C# – ref readonly parameters

The ref readonly modifier enables a more transparent way of passing read-only references to a method. Passing of readonly references was already possible in C# by using the in modifier ever since version 7.2, but that syntax had some limitations, or rather too little constraints.

So how does the new modifier work? Let’s assume the following method signature:

void FooRef(ref readonly int bar) { }

Simply calling the method by passing an integer variable or a value, will result in a compiler warning. Note, this is only a warning, it highlights an ambiguity in your implementation, but it will still allow your code to run if you insist.

var x = 42;

FooRef(x);
FooRef(42);

• FooRef(x) will trigger warning CS9192: Argument 1 should be passed with ‘ref’ or ‘in’ keyword
• FooRef(42) will trigger warning CS9193: Argument 1 should be a variable because it is passed to a ‘ref readonly’ parameter

Let’s take them one by one.

FooRef(x) – using ref or in

This is one of the improvements over using the in modifier. ref readonly make it explicit to the caller that the value is being passed as a reference. With in, this was not transparent to the caller and could lead to confusion.

To fix CS9192, simply change the call to explicitly specify FooRef(ref x) or FooRef(in x). The two annotations are mostly equivalent, the main difference being that in is more permissive and allows for unassignable values to be passed, while ref requires an assignable variable.

For example:

readonly int y = 43;

FooRef(in y);
FooRef(ref y);

FooRef(in y) will work without any issues, while FooRef(ref y) will trigger a compiler error saying that the ref value must be an assignable variable.

FooRef(42) – only variables are allowed

This is the other improvement which ref readonly brings over in – it will start complaining when you try to pass it an rvalue — a value without a location. This plays hand in hand with the warning above, because if you try to use FooRef(ref 42) you will instantly get a compiler error saying CS1510: A ref or out value must be an assignable variable.