C# 14: Null conditional assignments. #21158

michaelnebel · 2026-01-13T10:39:37Z

In this PR, we:

Add support for C# 14 null conditional assignments. While the extractor already supported this feature, updates to the C# control flow implementation were needed.
Simplify the control flow logic for qualified write access, which also helps with the addition of null conditional assignments.
Make a small improvement to the MaybeNullExpr class so that it now correctly handles null conditional (read) accesses.

Changes to existing control flow

Previously, the control flow for

ca.IntField = 42

looked like this:

ca -> 42 -> ca.IntField = 42

With this PR, we now include the field access after evaluating the right-hand side, so the control flow becomes:

ca -> 42 -> ca.IntField -> ca.IntField = 42

This matches how property assignments are handled, where it’s important for the property access to be included in the control flow after evaluating the right-hand side, since accessing the property corresponds to invoking the property setter.

Previously, the control flow for

M(out ca.IntField)

looked like this:

ca -> M(out ca.IntField)

With this PR, we now include the field access before the call, so the control flow becomes:

ca -> ca.IntField -> M(out ca.IntField)

New control flow.

The changes to the control flow graph implementation mean that for a statement like

ca?.Prop?.StringProp = "World";

the control flow is now built correctly to reflect the sequence of null conditional accesses.

That is, if the qualifier uses a null conditional, we add a null edge from the qualifier to the point after the assignment. This reflects the fact that if the qualifier is null, the right-hand side of the assignment isn’t evaluated.

michaelnebel · 2026-01-16T10:25:45Z

@hvitved : Would it be possible to get some early feedback on the contents of the PR (in case we should design it differently)?

…field access in the CFG.

hvitved

Overall LGTM, I mostly have a concern about CFGs for out arguments.

csharp/ql/lib/semmle/code/csharp/controlflow/internal/ControlFlowGraphImpl.qll

hvitved · 2026-01-16T12:51:28Z

csharp/ql/lib/semmle/code/csharp/dataflow/Nullness.qll

+  result =
+    any(QualifiableExpr qe |
+      qe.isConditional() and
+      qe.getQualifier() = maybeNullExpr(reason)


Is this actually needed? I would think that an expression like x?.M() can always potentially be null, regardless of what we know about x.

Yes, if x is variable (or other stuff that is not special cased in the MaybeNullExpr class), but it appears we need to propgate the maybe null information for casts and conditionals (stuff that is explicitly handled in the MaybeNullExpr class itself). That is, to properly handle (x as C).GetInt().
An example of an extra finding: https://github.com/dotnet/roslyn/blob/6afbfb45ccc9691167206bf29482a99b1d6d469c/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenTupleTest.cs#L24398

What I meant was why not

result = any(QualifiableExpr qe | qe.isConditional() and reason = qe.getQualifier())

That should give us strictly more results, and I would assume that any x?.M() expression can be potentially null because it is conditionally qualified.

That is somewhat less conservative. It means that stuff like

void M1(object o) { var t = o?.GetType(); Console.WriteLine(t.FullName); }

gets flagged (which is good), but it also means that we introduce false positives in cases like

void M2() { var o = new object(); var t = o?.GetType(); Console.WriteLine(t.FullName); // GOOD }

Maybe that is acceptable for a "maybe" query (to consider the intent of the use of ?). I will try and make the change and see what results DCA produce.

I would think that was OK; if the qualifier cannot be null then there is no need to use ?.

csharp/ql/lib/semmle/code/csharp/controlflow/internal/ControlFlowGraphImpl.qll

Copilot

Pull request overview

This pull request adds support for C# 14 null conditional assignments and improves control flow graph handling for qualified write access. The changes update the control flow logic to match how property assignments work, ensuring field/property accesses are included in the control flow graph after evaluating the right-hand side.

Changes:

Updated control flow graph to include field/property access nodes before assignments
Added support for null conditional assignments (e.g., ca?.Prop = value)
Improved MaybeNullExpr class to handle null conditional read accesses
Added comprehensive test coverage for the new features

Reviewed changes

Copilot reviewed 23 out of 24 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
csharp/ql/test/library-tests/standalone/controlflow/cfg.expected	Updated expected control flow for array element access assignments
csharp/ql/test/library-tests/obinit/ObInit.expected	Updated control flow for object initializer field assignments
csharp/ql/test/library-tests/dataflow/signanalysis/SignAnalysis.expected	Added entries for field access in assignments
csharp/ql/test/library-tests/dataflow/nullness/options	Added extractor options for nullness test
csharp/ql/test/library-tests/dataflow/nullness/maybeNullExpr.ql	New query for testing MaybeNullExpr class
csharp/ql/test/library-tests/dataflow/nullness/maybeNullExpr.expected	Expected results for MaybeNullExpr test
csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs	Test cases for null conditional expressions
csharp/ql/test/library-tests/dataflow/local/TaintTrackingStep.expected	Updated taint tracking for array element assignments
csharp/ql/test/library-tests/csharp8/*.expected	Updated control flow for various C# 8 features with new assignment handling
csharp/ql/test/library-tests/controlflow/graph/*.expected	Comprehensive updates to control flow graph expected results
csharp/ql/test/library-tests/controlflow/graph/ConditionalAccess.cs	New test cases for null conditional assignments (M9 method)
csharp/ql/test/library-tests/controlflow/graph/Assignments.cs	New test cases for out parameter assignments with fields

Comments suppressed due to low confidence (3)

csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs:13

This assignment to nullCoalescing is useless, since its value is never read.

        var nullCoalescing = o ?? null;

csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs:16

This assignment to c is useless, since its value is never read.

        var c = o as C;

csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs:19

This assignment to s1 is useless, since its value is never read.

        var s1 = (o as C)?.Prop;

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Copilot · 2026-01-19T09:59:19Z

csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs

+        var c = o as C;
+
+        // Conditional access might be null as the qualifier might be null.
+        var s1 = (o as C)?.Prop;


Local scope variable 's1' shadows C.s1.

Suggested change

var s1 = (o as C)?.Prop;

var propValue = (o as C)?.Prop;

Copilot · 2026-01-19T09:59:19Z

csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs

+    void M(object o, bool b)
+    {
+        // Conditional expr might be null.
+        var conditionalExpr = b ? new object() : null;


This assignment to conditionalExpr is useless, since its value is never read.

This issue also appears in the following locations of the same file:

line 13

line 16

line 19

Copilot · 2026-01-19T09:59:20Z

csharp/ql/test/library-tests/controlflow/graph/ConditionalAccess.cs

+    string this[int index]
+    {
+        get { return null; }
+        set { }


Value ignored when setting property.

Suggested change

string this[int index]

{

get { return null; }

set { }

string[] _items = new string[1];

string this[int index]

{

get { return index == 0 ? _items[0] : null; }

set

{

if (index == 0)

_items[0] = value;

}

hvitved

LGTM, just the one comment in Nullness.qll.

michaelnebel · 2026-01-19T12:27:51Z

@hvitved : Thank you so much for all the feedback and the time you have spent on this!
Running DCA one more time.

github-actions bot added the C# label Jan 13, 2026

michaelnebel force-pushed the csharp/cfgforaccess branch 3 times, most recently from 30e2a16 to 15848f3 Compare January 14, 2026 09:55

michaelnebel changed the title ~~C#: Add CFG support for null conditional assignments and include eg. …~~ C# 14: Null conditional assignments. Jan 14, 2026

michaelnebel force-pushed the csharp/cfgforaccess branch from 438153d to 15e9685 Compare January 14, 2026 13:41

github-actions bot added the documentation label Jan 14, 2026

michaelnebel mentioned this pull request Jan 14, 2026

C# 14: Null conditional assignment. #21118

Closed

michaelnebel force-pushed the csharp/cfgforaccess branch from 8877b1e to 3c42988 Compare January 16, 2026 10:20

michaelnebel requested a review from hvitved January 16, 2026 10:26

michaelnebel added 11 commits January 16, 2026 11:53

C#: Add CFG support for null conditional assignments and include eg. …

9894993

…field access in the CFG.

C#: Update test expected output.

b061c4d

C#: Add some null conditional assignments CFG tests.

0bf0cba

C#: Update test expected output.

4ba8923

C#: Add a tests for MaybeNullExpr.

f0135e9

C#: Take null conditional assignments into account in MaybeNullExpr.

5942edf

C#: Update test expected output.

ab432ec

C#: Add change-note.

812fdbe

C#: Add field access for out assignments in the CFG.

3d988e8

C#: Exclude the field access cases from missing argument call.

bd1c6e6

C#: Add tests for out writeacceses.

86198e3

michaelnebel force-pushed the csharp/cfgforaccess branch from 3c42988 to 86198e3 Compare January 16, 2026 10:54

hvitved reviewed Jan 16, 2026

View reviewed changes

michaelnebel added 2 commits January 19, 2026 10:06

C#: Address review comments.

beb7750

C#: Update test expected output.

33fc2ba

michaelnebel marked this pull request as ready for review January 19, 2026 09:49

Copilot AI review requested due to automatic review settings January 19, 2026 09:49

michaelnebel requested a review from a team as a code owner January 19, 2026 09:49

Copilot started reviewing on behalf of michaelnebel January 19, 2026 09:49 View session

Copilot AI reviewed Jan 19, 2026

View reviewed changes

hvitved previously approved these changes Jan 19, 2026

View reviewed changes

michaelnebel added 2 commits January 19, 2026 13:17

C#: Relax the condition for when a qualified expression might be null.

7ae2b76

C#: Add some more tests.

7ff1c12

michaelnebel dismissed hvitved’s stale review via 7ff1c12 January 19, 2026 12:18

hvitved approved these changes Jan 19, 2026

View reviewed changes

-    string this[int index]
-    {
-        get { return null; }
-        set { }
+    string[] _items = new string[1];
+    string this[int index]
+    {
+        get { return index == 0 ? _items[0] : null; }
+        set
+        {
+            if (index == 0)
+                _items[0] = value;
+        }

C# 14: Null conditional assignments. #21158

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C# 14: Null conditional assignments. #21158

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Changes to existing control flow

New control flow.

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Pull request overview

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michaelnebel commented Jan 13, 2026 •

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