This document lists known breaking changes in Roslyn after .NET 6 all the way to .NET 7.

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11/17/2022
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Inferred delegate type for methods includes default parameter values and `params` modifier

Introduced in Visual Studio 2022 version 17.5

In .NET SDK 7.0.100 or earlier, delegate types inferred from methods ignored default parameter values and params modifiers as demonstrated in the following code:

void Method(int i = 0, params int[] xs) { }
var action = Method; // System.Action<int, int[]>
DoAction(action, 1); // ok
void DoAction(System.Action<int, int[]> a, int p) => a(p, new[] { p });

In .NET SDK 7.0.200 or later, such methods are inferred as anonymous synthesized delegate types with the same default parameter values and params modifiers. This change can break the code above as demonstrated below:

void Method(int i = 0, params int[] xs) { }
var action = Method; // delegate void <anonymous delegate>(int arg1 = 0, params int[] arg2)
DoAction(action, 1); // error CS1503: Argument 1: cannot convert from '<anonymous delegate>' to 'System.Action<int, int[]>'
void DoAction(System.Action<int, int[]> a, int p) => a(p, new[] { p });

You can learn more about this change in the associated proposal.

For the purpose of definite assignment analysis, invocations of async local functions are no longer treated as being awaited

Introduced in Visual Studio 2022 version 17.5

For the purpose of definite assignment analysis, invocations of an async local function is no longer treated as being awaited and, therefore, the local function is not considered to be fully executed. See https://github.com/dotnet/roslyn/issues/43697 for the rationale.

The code below is now going to report a definite assignment error:

    public async Task M()
    {
        bool a;
        await M1();
        Console.WriteLine(a); // error CS0165: Use of unassigned local variable 'a'  

        async Task M1()
        {
            if ("" == String.Empty)
            {
                throw new Exception();
            }
            else
            {
                a = true;
            }
        }
    }

`INoneOperation` nodes for attributes are now `IAttributeOperation` nodes.

Introduced in Visual Studio 2022 version 17.5, .NET SDK version 7.0.200

In previous versions of the compiler, the IOperation tree for an attribute was rooted with an INoneOperation node. We have added native support for attributes, which means that the root of the tree is now an IAttributeOperation. Some analyzers, including older versions of the .NET SDK analyzers, are not expecting this tree shape, and will incorrectly warn (or potentially fail to warn) when encountering it. The workarounds for this are:

Update your analyzer version, if possible. If using the .NET SDK or older versions of Microsoft.CodeAnalysis.FxCopAnalyzers, update to Microsoft.CodeAnalysis.NetAnalyzers 7.0.0-preview1.22464.1 or newer.
Suppress any false-positives from the analyzers until they can be updated with a version that takes this change into account.

Type tests for `ref` structs are not supported.

Introduced in Visual Studio 2022 version 17.4

When a ref struct type is used in an 'is' or 'as' operator, in some scenarios compiler was previously reporting an erroneous warning about the type test always failing at runtime, omitting the actual type check, and leading to incorrect behavior. When incorrect behavior at execution time was possible, compiler will now produce an error instead.

ref struct G<T>
{
    public void Test()
    {
        if (this is G<int>) // Will now produce an error, used to be treated as always `false`.
        {

Unused results from ref local are dereferences.

Introduced in Visual Studio 2022 version 17.4

When a ref local variable is referenced by value, but the result is not used (such as being assigned to a discard), the result was previously ignored. The compiler will now dereference that local, ensuring that any side effects are observed.

ref int local = Unsafe.NullRef<int>();
_ = local; // Will now produce a `NullReferenceException`

Types cannot be named `scoped`

Introduced in Visual Studio 2022 version 17.4. Starting in C# 11, types cannot be named scoped. The compiler will report an error on all such type names. To work around this, the type name and all usages must be escaped with an @:

class scoped {} // Error CS9056
class @scoped {} // No error

ref scoped local; // Error
ref scoped.nested local; // Error
ref @scoped local2; // No error

This was done as scoped is now a modifier for variable declarations and reserved following ref in a ref type.

Types cannot be named `file`

Introduced in Visual Studio 2022 version 17.4. Starting in C# 11, types cannot be named file. The compiler will report an error on all such type names. To work around this, the type name and all usages must be escaped with an @:

class file {} // Error CS9056
class @file {} // No error

This was done as file is now a modifier for type declarations.

You can learn more about this change in the associated csharplang issue.

Required spaces in #line span directives

Introduced in .NET SDK 6.0.400, Visual Studio 2022 version 17.3.

When the #line span directive was introduced in C# 10, it required no particular spacing.
For example, this would be valid: #line(1,2)-(3,4)5"file.cs".

In Visual Studio 17.3, the compiler requires spaces before the first parenthesis, the character offset, and the file name.
So the above example fails to parse unless spaces are added: #line (1,2)-(3,4) 5 "file.cs".

Checked operators on System.IntPtr and System.UIntPtr

Introduced in .NET SDK 7.0.100, Visual Studio 2022 version 17.3.

When the platform supports numeric IntPtr and UIntPtr types (as indicated by the presence of System.Runtime.CompilerServices.RuntimeFeature.NumericIntPtr) the built-in operators from nint and nuint apply to those underlying types. This means that on such platforms, IntPtr and UIntPtr have built-in checked operators, which can now throw when an overflow occurs.

IntPtr M(IntPtr x, int y)
{
    checked
    {
        return x + y; // may now throw
    }
}

unsafe IntPtr M2(void* ptr)
{
    return checked((IntPtr)ptr); // may now throw
}

Possible workarounds are:

Specify unchecked context
Downgrade to a platform/TFM without numeric IntPtr/UIntPtr types

Also, implicit conversions between IntPtr/UIntPtr and other numeric types are treated as standard conversions on such platforms. This can affect overload resolution in some cases.

These changes could cause a behavioral change if the user code was depending on overflow exceptions in an unchecked context, or if it was not expecting overflow exceptions in a checked context. An analyzer was added in 7.0 to help detect such behavioral changes and take appropriate action. The analyzer will produce diagnostics on potential behavioral changes, which default to info severity but can be upgraded to warnings via editorconfig.

Addition of System.UIntPtr and System.Int32

Introduced in .NET SDK 7.0.100, Visual Studio 2022 version 17.3.

When the platform supports numeric IntPtr and UIntPtr types (as indicated by the presence of System.Runtime.CompilerServices.RuntimeFeature.NumericIntPtr), the operator +(UIntPtr, int) defined in System.UIntPtr can no longer be used. Instead, adding expressions of types System.UIntPtr and a System.Int32 results in an error:

UIntPtr M(UIntPtr x, int y)
{
    return x + y; // error: Operator '+' is ambiguous on operands of type 'nuint' and 'int'
}

Possible workarounds are:

Use the UIntPtr.Add(UIntPtr, int) method: UIntPtr.Add(x, y)
Apply an unchecked cast to type nuint on the second operand: x + unchecked((nuint)y)

Nameof operator in attribute on method or local function

Introduced in .NET SDK 6.0.400, Visual Studio 2022 version 17.3.

When the language version is C# 11 or later, a nameof operator in an attribute on a method brings the type parameters of that method in scope. The same applies for local functions.
A nameof operator in an attribute on a method, its type parameters or parameters brings the parameters of that method in scope. The same applies to local functions, lambdas, delegates and indexers.

For instance, these will now be errors:

class C
{
  class TParameter
  {
    internal const string Constant = """";
  }
  [MyAttribute(nameof(TParameter.Constant))]
  void M<TParameter>() { }
}

class C
{
  class parameter
  {
    internal const string Constant = """";
  }
  [MyAttribute(nameof(parameter.Constant))]
  void M(int parameter) { }
}

Possible workarounds are:

Rename the type parameter or parameter to avoid shadowing the name from outer scope.
Use a string literal instead of the nameof operator.

Cannot return an out parameter by reference

Introduced in .NET SDK 7.0.100, Visual Studio 2022 version 17.3.

With language version C# 11 or later, or with .NET 7.0 or later, an out parameter cannot be returned by reference.

static ref T ReturnOutParamByRef<T>(out T t)
{
    t = default;
    return ref t; // error CS8166: Cannot return a parameter by reference 't' because it is not a ref parameter
}

Possible workarounds are:

Use System.Diagnostics.CodeAnalysis.UnscopedRefAttribute to mark the reference as unscoped.

static ref T ReturnOutParamByRef<T>([UnscopedRef] out T t)
{
    t = default;
    return ref t; // ok
}

Change the method signature to pass the parameter by ref.

static ref T ReturnRefParamByRef<T>(ref T t)
{
    t = default;
    return ref t; // ok
}

Instance method on ref struct may capture unscoped ref parameters

Introduced in .NET SDK 7.0.100, Visual Studio 2022 version 17.4.

With language version C# 11 or later, or with .NET 7.0 or later, a ref struct instance method invocation is assumed to capture unscoped ref or in parameters.

R<int> Use(R<int> r)
{
    int i = 42;
    r.MayCaptureArg(ref i); // error CS8350: may expose variables referenced by parameter 't' outside of their declaration scope
    return r;
}

ref struct R<T>
{
    public void MayCaptureArg(ref T t) { }
}

A possible workaround, if the ref or in parameter is not captured in the ref struct instance method, is to declare the parameter as scoped ref or scoped in.

R<int> Use(R<int> r)
{
    int i = 42;
    r.CannotCaptureArg(ref i); // ok
    return r;
}

ref struct R<T>
{
    public void CannotCaptureArg(scoped ref T t) { }
}

Method ref struct return escape analysis depends on ref escape of ref arguments

Introduced in .NET SDK 7.0.100, Visual Studio 2022 version 17.4.

With language version C# 11 or later, or with .NET 7.0 or later, a ref struct returned from a method invocation, either as a return value or in an out parameters, is only safe-to-escape if all the ref and in arguments to the method invocation are ref-safe-to-escape. The in arguments may include implicit default parameter values.

ref struct R { }

static R MayCaptureArg(ref int i) => new R();

static R MayCaptureDefaultArg(in int i = 0) => new R();

static R Create()
{
    int i = 0;
    // error CS8347: Cannot use a result of 'MayCaptureArg(ref int)' because it may expose
    // variables referenced by parameter 'i' outside of their declaration scope
    return MayCaptureArg(ref i);
}

static R CreateDefault()
{
    // error CS8347: Cannot use a result of 'MayCaptureDefaultArg(in int)' because it may expose
    // variables referenced by parameter 'i' outside of their declaration scope
    return MayCaptureDefaultArg();
}

A possible workaround, if the ref or in argument is not captured in the ref struct return value, is to declare the parameter as scoped ref or scoped in.

static R CannotCaptureArg(scoped ref int i) => new R();

static R Create()
{
    int i = 0;
    return CannotCaptureArg(ref i); // ok
}

`ref` to `ref struct` argument considered unscoped in `__arglist`

Introduced in .NET SDK 7.0.100, Visual Studio 2022 version 17.4.

With language version C# 11 or later, or with .NET 7.0 or later, a ref to a ref struct type is considered an unscoped reference when passed as an argument to __arglist.

ref struct R { }

class Program
{
    static void MayCaptureRef(__arglist) { }

    static void Main()
    {
        var r = new R();
        MayCaptureRef(__arglist(ref r)); // error: may expose variables outside of their declaration scope
    }
}

Unsigned right shift operator

Introduced in .NET SDK 6.0.400, Visual Studio 2022 version 17.3. The language added support for an "Unsigned Right Shift" operator (>>>). This disables the ability to consume methods implementing user-defined "Unsigned Right Shift" operators as regular methods.

For example, there is an existing library developed in some language (other than VB or C#) that exposes an "Unsigned Right Shift" user-defined operator for type C1. The following code used to compile successfully before:

static C1 Test1(C1 x, int y) => C1.op_UnsignedRightShift(x, y); //error CS0571: 'C1.operator >>>(C1, int)': cannot explicitly call operator or accessor

A possible workaround is to switch to using >>> operator:

static C1 Test1(C1 x, int y) => x >>> y;

Foreach enumerator as a ref struct

Introduced in .NET SDK 6.0.300, Visual Studio 2022 version 17.2. A foreach using a ref struct enumerator type reports an error if the language version is set to 7.3 or earlier.

This fixes a bug where the feature was supported in newer compilers targeting a version of C# prior to its support.

Possible workarounds are:

Change the ref struct type to a struct or class type.
Upgrade the <LangVersion> element to 7.3 or later.

Async `foreach` prefers pattern based `DisposeAsync` to an explicit interface implementation of `IAsyncDisposable.DisposeAsync()`

Introduced in .NET SDK 6.0.300, Visual Studio 2022 version 17.2. An async foreach prefers to bind using a pattern-based DisposeAsync() method rather than IAsyncDisposable.DisposeAsync().

For instance, the DisposeAsync() will be picked, rather than the IAsyncEnumerator<int>.DisposeAsync() method on AsyncEnumerator:

await foreach (var i in new AsyncEnumerable())
{
}

struct AsyncEnumerable
{
    public AsyncEnumerator GetAsyncEnumerator() => new AsyncEnumerator();
}

struct AsyncEnumerator : IAsyncDisposable
{
    public int Current => 0;
    public async ValueTask<bool> MoveNextAsync()
    {
        await Task.Yield();
        return false;
    }
    public async ValueTask DisposeAsync()
    {
        Console.WriteLine("PICKED");
        await Task.Yield();
    }
    ValueTask IAsyncDisposable.DisposeAsync() => throw null; // no longer picked
}

This change fixes a spec violation where the public DisposeAsync method is visible on the declared type, whereas the explicit interface implementation is only visible using a reference to the interface type.

To workaround this error, remove the pattern based DisposeAsync method from your type.

Disallow converted strings as a default argument

Introduced in .NET SDK 6.0.300, Visual Studio 2022 version 17.2. The C# compiler would accept incorrect default argument values involving a reference conversion of a string constant, and would emit null as the constant value instead of the default value specified in source. In Visual Studio 17.2, this becomes an error. See roslyn#59806.

This change fixes a spec violation in the compiler. Default arguments must be compile time constants. Previous versions allowed the following code:

void M(IEnumerable<char> s = "hello")

The preceding declaration required a conversion from string to IEnumerable<char>. The compiler allowed this construct, and would emit null as the value of the argument. The preceding code produces a compiler error starting in 17.2.

To work around this change, you can make one of the following changes:

Change the parameter type so a conversion isn't required.
Change the value of the default argument to null to restore the previous behavior.

The contextual keyword `var` as an explicit lambda return type

Introduced in .NET SDK 6.0.200, Visual Studio 2022 version 17.1. The contextual keyword var cannot be used as an explicit lambda return type.

This change enables potential future features by ensuring that the var remains the natural type for the return type of a lambda expression.

You can encounter this error if you have a type named var and define a lambda expression using an explicit return type of var (the type).

using System;

F(var () => default);  // error CS8975: The contextual keyword 'var' cannot be used as an explicit lambda return type
F(@var () => default); // ok
F(() => default);      // ok: return type is inferred from the parameter to F()

static void F(Func<var> f) { }

public class var
{
}

Workarounds include the following changes:

Use @var as the return type.
Remove the explicit return type so that the compiler determines the return type.

Interpolated string handlers and indexer initialization

Introduced in .NET SDK 6.0.200, Visual Studio 2022 version 17.1. Indexers that take an interpolated string handler and require the receiver as an input for the constructor cannot be used in an object initializer.

This change disallows an edge case scenario where indexer initializers use an interpolated string handler and that interpolated string handler takes the receiver of the indexer as a parameter of the constructor. The reason for this change is that this scenario could result in accessing variables that haven't yet been initialized. Consider this example:

using System.Runtime.CompilerServices;

// error: Interpolated string handler conversions that reference
// the instance being indexed cannot be used in indexer member initializers.
var c = new C { [$""] = 1 }; 

class C
{
    public int this[[InterpolatedStringHandlerArgument("")] CustomHandler c]
    {
        get => ...;
        set => ...;
    }
}

[InterpolatedStringHandler]
class CustomHandler
{
    // The constructor of the string handler takes a "C" instance:
    public CustomHandler(int literalLength, int formattedCount, C c) {}
}

Workarounds include the following changes:

Remove the receiver type from the interpolated string handler.
Change the argument to the indexer to be a string

ref, readonly ref, in, out not allowed as parameters or return on methods with Unmanaged callers only

Introduced in .NET SDK 6.0.200, Visual Studio 2022 version 17.1. ref/ref readonly/in/out are not allowed to be used on return/parameters of a method attributed with UnmanagedCallersOnly.

This change is a bug fix. Return values and parameters aren't blittable. Passing arguments or return values by reference can cause undefined behavior. None of the following declarations will compile:

using System.Runtime.InteropServices;
[UnmanagedCallersOnly]
static ref int M1() => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.

[UnmanagedCallersOnly]
static ref readonly int M2() => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.

[UnmanagedCallersOnly]
static void M3(ref int o) => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.

[UnmanagedCallersOnly]
static void M4(in int o) => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.

[UnmanagedCallersOnly]
static void M5(out int o) => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.

The workaround is to remove the by reference modifier.

Length, Count assumed to be non-negative in patterns

Introduced in .NET SDK 6.0.200, Visual Studio 2022 version 17.1. Length and Count properties on countable and indexable types are assumed to be non-negative for purpose of subsumption and exhaustiveness analysis of patterns and switches. Those types can be used with implicit Index indexer and list patterns.

The Length and Count properties, even though typed as int, are assumed to be non-negative when analyzing patterns. Consider this sample method:

string SampleSizeMessage<T>(IList<T> samples)
{
    return samples switch
    {
        // This switch arm prevents a warning before 17.1, but will never happen in practice.
        // Starting with 17.1, this switch arm produces a compiler error.
        // Removing it won't introduce a warning.
        { Count: < 0 }    => throw new InvalidOperationException(),
        { Count:  0 }     => "Empty collection",
        { Count: < 5 }    => "Too small",
        { Count: < 20 }   => "reasonable for the first pass",
        { Count: < 100 }  => "reasonable",
        { Count: >= 100 } => "fine",
    };
}

void M(int[] i)
{
    if (i is { Length: -1 }) {} // error: impossible under assumption of non-negative length
}

Prior to 17.1, The first switch arm, testing that Count is negative was necessary to avoid a warning that all possible values weren't covered. Starting with 17.1, the first switch arm generates a compiler error. The workaround is to remove the switch arms added for the invalid cases.

This change was made as part of adding list patterns. The processing rules are more consistent if every use of a Length or Count property on a collection are considered non-negative. You can read more details about the change in the language design issue.

The workaround is to remove the switch arms with unreachable conditions.

Adding field initializers to a struct requires an explicitly declared constructor

Introduced in .NET SDK 6.0.200, Visual Studio 2022 version 17.1. struct type declarations with field initializers must include an explicitly declared constructor. Additionally, all fields must be definitely assigned in struct instance constructors that do not have a : this() initializer so any previously unassigned fields must be assigned from the added constructor or from field initializers. See dotnet/csharplang#5552, dotnet/roslyn#58581.

There are two ways to initialize a variable to its default value in C#: new() and default. For classes, the difference is evident since new creates a new instance and default returns null. The difference is more subtle for structs, since for default, structs return an instance with each field/property set to its own default. We added field initializers for structs in C# 10. Field initializers are executed only when an explicitly declared constructor runs. Significantly, they don't execute when you use default or create an array of any struct type.

In 17.0, if there are field initializers but no declared constructors, a parameterless constructor is synthesized that runs field initializers. However, that meant adding or removing a constructor declaration may affect whether a parameterless constructor is synthesized, and as a result, may change the behavior of new().

To address the issue, in .NET SDK 6.0.200 (VS 17.1) the compiler no longer synthesizes a parameterless constructor. If a struct contains field initializers and no explicit constructors, the compiler generates an error. If a struct has field initializers it must declare a constructor, because otherwise the field initializers are never executed.

Additionally, all fields that do not have field initializers must be assigned in each struct constructor unless the constructor has a : this() initializer.

For instance:

struct S // error CS8983: A 'struct' with field initializers must include an explicitly declared constructor.
{
    int X = 1; 
    int Y;
}

The workaround is to declare a constructor. Unless fields were previously unassigned, this constructor can, and often will, be an empty parameterless constructor:

struct S
{
    int X = 1;
    int Y;

    public S() { Y = 0; } // ok
}

Format specifiers can't contain curly braces

Introduced in .NET SDK 6.0.200, Visual Studio 2022 version 17.1. Format specifiers in interpolated strings can not contain curly braces (either { or }). In previous versions {{ was interpreted as an escaped { and }} was interpreted as an escaped } char in the format specifier. Now the first } char in a format specifier ends the interpolation, and any { char is an error.

This makes interpolated string processing consistent with the processing for System.String.Format:

using System;
Console.WriteLine($"{{{12:X}}}");
//prints now: "{C}" - not "{X}}"

X is the format for uppercase hexadecimal and C is the hexadecimal value for 12.

The workaround is to remove the extra braces in the format string.

You can learn more about this change in the associated roslyn issue.

Types cannot be named `required`

Introduced in Visual Studio 2022 version 17.3. Starting in C# 11, types cannot be named required. The compiler will report an error on all such type names. To work around this, the type name and all usages must be escaped with an @:

class required {} // Error CS9029
class @required {} // No error

This was done as required is now a member modifier for properties and fields.

You can learn more about this change in the associated csharplang issue.

This document lists known breaking changes in Roslyn after .NET 6 all the way to .NET 7.

Inferred delegate type for methods includes default parameter values and params modifier

For the purpose of definite assignment analysis, invocations of async local functions are no longer treated as being awaited

INoneOperation nodes for attributes are now IAttributeOperation nodes.

Type tests for ref structs are not supported.

Unused results from ref local are dereferences.

Types cannot be named scoped

Types cannot be named file

Required spaces in #line span directives

Checked operators on System.IntPtr and System.UIntPtr

Addition of System.UIntPtr and System.Int32

Nameof operator in attribute on method or local function

Cannot return an out parameter by reference

Instance method on ref struct may capture unscoped ref parameters

Method ref struct return escape analysis depends on ref escape of ref arguments

ref to ref struct argument considered unscoped in __arglist

Unsigned right shift operator

Foreach enumerator as a ref struct

Async foreach prefers pattern based DisposeAsync to an explicit interface implementation of IAsyncDisposable.DisposeAsync()

Disallow converted strings as a default argument

The contextual keyword var as an explicit lambda return type

Interpolated string handlers and indexer initialization

ref, readonly ref, in, out not allowed as parameters or return on methods with Unmanaged callers only

Length, Count assumed to be non-negative in patterns

Adding field initializers to a struct requires an explicitly declared constructor

Format specifiers can't contain curly braces

Types cannot be named required

反馈

Inferred delegate type for methods includes default parameter values and `params` modifier

`INoneOperation` nodes for attributes are now `IAttributeOperation` nodes.

Type tests for `ref` structs are not supported.

Types cannot be named `scoped`

Types cannot be named `file`

`ref` to `ref struct` argument considered unscoped in `__arglist`

Async `foreach` prefers pattern based `DisposeAsync` to an explicit interface implementation of `IAsyncDisposable.DisposeAsync()`

The contextual keyword `var` as an explicit lambda return type

Types cannot be named `required`