in C# 7.2

With the release of C# 7.2 there is now the ability to have readonly structs, which in many cases can improve performance.

For one of my structs I am using a fixed-size byte array to actually hold the data. However when I marked the struct and the byte array field readonly, C# compiler complained that readonly is not valid on the field. Why can't I have both fixed and readonly on a field in a struct?

readonly unsafe struct MyStruct {
  readonly fixed byte _Value[6]; //The modifier 'readonly' is not valid for this item.
}

Upvotes: 7

Views: 1164

Answers (2)

Alexis Martial
Alexis Martial

Reputation: 65

Actually there is a trick to have a readonly fixed field in a readonly struct: just embed your fixed inside a dedicated struct ;-)

unsafe struct MyFixed {
    public fixed byte _v[6];

    public MyFixed(byte[] bs)
    {
        for (int i=0;i!=6;i++)
            _v[i]=bs[i];
    }
 }

readonly struct MyStruct {
    readonly public MyFixed Values;

    public MyStruct(byte[] bs){
        Values = new MyFixed(bs);
    }

static unsafe byte GetFixed(in MyStruct _ms)
=> _ms.Values._v[1];
        
static unsafe byte SetFixed(in MyStruct _ms)
=> _ms.Values._v[1] = 0;

I added:

  • a constructor just to be able to test sharplab.
  • Get and Set from outside, the compiler detect and report a read-only access in the Set.

This clearly demonstrate that the limitation you faced is just arbitrary and could easily be fixed (pun intended) in Roslyn ;-)

Upvotes: 0

Evk
Evk

Reputation: 101543

Because C# specification says so (and it always did, even before c# 7.2). In 18.7.1 section, named "Fixed size buffer declarations", the following modifiers are allowed on fixed buffer declaration:

new

public

protected

internal

private

unsafe

No readonly here. If you think about it - it doesn't make much sense anyway, because fixed buffer size is represented by a pointer, and you cannot restict write access to a pointer. For example:

var s = new MyStruct();
byte* value = s._Value;
// how can you prevent writing to `byte*`?

Upvotes: 1

Related Questions