Handle potential null in extension lookup #80621

333fred · 2025-10-10T00:42:43Z

There are specific scenarios where applicability will remove a symbol from lookup entirely; no error, just silent removal. This introduces a null that consumers were not expecting and caused an NRE. I've handled that NRE here and annotated the API. Fixes https://developercommunity.visualstudio.com/t/NRE-in-Roslyn-v500-225451107/10979295.

There are specific scenarios where applicability will remove a symbol from lookup entirely; no error, just silent removal. This introduces a `null` that consumers were not expecting and caused an NRE. I've handled that NRE here and annotated the API. Fixes https://developercommunity.visualstudio.com/t/NRE-in-Roslyn-v500-225451107/10979295.

333fred · 2025-10-10T00:45:34Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

+        var code2 = """
+            namespace N2;
+
+            file static class E


One such "remove without any errors" scenario is when a file class is out of scope:

https://github.com/333fred/roslyn/blob/1a6213d6fe1b37b01bd8816f5a93d681d070e435/src/Compilers/CSharp/Portable/Binder/Binder_Lookup.cs#L1471-L1475

There are other scenarios in which lookup can return a result with a null symbol, but this proved to be the easiest to replicate as @viceroypenguin gave me a dump where he hit it. #Closed

Thanks for fixing this. I don't quite understand how the scenario differs from existing InstanceMethodInvocation_VariousScopes_Errors which also has a file-scoped extension (therefore inaccessible) :-/

Update: ah, the receiver is non existent...

333fred · 2025-10-10T00:45:54Z

@dotnet/roslyn-compiler for review

AlekseyTs · 2025-10-10T04:35:01Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

 using System.Globalization;
 using System.Linq;
 using System.Reflection.Metadata.Ecma335;
+using System.Text;


Is this using necessary? #Closed

Yes, for Encoding.

AlekseyTs · 2025-10-10T04:39:48Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

+
+        var parseOptions = TestOptions.Regular.WithLanguageVersion(languageVersion);
+        var comp = CreateCompilation([
+            CSharpSyntaxTree.ParseText(code, path: "file1.cs", encoding: Encoding.UTF8, options: parseOptions),


CSharpSyntaxTree.ParseText(code, path: "file1.cs", encoding: Encoding.UTF8, options: parseOptions)

It looks like this can be simplified to (code, "file1.cs"). Similar the next item. #Closed

parseOptions can be passed to CreateCompilation.

AlekseyTs · 2025-10-10T04:46:07Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

+            }
+            """;
+
+        var comp = CreateCompilation([CSharpSyntaxTree.ParseText(code, path: "file1.cs", encoding: Encoding.UTF8), CSharpSyntaxTree.ParseText(code2, path: "file2.cs", encoding: Encoding.UTF8)]);


[CSharpSyntaxTree.ParseText(code, path: "file1.cs", encoding: Encoding.UTF8), CSharpSyntaxTree.ParseText(code2, path: "file2.cs", encoding: Encoding.UTF8)

Similar suggestion to simplify compilation creation code. Encoding.UTF8 is probably not required to reproduce the problem and even if it is, it looks like it is what our test helpers use by default anyway. #Closed

AlekseyTs · 2025-10-10T04:53:00Z

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

                    {
-                        Symbol extensionMember = singleLookupResult.Symbol;
+                        Symbol? extensionMember = singleLookupResult.Symbol;
+                        if (extensionMember is null)


extensionMember is null

It is probably preferrable to check that singleLookupResult.Kind is LookupResultKind.Empty. I do not think we expect null symbol otherwise. #Closed

AlekseyTs · 2025-10-10T04:54:42Z

src/Compilers/CSharp/Portable/Binder/SingleLookupResult.cs

-        internal SingleLookupResult(LookupResultKind kind, Symbol symbol, DiagnosticInfo error)
+        internal SingleLookupResult(LookupResultKind kind, Symbol? symbol, DiagnosticInfo? error)
        {
            this.Kind = kind;


this.Kind = kind;

Consider adding an assert that symbol is not null unless kind is empty. #Closed

AlekseyTs · 2025-10-10T04:59:32Z

                scope.Binder.EnumerateAllExtensionMembersInSingleBinder(singleLookupResults, node.Name, arity, options, originalBinder: this, ref discardedUseSiteInfo, ref discardedUseSiteInfo);

I think it would be better long term to filter out empty results in EnumerateAllExtensionMembersInSingleBinder instead of having all existing and future consumers to worry about specially handling this situation. #Closed

Refers to: src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs:10895 in 1a6213d. [](commit_id = 1a6213d, deletion_comment = False)

AlekseyTs · 2025-10-10T05:00:34Z

Done with review pass (commit 1) #Closed

AlekseyTs · 2025-10-10T16:32:58Z

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

                    foreach (SingleLookupResult singleLookupResult in singleLookupResults)
                    {
-                        Symbol extensionMember = singleLookupResult.Symbol;
+                        var extensionMember = singleLookupResult.Symbol;


var

Consider reverting change on this line. #Closed

I realized that it is probably driven by added nullable annotations

Yep, that's the reason.

AlekseyTs · 2025-10-10T16:35:18Z

src/Compilers/CSharp/Portable/Binder/Binder_Lookup.cs

+                    result.Add(resultOfThisMember);
+                }

                if (candidate is MethodSymbol { IsStatic: false } shadows &&


if (candidate is MethodSymbol { IsStatic: false } shadows &&

It looks like this if belongs inside the if added above #Closed

AlekseyTs · 2025-10-10T16:36:51Z

Done with review pass (commit 2) #Closed

AlekseyTs

LGTM (commit 3)

333fred · 2025-10-10T20:07:00Z

@dotnet/roslyn-compiler for a second review.

jcouv

LGTM Thanks (commit 3)

MaceWindu · 2025-11-21T14:43:24Z

~~@333fred could you please tell, which release contains this fix. We still see issue with v10 SDK (10.0.100)~~

nevermind, will be in december release

333fred · 2025-12-17T19:12:03Z

/backport to release/dev18.0

github-actions · 2025-12-17T19:12:13Z

Started backporting to release/dev18.0 (link to workflow run)

333fred · 2025-12-17T19:12:58Z

@MaceWindu apologies, I got my milestones mixed up and this is not in 10.0.1xx. I'm working on it now.

github-actions · 2025-12-17T19:14:46Z

@333fred backporting to release/dev18.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Handle potential null in extension lookup
Using index info to reconstruct a base tree...
M	src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs
M	src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs
Falling back to patching base and 3-way merge...
Auto-merging src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs
Auto-merging src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs
CONFLICT (content): Merge conflict in src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 Handle potential null in extension lookup
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output

There are specific scenarios where applicability will remove a symbol from lookup entirely; no error, just silent removal. This introduces a `null` that consumers were not expecting and caused an NRE. I've handled that NRE here and annotated the API. Fixes https://developercommunity.visualstudio.com/t/NRE-in-Roslyn-v500-225451107/10979295. (cherry picked from commit 16883a5)

333fred requested a review from a team as a code owner October 10, 2025 00:42

github-actions bot added the Area-Compilers label Oct 10, 2025

333fred commented Oct 10, 2025

View reviewed changes

AlekseyTs reviewed Oct 10, 2025

View reviewed changes

jcouv self-requested a review October 10, 2025 15:13

jcouv added the Feature - Extension Everything The extension everything feature label Oct 10, 2025

Address feedback.

15896a3

AlekseyTs reviewed Oct 10, 2025

View reviewed changes

Invert if

e58dc95

AlekseyTs approved these changes Oct 10, 2025

View reviewed changes

jcouv self-assigned this Oct 10, 2025

jcouv approved these changes Oct 10, 2025

View reviewed changes

333fred merged commit 16883a5 into dotnet:main Oct 13, 2025
24 checks passed

333fred deleted the nre branch October 13, 2025 23:02

dotnet-policy-service bot added this to the Next milestone Oct 13, 2025

dotnet-maestro bot mentioned this pull request Oct 15, 2025

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Handle potential null in extension lookup #80621

Handle potential null in extension lookup #80621

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