C# 14: Support `extension` types. #21220

michaelnebel · 2026-01-26T15:36:27Z

In this PR we introduce support for extension types (blocks). The feature is explained in detail here.
The feature generalises extensions to operators and properties.

Here is a small example of an declaration and how it can be invoked.

public static class MyExtensions
{
    extension(string s)
    {
        public bool IsValid() { return s is not null; }
    }
}

public class A
{
    public void M()
    {
        var s = "Hello World";
        s.IsValid(); // Call to extension method.
        MyExtensions.IsValid(s) // Call to compiler generated static method.
    }
}

It turns out that Roslyn generates a static method corresponding to the extension method. To avoid extracting multiple methods with identical bodies (which would further complicate the QL implementation) we

Only extract the IsValid() method, which has the qualified name MyExtensions.extension(string).IsValid.
Add a parameter s to IsValid() corresponding to the receiver parameter s. This needs to be synthesised as we can't re-use the parameter from the extension declaration.
Replace invocations of MyExtensions.IsValid with MyExtensions.extensions(string).IsValid.

DCA looks good.

Performance appears to be un-affected.
Nice reduction in the number of missing call targets for a range of projects (and an increase in the number of alerts).
There are some "new" extraction errors for the dotnet/runtime project. These extraction errors are because some of the extensions (that we now extract) adds user-defined compound assignment operators (described here), which are not supported yet.

csharp/extractor/Semmle.Extraction.CSharp/Entities/Method.cs

csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll

csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

…meter and make it possible to specify a parameter position offset.

…tic generated methods with invocation of extension type member.

…ic parameter.

michaelnebel · 2026-02-05T15:08:38Z

@hvitved : The PR is ready for review. However, I haven't added any upgrade/downgrade scripts yet in case the review leads to a change in the DB scheme. Review on commit by commit basis is encouraged.

hvitved

Impressive work! Some mostly minor comments

hvitved · 2026-02-06T12:54:45Z

csharp/ql/lib/semmlecode.csharp.dbscheme

         | @xmllocatable | @asp_element | @namespace | @preprocessor_directive;

-@declaration = @callable | @generic | @assignable | @namespace;
+@declaration = @callable | @generic | @assignable | @namespace | @extension_type;


Is this needed? I would think that @extension_type is part of @type, which is part of @generic.

hvitved · 2026-02-06T12:59:25Z

csharp/extractor/Semmle.Extraction.CSharp/Entities/SyntheticExtensionParameter.cs

+{
+    /// <summary>
+    /// Synthetic parameter for extension methods declared using the extension syntax.
+    /// That is, we add a synthetic parameter s to IsValid in the following example:


backticks around s and IsValid makes this easier to parse.

hvitved · 2026-02-06T13:02:25Z

csharp/extractor/Semmle.Extraction.CSharp/Entities/SyntheticExtensionParameter.cs

+                switch (ExtensionParameter.RefKind)
+                {
+                    case RefKind.Ref:
+                        return Parameter.Kind.Ref;
+                    case RefKind.In:
+                        return Parameter.Kind.In;
+                    case RefKind.RefReadOnlyParameter:
+                        return Parameter.Kind.RefReadOnly;
+                    default:
+                        return Parameter.Kind.None;
+                }


Should we share this logic with the logic in Parameter.cs?

hvitved · 2026-02-06T13:05:58Z

csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

+        /// Returns true if this method is a compiler-generated extension method,
+        /// and outputs the original extension method declaration.
+        /// </summary>
+        public static bool TryGetExtensionMethod(this IMethodSymbol method, out IMethodSymbol? declaration)


Why not simply public static IMethodSymbol? TryGetExtensionMethod(this IMethodSymbol method)?

hvitved · 2026-02-06T13:22:36Z

csharp/ql/lib/semmle/code/csharp/Callable.qll

+ * Either an extension method (`ExtensionMethod`), an extension operator
+ * (`ExtensionOperator`) or an extension accessor (`ExtensionAccessor`).
+ */
+abstract class ExtensionCallable extends Callable {


We don't want to expose abstract classes, so better to (1) rename it to ExtensionCallableImpl, (2) move it inside internal/Callable.qll, (3) update all existing references to ExtensionCallableImpl, and (4) add here final class ExtensionCallable = ExtensionCallableImpl.

hvitved · 2026-02-06T13:31:06Z

csharp/ql/lib/semmle/code/csharp/Callable.qll

  Call getACall() { this = result.getTarget() }
+
+  /** Holds if this callable is declared in an extension type. */
+  predicate isInExtension() { this.getDeclaringType() instanceof ExtensionType }


If you move this inside the Declaration class, it can be used also in the charpreds of ExtensionProperty and ExtensionAccessor.

hvitved · 2026-02-06T13:31:42Z

csharp/ql/lib/semmle/code/csharp/Callable.qll

+ * Either a classic extension method (`ClassicExtensionMethod`) or an extension
+ * type extension method (`ExtensionTypeExtensionMethod`).
+ */
+abstract class ExtensionMethod extends ExtensionCallable, Method {


hvitved · 2026-02-06T13:35:50Z

csharp/ql/lib/semmle/code/csharp/exprs/Access.qll

+ * ```
+ */
+class SyntheticExtensionParameterAccess extends ParameterAccess {
+  private Parameter p;


No need to have this as a field.

hvitved · 2026-02-06T13:41:40Z

csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.ext.yml

      - ["My.Qltest", "K", false, "GetMyFieldOnSyntheticField", "()", "", "Argument[this].SyntheticField[My.Qltest.K.MySyntheticField2].Field[My.Qltest.K.MyField]", "ReturnValue", "value", "manual"]
      - ["My.Qltest", "Library", false, "SetValue", "(System.Object)", "", "Argument[0]", "Argument[this].SyntheticField[X]", "value", "dfc-generated"]
      - ["My.Qltest", "Library", false, "GetValue", "()", "", "Argument[this].SyntheticField[X]", "ReturnValue", "value", "dfc-generated"]
+      - ["My.Qltest", "TestExtensions+extension(Object)", false, "Method1", "(System.Object)", "", "Argument[0]", "ReturnValue", "value", "manual"]


This ought to use the FQN of the type being extended, i.e. TestExtensions+extension(System.Object)

… extension types.

csharp/ql/lib/semmle/code/csharp/internal/Callable.qll

csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

+                    catch
+                    {
+                        // If anything goes wrong, fall back to the unbound declaration.
+                        return unboundDeclaration;
+                    }


csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

+                    if (parameter.Ordinal == 0)
+                    {
+                        if (parameter.ContainingSymbol is IMethodSymbol method && method.IsExtensionMethod)
+                        {
+                            return Parameter.Kind.This;
+                        }
+                    }


…pl suffix.

michaelnebel · 2026-02-09T16:29:43Z

@hvitved Added some commits which addresses your comments, added DB upgrade/downgrade scripts (not intending to make the DB downgrade script delete all the extension members and types from the DB). Also made a minor modification to one of the QL4QL queries.

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michaelnebel added 3 commits February 4, 2026 12:51

C#: Add extended_type to the DB scheme.

dca10f8

C#: Add parameter marker interface, allow a type to a parent for para…

c68cd58

…meter and make it possible to specify a parameter position offset.

C#: Move some populate methods and location writing methods.

60bb9a9

michaelnebel force-pushed the csharp14/extension branch from a03f985 to 2a9166f Compare February 4, 2026 15:35

michaelnebel added 11 commits February 5, 2026 15:38

C#: Add class for making synthetic parameter entities.

ab505e3

C#: Extract extension types and members. Replacing invocations to sta…

edfdc98

…tic generated methods with invocation of extension type member.

C#: Add ExtensionType to the QL library.

9a4a6cf

C#: Add extension callable and accessor classes.

b9f36f3

C#: Add extension call classes.

5e02a86

C#: Replace extension parameter access with the corresponding synthet…

e831c80

…ic parameter.

C#: Add dispatch logic for calling extensions accessors as methods.

849823e

C#: Add extensions test.

c040daa

C#: Add PrintAst test for extensions.

6cbe000

C#: Add extension data flow test.

4b6a53b

C#: Add MaD tests for extensions.

bd3e4d3

michaelnebel changed the title ~~[WIP] C# 14: Support extension blocks.~~ C# 14: Support extension types. Feb 5, 2026

C#: Add change-note.

02e4a8b

michaelnebel force-pushed the csharp14/extension branch from 2a9166f to 02e4a8b Compare February 5, 2026 14:53

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michaelnebel added 2 commits February 9, 2026 11:46

C#: Address review comments.

fe94b3b

C#: Use the fully qualified name for the extension type when printing…

bcdbd6e

… extension types.

michaelnebel added 4 commits February 9, 2026 13:28

C#: Update MaD models for extension members.

d9fea15

C#: Update test expected output.

eff9f99

C#: Add DB upgrade script.

42d2de8

C#: Add DB downgrade script.

3e914f7

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C# 14: Support extension types. #21220

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C# 14: Support `extension` types. #21220

C# 14: Support `extension` types. #21220

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