• [x] Proposed
• [x] Prototype
• [x] Implementation: Started
• [ ] Specification: Not Started

Summary

The "readonly references" feature is actually a group of features that leverage the efficiency of passing variables by reference, but without exposing the data to modifications:

• in parameters
• ref readonly returns
• readonly structs
• ref/in extension methods
• ref readonly locals
• ref conditional expressions

There is an existing proposal that touches this topic https://github.com/dotnet/roslyn/issues/115 as a special case of readonly parameters without going into many details. Here I just want to acknowledge that the idea by itself is not very new.

Motivation

Prior to this feature C# did not have an efficient way of expressing a desire to pass struct variables into method calls for readonly purposes with no intention of modifying. Regular by-value argument passing implies copying, which adds unnecessary costs. That drives users to use by-ref argument passing and rely on comments/documentation to indicate that the data is not supposed to be mutated by the callee. It is not a good solution for many reasons.
The examples are numerous - vector/matrix math operators in graphics libraries like XNA are known to have ref operands purely because of performance considerations. There is code in Roslyn compiler itself that uses structs to avoid allocations and then passes them by reference to avoid copying costs.

Solution (in parameters)

Similarly to the out parameters, in parameters are passed as managed references with additional guarantees from the callee.
Unlike out parameters which must be assigned by the callee before any other use, in parameters cannot be assigned by the callee at all.

As a result in parameters allow for effectiveness of indirect argument passing without exposing arguments to mutations by the callee.

Declaring in parameters

in parameters are declared by using in keyword as a modifier in the parameter signature.

For all purposes the in parameter is treated as a readonly variable. Most of the restrictions on the use of in parameters inside the method are the same as with readonly fields.

Indeed an in parameter may represent a readonly field. Similarity of restrictions is not a coincidence.

For example fields of an in parameter which has a struct type are all recursively classified as readonly variables .

static Vector3 Add (in Vector3 v1, in Vector3 v2)
{
// not OK!!
v1 = default(Vector3);

// not OK!!
v1.X = 0;

// not OK!!
foo(ref v1.X);

// OK
return new Vector3(v1.X + v2.X, v1.Y + v2.Y, v1.Z + v2.Z);
}

• in parameters are allowed anywhere where ordinary byval parameters are allowed. This includes indexers, operators (including conversions), delegates, lambdas, local functions.
 (in int x) => x                                                     // lambda expression
TValue this[in TKey index];                                         // indexer
public static Vector3 operator +(in Vector3 x, in Vector3 y) => ... // operator

• in is not allowed in combination with out or with anything that out does not combine with.

• It is not permitted to overload on ref/out/in differences.

• It is permitted to overload on ordinary byval and in differences.

• For the purpose of OHI (Overloading, Hiding, Implementing), in behaves similarly to an out parameter. All the same rules apply. For example the overriding method will have to match in parameters with in parameters of an identity-convertible type.

• For the purpose of delegate/lambda/method group conversions, in behaves similarly to an out parameter. Lambdas and applicable method group conversion candidates will have to match in parameters of the target delegate with in parameters of an identity-convertible type.

• For the purpose of generic variance, in parameters are nonvariant.

NOTE: There are no warnings on in parameters that have reference or primitives types. It may be pointless in general, but in some cases user must/want to pass primitives as in. Examples - overriding a generic method like Method(in T param) when T was substituted to be int, or when having methods like Volatile.Read(in int location)

It is conceivable to have an analyzer that warns in cases of inefficient use of in parameters, but the rules for such analysis would be too fuzzy to be a part of a language specification.

Use of in at call sites. (in arguments)

There are two ways to pass arguments to in parameters.

in arguments can match in parameters:

An argument with an in modifier at the call site can match in parameters.

int x = 1;

void M1<T>(in T x)
{
// . . .
}

var x = M1(in x);  // in argument to a method

class D
{
public string this[in Guid index];
}

D dictionary = . . . ;
var y = dictionary[in Guid.Empty]; // in argument to an indexer

• in argument must be a readable LValue(*). Example: M1(in 42) is invalid

(*) The notion of LValue/RValue vary between languages.
Here, by LValue I mean an expression that represent a location that can be referred to directly. And RValue means an expression that yields a temporary result which does not persist on its own.

• In particular it is valid to pass readonly fields, in parameters or other formally readonly variables as in arguments. Example: dictionary[in Guid.Empty] is legal. Guid.Empty is a static readonly field.

• in argument must have type identity-convertible to the type of the parameter. Example: M1<object>(in Guid.Empty) is invalid. Guid.Empty is not identity-convertible to object

The motivation for the above rules is that in arguments guarantee aliasing of the argument variable. The callee always receives a direct reference to the same location as represented by the argument.

• in rare situations when in arguments must be stack-spilled due to await expressions used as operands of the same call, the behavior is the same as with out and ref arguments - if the variable cannot be spilled in referentially-transparent manner, an error is reported.

Examples:

1. M1(in staticField, await SomethingAsync()) is valid. staticField is a static field which can be accessed more than once without observable side effects. Therefore both the order of side effects and aliasing requirements can be provided.
2. M1(in RefReturningMethod(), await SomethingAsync()) will produce an error. RefReturningMethod() is a ref returning method. A method call may have observable side effects, therefore it must be evaluated before the SomethingAsync() operand. However the result of the invocation is a reference that cannot be preserved across the await suspension point which make the direct reference requirement impossible.

NOTE: the stack spilling errors are considered to be implementation-specific limitations. Therefore they do not have effect on overload resolution or lambda inference.

Ordinary byval arguments can match in parameters:

Regular arguments without modifiers can match in parameters. In such case the arguments have the same relaxed constraints as an ordinary byval arguments would have.

The motivation for this scenario is that in parameters in APIs may result in inconveniences for the user when arguments cannot be passed as a direct reference - ex: literals, computed or await-ed results or arguments that happen to have more specific types.
All these cases have a trivial solution of storing the argument value in a temporary local of appropriate type and passing that local as an in argument.
To reduce the need for such boilerplate code compiler can perform the same transformation, if needed, when in modifier is not present at the call site.

In addition, in some cases, such as invocation of operators, or in extension methods, there is no syntactical way to specify in at all. That alone requires specifying the behavior of ordinary byval arguments when they match in parameters.

In particular:

• it is valid to pass RValues. A reference to a temporary is passed in such case. Example:
Print("hello");      // not an error.

void Print<T>(in T x)
{
//. . .
}

• implicit conversions are allowed.

This is actually a special case of passing an RValue

A reference to a temporary holding converted value is passed in such case. Example:

Print<int>(Short.MaxValue)     // not an error.

• in a case of a receiver of an in extension method (as opposed to ref extension methods), RValues or implicit this-argument-conversions are allowed. A reference to a temporary holding converted value is passed in such case. Example:
public static IEnumerable<T> Concat<T>(in this (IEnumerable<T>, IEnumerable<T>) arg)  => . . .;

("aa", "bb").Concat<char>()    // not an error.


More information on ref/in extension methods is provided further in this document.

• argument spilling due to await operands could spill "by-value", if necessary. In scenarios where providing a direct reference to the argument is not possible due to intervening await a copy of the argument's value is spilled instead.
Example:
M1(RefReturningMethod(), await SomethingAsync())   // not an error.


Since the result of a side-effecting invocation is a reference that cannot be preserved across await suspension, a temporary containing the actual value will be preserved instead (as it would in an ordinary byval parameter case).

Omitted optional arguments

It is permitted for an in parameter to specify a default value. That makes the corresponding argument optional.

Omitting optional argument at the call site results in passing the default value via a temporary.

Print("hello");      // not an error, same as
Print("hello", c: Color.Black);

void Print(string s, in Color c = Color.Black)
{
// . . .
}


Aliasing behavior in general

Just like ref and out variables, in variables are references/aliases to existing locations.

While callee is not allowed to write into them, reading an in parameter can observe different values as a side effect of other evaluations.

Example:

static Vector3 v = Vector3.UnitY;

static void Main()
{
Test(v);
}

static void Test(in Vector3 v1)
{
Debug.Assert(v1 == Vector3.UnitY);
// changes v1 deterministically (no races required)
ChangeV();
Debug.Assert(v1 == Vector3.UnitX);
}

static void ChangeV()
{
v = Vector3.UnitX;
}


in parameters and capturing of local variables.

For the purpose of lambda/async capturing in parameters behave the same as out and ref parameters.

• in parameters cannot be captured in a closure
• in parameters are not allowed in iterator methods
• in parameters are not allowed in async methods

Temporary variables.

Some uses of in parameter passing may require indirect use of a temporary local variable:

• in arguments are always passed as direct aliases when call-site uses in. Temporary is never used in such case.
• in arguments are not required to be direct aliases when call-site does not use in. When argument is not an LValue, a temporary may be used.
• in parameter may have default value. When corresponding argument is omitted at the call site, the default value are passed via a temporary.
• in arguments may have implicit conversions, including those that do not preserve identity. A temporary is used in those cases.
• receivers of ordinary struct calls may not be writeable LValues (existing case!). A temporary is used in those cases.

The life time of the argument temporaries matches the closest encompassing scope of the call-site.

The formal life time of temporary variables is semantically significant in scenarios involving escape analysis of variables returned by reference.

Metadata representation of in parameters.

When System.Runtime.CompilerServices.IsReadOnlyAttribute is applied to a byref parameter, it means that the parameter is an in parameter.

In addition, if the method is abstract or virtual, then the signature of such parameters (and only such parameters) must have modreq[System.Runtime.InteropServices.InAttribute].

Motivation: this is done to ensure that in a case of method overriding/implementing the in parameters match.

Same requirements apply to Invoke methods in delegates.

Motivation: this is to ensure that existing compilers cannot simply ignore readonly when creating or assigning delegates.

Motivation

The motivation for this sub-feature is roughly symmetrical to the reasons for the in parameters - avoiding copying, but on the returning side. Prior to this feature, a method or an indexer had two options: 1) return by reference and be exposed to possible mutations or 2) return by value which results in copying.

Solution (ref readonly returns)

The feature allows a member to return variables by reference without exposing them to mutations.

Declaring ref readonly returning members

A combination of modifiers ref readonly on the return signature is used to to indicate that the member returns a readonly reference.

For all purposes a ref readonly member is treated as a readonly variable - similar to readonly fields and in parameters.

For example fields of ref readonly member which has a struct type are all recursively classified as readonly variables. - It is permitted to pass them as in arguments, but not as ref or out arguments.

ref readonly Guid Method1()
{
}

Method2(in Method1()); // valid. Can pass as in argument.

Method3(ref Method1()); // not valid. Cannot pass as ref argument

• ref readonly returns are allowed in the same places were ref returns are allowed. This includes indexers, delegates, lambdas, local functions.

• It is not permitted to overload on ref/ref readonly / differences.

• It is permitted to overload on ordinary byval and ref readonly return differences.

• For the purpose of OHI (Overloading, Hiding, Implementing), ref readonly is similar but distinct from ref. For example the a method that overrides ref readonly one, must itself be ref readonly and have identity-convertible type.

• For the purpose of delegate/lambda/method group conversions, ref readonly is similar but distinct from ref. Lambdas and applicable method group conversion candidates have to match ref readonly return of the target delegate with ref readonly return of the type that is identity-convertible.

• For the purpose of generic variance, ref readonly returns are nonvariant.

NOTE: There are no warnings on ref readonly returns that have reference or primitives types. It may be pointless in general, but in some cases user must/want to pass primitives as in. Examples - overriding a generic method like ref readonly T Method() when T was substituted to be int.

It is conceivable to have an analyzer that warns in cases of inefficient use of ref readonly returns, but the rules for such analysis would be too fuzzy to be a part of a language specification.

Returning from ref readonly members

Inside the method body the syntax is the same as with regular ref returns. The readonly will be inferred from the containing method.

The motivation is that return ref readonly <expression> is unnecessary long and only allows for mismatches on the readonly part that would always result in errors. The ref is, however, required for consistency with other scenarios where something is passed via strict aliasing vs. by value.

Unlike the case with in parameters, ref readonly returns never return via a local copy. Considering that the copy would cease to exist immediately upon returning such practice would be pointless and dangerous. Therefore ref readonly returns are always direct references.

Example:

struct ImmutableArray<T>
{

public ref readonly T ItemRef(int i)
{
// returning a readonly reference to an array element
return ref this.array[i];
}
}


• An argument of return ref must be an LValue (existing rule)
• An argument of return ref must be "safe to return" (existing rule)
• In a ref readonly member an argument of return ref is not required to be writeable . For example such member can ref-return a readonly field or one of its in parameters.

Safe to Return rules.

Normal safe to return rules for references will apply to readonly references as well.

Note that a ref readonly can be obtained from a regular ref local/parameter/return, but not the other way around. Otherwise the safety of ref readonly returns is inferred the same way as for regular ref returns.

Considering that RValues can be passed as in parameter and returned as ref readonly we need one more rule - RValues are not safe-to-return by reference.

Consider the situation when an RValue is passed to an in parameter via a copy and then returned back in a form of a ref readonly. In the context of the caller the result of such invocation is a reference to local data and as such is unsafe to return. Once RValues are not safe to return, the existing rule #6 already handles this case.

Example:

ref readonly Vector3 Test1()
{
// can pass an RValue as "in" (via a temp copy)
// but the result is not safe to return
// because the RValue argument was not safe to return by reference
return ref Test2(default(Vector3));
}

ref readonly Vector3 Test2(in Vector3 r)
{
// this is ok, r is returnable
return ref r;
}


Updated safe to return rules:

1. refs to variables on the heap are safe to return
2. ref/in parameters are safe to return in parameters naturally can only be returned as readonly.
3. out parameters are safe to return (but must be definitely assigned, as is already the case today)
4. instance struct fields are safe to return as long as the receiver is safe to return
5. 'this' is not safe to return from struct members
6. a ref, returned from another method is safe to return if all refs/outs passed to that method as formal parameters were safe to return. Specifically it is irrelevant if receiver is safe to return, regardless whether receiver is a struct, class or typed as a generic type parameter.
7. RValues are not safe to return by reference. Specifically RValues are safe to pass as in parameters.

NOTE: There are additional rules regarding safety of returns that come into play when ref-like types and ref-reassignments are involved. The rules equally apply to ref and ref readonly members and therefore are not mentioned here.

Aliasing behavior.

ref readonly members provide the same aliasing behavior as ordinary ref members (except for being readonly). Therefore for the purpose of capturing in lambdas, async, iterators, stack spilling etc... the same restrictions apply. - I.E. due to inability to capture the actual references and due to side-effecting nature of member evaluation such scenarios are disallowed.

It is permitted and required to make a copy when ref readonly return is a receiver of regular struct methods, which take this as an ordinary writeable reference. Historically in all cases where such invocations are applied to readonly variable a local copy is made.

When System.Runtime.CompilerServices.IsReadOnlyAttribute is applied to the return of a byref returning method, it means that the method returns a readonly reference.

In addition, the result signature of such methods (and only those methods) must have modreq[System.Runtime.CompilerServices.IsReadOnlyAttribute].

Motivation: this is to ensure that existing compilers cannot simply ignore readonly when invoking methods with ref readonly returns

In short - a feature that makes this parameter of all instance members of a struct, except for constructors, an in parameter.

Motivation

Compiler must assume that any method call on a struct instance may modify the instance. Indeed a writeable reference is passed to the method as this parameter and fully enables this behavior. To allow such invocations on readonly variables, the invocations are applied to temp copies. That could be unintuitive and sometimes forces people to abandon readonly for performance reasons.

After adding support for in parameters and ref readonly returns the problem of defensive copying will get worse since readonly variables will become more common.

Solution

Allow readonly modifier on struct declarations which would result in this being treated as in parameter on all struct instance methods except for constructors.

static void Test(in Vector3 v1)
{
// no need to make a copy of v1 since Vector3 is a readonly struct
System.Console.WriteLine(v1.ToString());
}

{
. . .

public override string ToString()
{
// not OK!!  this is an in parameter
foo(ref this.X);

// OK
return \$"X: {X}, Y: {Y}, Z: {Z}";
}
}


Restrictions on members of readonly struct

Motivation: can only be written to externally, but not through members.
• Instance autoproperties of a readonly struct must be get-only.
Motivation: consequence of restriction on instance fields.
• Readonly struct may not declare field-like events.
Motivation: consequence of restriction on instance fields.

When System.Runtime.CompilerServices.IsReadOnlyAttribute is applied to a value type, it means that the type is a readonly struct.

In particular:

• The identity of the IsReadOnlyAttribute type is unimportant. In fact it can be embedded by the compiler in the containing assembly if needed.

ref/in extension methods

There is actually an existing proposal (https://github.com/dotnet/roslyn/issues/165) and corresponding prototype PR (https://github.com/dotnet/roslyn/pull/15650). I just want to acknowledge that this idea is not entirely new. It is, however, relevant here since ref readonly elegantly removes the most contentious issue about such methods - what to do with RValue receivers.

The general idea is allowing extension methods to take the this parameter by reference, as long as the type is known to be a struct type.

public static void Extension(ref this Guid self)
{
// do something
}


The reasons for writing such extension methods are primarily:

1. Avoid copying when receiver is a large struct
2. Allow mutating extension methods on structs

The reasons why we do not want to allow this on classes

1. It would be of very limited purpose.
2. It would break long standing invariant that a method call cannot turn non-null receiver to become null after invocation.

In fact, currently a non-null variable cannot become null unless explicitly assigned or passed by ref or out. That greatly aids readability or other forms of "can this be a null here" analysis. 3. It would be hard to reconcile with "evaluate once" semantics of null-conditional accesses. Example: obj.stringField?.RefExtension(...) - need to capture a copy of stringField to make the null check meaningful, but then assignments to this inside RefExtension would not be reflected back to the field.

An ability to declare extension methods on structs that take the first argument by reference was a long-standing request. One of the blocking consideration was "what happens if receiver is not an LValue?".

• There is a precedent that any extension method could also be called as a static method (sometimes it is the only way to resolve ambiguity). It would dictate that RValue receivers should be disallowed.
• On the other hand there is a practice of making invocation on a copy in similar situations when struct instance methods are involved.

The reason why the "implicit copying" exists is because the majority of struct methods do not actually modify the struct while not being able to indicate that. Therefore the most practical solution was to just make the invocation on a copy, but this practice is known for harming performance and causing bugs.

Now, with availability of in parameters, it is possible for an extension to signal the intent. Therefore the conundrum can be resolved by requiring ref extensions to be called with writeable receivers while in extensions permit implicit copying if necessary.

// this can be called on either RValue or an LValue
public static void Reader(in this Guid self)
{
// do something nonmutating.
WriteLine(self == default(Guid));
}

// this can be called only on an LValue
public static void Mutator(ref this Guid self)
{
// can mutate self
self = new Guid();
}


in extensions and generics.

The purpose of ref extension methods is to mutate the receiver directly or by invoking mutating members. Therefore ref this T extensions are allowed as long as T is constrained to be a struct.

On the other hand in extension methods exist specifically to reduce implicit copying. However any use of an in T parameter will have to be done through an interface member. Since all interface members are considered mutating, any such use would require a copy. - Instead of reducing copying, the effect would be the opposite. Therefore in this T is not allowed when T is a generic type parameter regardless of constraints.

Valid kinds of extension methods (recap):

The following forms of this declaration in an extension method are now allowed:

1. this T arg - regular byval extension. (existing case)
• T can be any type, including reference types or type parameters. Instance will be the same variable after the call. Allows implicit conversions of this-argument-conversion kind. Can be called on RValues.

• in this T self - in extension. T must be an actual struct type. Instance will be the same variable after the call. Allows implicit conversions of this-argument-conversion kind. Can be called on RValues (may be invoked on a temp if needed).

• ref this T self - ref extension. T must be a struct type or a generic type parameter constrained to be a struct. Instance may be written to by the invocation. Allows only identity conversions. Must be called on writeable LValue. (never invoked via a temp).

Motivation.

Once ref readonly members were introduced, it was clear from the use that they need to be paired with appropriate kind of local. Evaluation of a member may produce or observe side effects, therefore if the result must be used more than once, it needs to be stored. Ordinary ref locals do not help here since they cannot be assigned a readonly reference.

Solution.

Allow declaring ref readonly locals. This is a new kind of ref locals that is not writeable. As a result ref readonly locals can accept references to readonly variables without exposing these variables to writes.

Declaring and using ref readonly locals.

The syntax of such locals uses ref readonly modifiers at declaration site (in that specific order). Similarly to ordinary ref locals, ref readonly locals must be ref-initialized at declaration. Unlike regular ref locals, ref readonly locals can refer to readonly LValues like in parameters, readonly fields, ref readonly methods.

For all purposes a ref readonly local is treated as a readonly variable. Most of the restrictions on the use are the same as with readonly fields or in parameters.

For example fields of an in parameter which has a struct type are all recursively classified as readonly variables .

static readonly ref Vector3 M1() => . . .

{
// OK
ref readonly var r1 = ref M1();

// Not valid. Need an LValue
ref readonly Vector3 r2 = ref default(Vector3);

// Not valid. r1 is readonly.
Mutate(ref r1);

// OK.
Print(in r1);

// OK.
return ref r1;
}


Restrictions on use of ref readonly locals

Except for their readonly nature, ref readonly locals behave like ordinary ref locals and are subject to exactly same restrictions.
For example restrictions related to capturing in closures, declaring in async methods or the safe-to-return analysis equally applies to ref readonly locals.

Ternary ref expressions. (aka "Conditional LValues")

Motivation

Use of ref and ref readonly locals exposed a need to ref-initialize such locals with one or another target variable based on a condition.

A typical workaround is to introduce a method like:

ref T Choice(bool condition, ref T consequence, ref T alternative)
{
if (condition)
{
return ref consequence;
}
else
{
return ref alternative;
}
}


Note that Choice is not an exact replacement of a ternary since all arguments must be evaluated at the call site, which was leading to unintuitive behavior and bugs.

The following will not work as expected:

    // will crash with NRE because 'arr[0]' will be executed unconditionally
ref var r = ref Choice(arr != null, ref arr[0], ref otherArr[0]);


Solution

Allow special kind of conditional expression that evaluates to a reference to one of LValue argument based on a condition.

Using ref ternary expression.

The syntax for the ref flavor of a conditional expression is <condition> ? ref <consequence> : ref <alternative>;

Just like with the ordinary conditional expression only <consequence> or <alternative> is evaluated depending on result of the boolean condition expression.

Unlike ordinary conditional expression, ref conditional expression:

• requires that <consequence> and <alternative> are LValues.
• ref conditional expression itself is an LValue and
• ref conditional expression is writeable if both <consequence> and <alternative> are writeable LValues

Examples:
ref ternary is an LValue and as such it can be passed/assigned/returned by reference;

     // pass by reference
foo(ref (arr != null ? ref arr[0]: ref otherArr[0]));

// return by reference
return ref (arr != null ? ref arr[0]: ref otherArr[0]);


Being an LValue, it can also be assigned to.

     // assign to
(arr != null ? ref arr[0]: ref otherArr[0]) = 1;

(arr != null ? ref arr[0]: ref obj.readOnlyField) = 1;


Can be used as a receiver of a method call and skip copying if necessary.

     // no copies
(arr != null ? ref arr[0]: ref otherArr[0]).StructMethod();

// invoked on a copy.
// The receiver is readonly because readOnlyField is readonly.
(arr != null ? ref arr[0]: ref obj.readOnlyField).StructMethod();

// no copies. ReadonlyStructMethod is a method on a readonly struct


ref ternary can be used in a regular (not ref) context as well.

     // only an example
// a regular ternary could work here just the same
int x = (arr != null ? ref arr[0]: ref otherArr[0]);


Drawbacks

I can see two major arguments against enhanced support for references and readonly references:

1. The problems that are solved here are very old. Why suddenly solve them now, especially since it would not help existing code?

As we find C# and .Net used in new domains, some problems become more prominent.
As examples of environments that are more critical than average about computation overheads, I can list

• cloud/datacenter scenarios where computation is billed for and responsiveness is a competitive advantage.
• Games/VR/AR with soft-realtime requirements on latencies

This feature does not sacrifice any of the existing strengths such as type-safety, while allowing to lower overheads in some common scenarios.

1. Can we reasonably guarantee that the callee will play by the rules when it opts into readonly contracts?

We have similar trust when using out. Incorrect implementation of out can cause unspecified behavior, but in reality it rarely happens.

Making the formal verification rules familiar with ref readonly would further mitigate the trust issue.

Alternatives

The main competing design is really "do nothing".