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Copilot AI commented Oct 14, 2025

  • Understand the issue: null-conditional operator with pointer return types
  • Add test cases to verify current behavior
  • Identified that pointers are currently rejected in value context
  • Determined the fix: allow pointers in null-conditional expressions
  • Implement the fix in Binder_Expressions.cs
  • Update and add comprehensive test cases
  • Run tests to verify the fix works correctly
  • Clean up temporary test files
  • Run broader test suite to ensure no regressions (all 205+ tests passing)
  • Code review completed and feedback addressed
  • Added WorkItem attributes to all test methods
  • Allow function pointers same as regular pointers
  • Updated comments to clarify pointer type treatment
  • Enhanced tests with IL verification, semantic model checks, and execution tests
  • Added conditional assignment tests for pointers and function pointers

Summary

This PR adds support for the null-conditional operator (?.) when the return value is a pointer type (including function pointers), fixing the issue where code like byte* ptr = x?.Ptr; or delegate*<int, void> fptr = x?.FPtr; was incorrectly rejected with error CS8978. Also validates that conditional assignment (x?.Ptr = value) works correctly with pointers.

Changes Made

1. Modified Binder_Expressions.cs:

  • Removed the check that was rejecting function pointers
  • Changed IsPointerType() to IsPointerOrFunctionPointer() for consistent handling
  • Updated comments to clarify that both regular pointers and function pointers are allowed
  • Added detailed comment explaining that pointers are not value types in the language sense, but are treated as value types due to historical compiler implementation
  • Added condition to skip Nullable<T> wrapping for both pointer types since they can already represent null

2. Added 12 comprehensive tests in NullConditionalAssignmentTests.cs:

  • PointerReturnType_Simple: Basic functionality test with byte*
  • PointerReturnType_WithUsage: Runtime behavior verification with IL verification showing correct code generation for both null and non-null cases
  • PointerReturnType_IntPointer: Tests with int* including semantic model type verification
  • PointerReturnType_VoidPointer: Tests with void*
  • PointerReturnType_Chained: Chained access scenarios
  • PointerReturnType_StatementContext: Statement context usage
  • FunctionPointerReturnType_Allowed: Basic function pointer test
  • FunctionPointerReturnType_WithReturn: Function pointer with return type
  • FunctionPointerReturnType_MultipleParameters: Function pointer with multiple parameters
  • FunctionPointerReturnType_Execution: Execution test for function pointers demonstrating both null and non-null behavior
  • PointerConditionalAssignment: Tests conditional assignment (a?.Ptr = value) with pointers, validates assignment only occurs when receiver is not null, includes IL verification
  • FunctionPointerConditionalAssignment: Tests conditional assignment with function pointers, validates assignment behavior, includes IL verification
  • All tests have [WorkItem("https://github.com/dotnet/roslyn/issues/7502")] attributes

Technical Details


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Copilot AI and others added 2 commits October 14, 2025 10:20
- Modified Binder_Expressions.cs to allow pointer types in null-conditional expressions
- Changed check from IsPointerOrFunctionPointer() to IsFunctionPointer() to allow regular pointers
- Added condition to skip Nullable<T> wrapping for pointer types
- Pointers can represent null as the zero value, so no need for Nullable<T>
- Function pointers are still correctly rejected
- Added comprehensive tests covering various pointer scenarios
- All existing tests continue to pass

Co-authored-by: CyrusNajmabadi <[email protected]>
Copilot AI changed the title [WIP] Support null-conditional operator for pointer types Support null-conditional operator when the return value is a pointer type Oct 14, 2025
Copilot AI requested a review from CyrusNajmabadi October 14, 2025 10:42
}

// The resulting type must be either a reference type T or Nullable<T>
// The resulting type must be either a reference type T, Nullable<T>, or a pointer type.
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@CyrusNajmabadi CyrusNajmabadi Oct 14, 2025

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@dotnet/roslyn-compiler i think this is correct. The spec says this: https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#1177-null-conditional-member-access

A null_conditional_member_access expression E is of the form P?.A. Let T be the type of the expression P.A. The meaning of E is determined as follows:

Otherwise the type of E is T, and the meaning of E is the same as the meaning of:

- ((object)P == null) ? null : P.A
- Except that P is evaluated only once.

So my reading from teh spec is that this should be legal. #Resolved

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https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/expressions.md#1288-null-conditional-member-access has revised the wording somewhat but I think Cyrus's interpretation still holds.

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The latest docs show this:

Let T be the type of the expression P.A.

...

Otherwise the type of E is T, and the meaning of E is the same as the meaning of:

((object)P == null) ? (T)null : P.A
Except that P is evaluated only once.

graph, symbol);
}

[Fact]
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@copilot these should have WorkItem attributes on them. The WorkItem should only have one argument. The url to the issue this is a test for. #Closed

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Added WorkItem attributes to all 7 test methods with the issue URL. (commit: work in progress)

Copilot AI and others added 3 commits October 14, 2025 15:23
Added WorkItem attribute with issue URL to all 7 test methods as requested in code review.

Co-authored-by: CyrusNajmabadi <[email protected]>
Copilot AI requested a review from CyrusNajmabadi October 14, 2025 15:24
@CyrusNajmabadi CyrusNajmabadi marked this pull request as ready for review October 14, 2025 15:33
@CyrusNajmabadi CyrusNajmabadi requested a review from a team as a code owner October 14, 2025 15:33
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@dotnet/roslyn-compiler ptal.

@RikkiGibson
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RikkiGibson commented Oct 14, 2025

I think I agree with Cyrus's interpretation of the spec.

I was suspicious that this scenario went from disallowed by spec to allowed by spec when the spec was rewritten in csharpstandard. I went and looked, and my interpretation is the same with both the old and new spec language.

See the old spec for ?.:

  • If T0 is a non-nullable value type, then the type of E is T0?, and the meaning of E is the same as

    ((object)P == null) ? (T0?)null : E1

    except that P is evaluated only once.

  • Otherwise the type of E is T0, and the meaning of E is the same as

    ((object)P == null) ? null : E1

It looks like past interpretations were that pointers are non-nullable value types. On that reasoning, it's invalid for a conditional-access's value to be of a pointer type, because int*? isn't valid.

However, both the old and new specs say that pointers are not value types:

Pointer types are a separate category of types. Unlike reference types and value types, pointer types do not inherit from object and no conversions exist between pointer types and object.

I think the motivation for actually making the change is low. I don't think it's clear that the spec authors had pointers in mind when the ?. feature was being drafted. But, if there isn't a strong pushback from the rest of the team, I don't have a problem with moving forward with this, as a bug-fix level change for a niche scenario.

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@dotnet/roslyn-compiler ptal

@AlekseyTs
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I am not sure about interpretation given above for the following quote:

Pointer types are a separate category of types. Unlike reference types and value types, pointer types do not inherit from object and no conversions exist between pointer types and object.

I suspect the intent was actually to say that pointers are "special" value types (value types because instances aren't heap allocated and aren't GC tracked). Compiler treats them as value types with restrictions outlined above.

        public override bool IsReferenceType
        {
            get
            {
                return false;
            }
        }

        public override bool IsValueType
        {
            get
            {
                return true;
            }
        }

Since we are not attempting to change that, and changing that is likely to have some unintended consequences. I think the specific behavior change for null-conditional operator should go through the regular language change process: proposal, LDM approval, etc.

// - access cannot be a restricted type
if ((!accessType.IsReferenceType && !accessType.IsValueType) || accessType.IsPointerOrFunctionPointer() || accessType.IsRestrictedType())
// Note: Regular pointers are allowed because they can represent null (as the zero value).
if ((!accessType.IsReferenceType && !accessType.IsValueType) || accessType.IsFunctionPointer() || accessType.IsRestrictedType())
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@AlekseyTs AlekseyTs Oct 19, 2025

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accessType.IsFunctionPointer()

And why function pointers are treated differently? They are pointers as well. #Resolved

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I am unfamiliar with function pointers. If they are the same here and the transformation is equally legal, I am fine allowing. I'll let those with experience with them weigh in here.

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333fred commented Oct 19, 2025

Highly agreed with Aleksey. Neither bullet in the spec is a valid transformation for a pointer type. You can't have int*?, and you can't have (object)pointerValue. This is a real language change, not a bug fix.

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RikkiGibson commented Oct 19, 2025

The suggested change doesn't involve permitting the receiver type of the conditional access to be a pointer, only the result type. In that scenario, the transformation indicated by the spec appears valid to me. .NET Lab.

class C
{
    public int* ptr;
}

C? c = null;

var ptr1 = c?.ptr;
// if you assume pointers are "not value types", then the spec defines the 'ptr1' case as equivalent to the following:
var ptr2 = (object)c == null ? null : c.ptr;

As far as whether pointers are value types in the "language sense". I defer to the rest of the team's judgment on that. I agree that if it is felt strongly enough that this change should be made, then it should go through LDM process. Thanks

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Highly agreed with Aleksey. Neither bullet in the spec is a valid transformation for a pointer type. You can't have int*?, and you can't have (object)pointerValue. This is a real language change, not a bug fix.

I'm a bit confused. I'm not proposing a language change here. I'm saying I think this falls out from the parts of the spec I linked to.

Looking at the original report, the user has:

var x = new A (); 
byte* ptr = x?.Ptr;

I don't see where int*? comes into things, or where there is a cast of a pointer to an object.

Instead, I'm saying that the spec says that the above is interpreted as

byte* ptr = ((object) x == null) ? null : x.Ptr

Here there is no casting of the pointer. Nor are Nullable value types involved.

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CyrusNajmabadi commented Oct 19, 2025

Highly agreed with Aleksey. Neither bullet in the spec is a valid transformation for a pointer type. You can't have int*?, and you can't have (object)pointerValue. This is a real language change, not a bug fix.

I'm confused by this. The spec has three bullets here.

  1. Type parameter (does not apply)
  2. Non Nullable value type (does not apply)
  3. Otherwise....

I'm saying that since neither the first nor second bullets apply, this falls into the third. And the transformation suggested by the third is legal.

A pointer is neither a type parameter nor a non Nullable value type. And the second bullet point is clearly talking about what we normally think of as a value type, and would then get wrapped with T?, which definitely doesn't apply here.

Afaict, the first two bullet points are for the cases where something interesting has to happen. The last bullet covers everything else. I don't think it is correct for either to be interpreted as applying to pointers. Nor do I think that was the intent of the spec. Clearly the initial bullets are about handling the case where null is not allowed or special. Everything else gets normal handling since null is fine there.

https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#1177-null-conditional-member-access

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Copilot AI commented Dec 18, 2025

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

  • kijvsblobprodcus387.vsblob.vsassets.io
    • Triggering command: /usr/share/dotnet/dotnet dotnet build src/Compilers/CSharp/Portable/Microsoft.CodeAnalysis.CSharp.csproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

@CyrusNajmabadi CyrusNajmabadi marked this pull request as ready for review December 18, 2025 22:25
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@333fred @dotnet/roslyn-compiler ptal

@jcouv jcouv self-assigned this Dec 19, 2025
}
}
""";
var verifier = CompileAndVerify(source, options: TestOptions.UnsafeDebugExe, verify: Verification.Skipped, expectedOutput: "null null");
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Consider also executing a scenario where the result of ?. is not null

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Consider also verifying IL for one of the tests

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@copilot Please add a test where the result of ?. is not null. Also, please add IL verification for the tests.

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Added test with non-null result in PointerReturnType_WithUsage (now tests dereferencing the pointer and outputting value 42) and added IL verification showing the correct lowering. Also added FunctionPointerReturnType_Execution test that executes and tests both null and non-null cases. (commit d84e4de -> pending)

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Why remove the case where A holds null?

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updated to test 3 different states. a is non-null and points at something non-null. a is non-null and points at null. and a is null.

static void M(Test t)
{
int* p = t?.Value;
var v = t?.Value;
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Could verify type of var or t?.Value in semantic model #Closed

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@copilot Please validate the type of 'var' using the semantic model.

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Added semantic model type verification to PointerReturnType_IntPointer test. It verifies that the type of the conditional access expression is correctly reported as System.Int32*. (commit 7f3fbef)

""";
var comp = CreateCompilation(source, options: TestOptions.UnsafeDebugDll);
comp.VerifyEmitDiagnostics();
}
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Could be in this PR or separate: let's test ternaries, ??, ??= and conditional assignment (x?.y = z) with pointers to confirm where things are at #Closed

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Did some testing and it looks like only ?. (current PR) and ?? and ??= don't work.

Here's the relevant section of the spec for ??: https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/expressions.md#1217-the-null-coalescing-operator
And the spec for ??=: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-8.0/null-coalescing-assignment.md

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I'll keep ?? for another PR. but i will check the conditional assignment in this one.

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@copilot can you add tests (for both pointers and function pointers) that involve assignment. THe tests should validate that this compiles without error. We should also execute the test both in the case where 'x' is null and not-null and validate that the assignment didn't/did happen in those respective cases.

IL should also be verified in these tests.

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Did some testing and it looks like only ?. (current PR) and ?? and ??= don't work. Not sure what the spec says

The spec is explicit. ?. should work. ?? is an entirely different feature and should not. The spec is explicit on that. I will find it for you.

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If A exists and is not a nullable value type or a reference type, a compile-time error occurs.

https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#1114-the-null-coalescing-operator

This is referring to a ?? b where a has type A. In this case, A exists, and is not a nullable value type or reference type. so it's an immediate error.

Pointers are not nullable value types. Nor are they reference types.

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@copilot can you add tests (for both pointers and function pointers) that involve assignment. THe tests should validate that this compiles without error. We should also execute the test both in the case where 'x' is null and not-null and validate that the assignment didn't/did happen in those respective cases.

IL should also be verified in these tests.

Specifically, this is testing the a?.b = c case, where the null conditional access is on the left side of an assignment. This is legal in the latest versions of C#, and we want to make sure that this executes properly both when 'a' is null or not null.

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Added PointerConditionalAssignment and FunctionPointerConditionalAssignment tests. Both tests verify that assignment happens when receiver is not null and doesn't happen when receiver is null. Tests include IL verification showing correct lowering. Both tests execute and produce expected output ("20 null" and "21 null" respectively). (commit 7f3fbef -> pending)

}

[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/7502")]
public void FunctionPointerReturnType_Allowed()
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Let's have one of the function pointer tests that executes as well #Closed

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@copilot please either add a function pointer test that will actually execute. Or have one of the tests you added actually execute to validate end behavior.

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Added FunctionPointerReturnType_Execution test that executes a function pointer obtained through null-conditional access. It tests both the case where the receiver is non-null (invokes the function pointer and outputs 42) and where it's null (outputs "null"). (commit 7f3fbef)

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Done with review pass (commit 11). Some test suggestions to consider

- Updated PointerReturnType_WithUsage to test non-null result and added IL verification
- Added semantic model type verification to PointerReturnType_IntPointer test
- Added FunctionPointerReturnType_Execution test that executes and tests both null and non-null cases
- All tests now provide more comprehensive coverage per review feedback

Co-authored-by: CyrusNajmabadi <[email protected]>
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Copilot AI commented Dec 19, 2025

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

  • kijvsblobprodcus387.vsblob.vsassets.io
    • Triggering command: /usr/share/dotnet/dotnet dotnet build src/Compilers/CSharp/Test/Semantic/Microsoft.CodeAnalysis.CSharp.Semantic.UnitTests.csproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

- Added PointerConditionalAssignment test with IL verification
- Added FunctionPointerConditionalAssignment test with IL verification
- Both tests verify assignment behavior when receiver is null vs non-null
- Tests validate that assignment only happens when receiver is not null
- All tests pass with expected output

Co-authored-by: CyrusNajmabadi <[email protected]>
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Copilot AI commented Dec 19, 2025

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

  • kijvsblobprodcus387.vsblob.vsassets.io
    • Triggering command: /usr/share/dotnet/dotnet dotnet build src/Compilers/CSharp/Test/Semantic/Microsoft.CodeAnalysis.CSharp.Semantic.UnitTests.csproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

""";
var verifier = CompileAndVerify(source, options: TestOptions.UnsafeDebugExe, verify: Verification.Skipped, expectedOutput: "42 null");
verifier.VerifyDiagnostics();
verifier.VerifyIL("Test.Main", """
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We only need to verify the IL for one of the cases. This is redundant and harder to review

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6 participants