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C#: Extract indexer when accessing a span with a range. #21877

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C#: Extract indexer when accessing a span with a range. #21877

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f2960a7
C#: Extract the indexer as the call target when using range expressio…
michaelnebel May 21, 2026
f6fe075
C#: Update DB quality expected test output.
michaelnebel May 21, 2026
9167814
C#: Add change-note.
michaelnebel May 21, 2026
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35 changes: 32 additions & 3 deletions csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/ElementAccess.cs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
using System.IO;
using System.Linq;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Semmle.Extraction.Kinds;
Expand All @@ -17,6 +18,35 @@ protected ElementAccess(ExpressionNodeInfo info, ExpressionSyntax qualifier, Bra
private readonly ExpressionSyntax qualifier;
private readonly BracketedArgumentListSyntax argumentList;


private IPropertySymbol? GetIndexerSymbol()
{
var symbol = Context.GetSymbolInfo(base.Syntax).Symbol;

if (symbol is IPropertySymbol { IsIndexer: true } indexer)
return indexer;

// In some cases, Roslyn translates the use of range expressions directly into method calls.
// E.g. `a[0..3]` is translated into `a.Slice(0, 3)`, if `a` is a `Span<T>`.
// In this case, we want to populate the indexer access as normal (as this reflects the source code more accurately).
if (symbol is IMethodSymbol method)
{
var indexers = method
.ContainingType
.GetMembers()
.OfType<IPropertySymbol>()
.Where(p => p.IsIndexer);

var intIndexer = indexers
.Where(i => i.Parameters.Length == 1 && i.Parameters[0].Type.SpecialType == SpecialType.System_Int32)
.FirstOrDefault();

return intIndexer;
}
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@michaelnebel michaelnebel May 28, 2026

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I think it is reasonable to assume that Roslyn only translates a[range] into a method call for integer range expressions (and thus we implicitly want to extract the indexer with an int parameter).


return null;
}

protected override void PopulateExpression(TextWriter trapFile)
{
if (Kind == ExprKind.POINTER_INDIRECTION)
Expand All @@ -32,9 +62,8 @@ protected override void PopulateExpression(TextWriter trapFile)
Create(Context, qualifier, this, -1);
PopulateArguments(trapFile, argumentList, 0);

var symbolInfo = Context.GetSymbolInfo(base.Syntax);

if (symbolInfo.Symbol is IPropertySymbol indexer)
var indexer = GetIndexerSymbol();
if (indexer is not null)
{
trapFile.expr_access(this, Indexer.Create(Context, indexer));
}
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4 changes: 4 additions & 0 deletions csharp/ql/lib/change-notes/2026-05-21-spanaccess-range.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* Improved extraction of span range-access expressions (for example, `a[0..3]`) so the indexer is recorded as the call target.
2 changes: 0 additions & 2 deletions csharp/ql/test/query-tests/Telemetry/DatabaseQuality/IsNotOkayCall.expected
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Original file line number Diff line number Diff line change
@@ -1,2 +0,0 @@
| Quality.cs:26:19:26:26 | access to indexer | Call without target $@. | Quality.cs:26:19:26:26 | access to indexer | access to indexer |
| Quality.cs:29:21:29:27 | access to indexer | Call without target $@. | Quality.cs:29:21:29:27 | access to indexer | access to indexer |
2 changes: 0 additions & 2 deletions csharp/ql/test/query-tests/Telemetry/DatabaseQuality/NoTarget.expected
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Expand Up @@ -7,7 +7,5 @@
| Quality.cs:20:13:20:23 | access to property MyProperty6 | Call without target $@. | Quality.cs:20:13:20:23 | access to property MyProperty6 | access to property MyProperty6 |
| Quality.cs:23:9:23:14 | access to event Event1 | Call without target $@. | Quality.cs:23:9:23:14 | access to event Event1 | access to event Event1 |
| Quality.cs:23:9:23:30 | delegate call | Call without target $@. | Quality.cs:23:9:23:30 | delegate call | delegate call |
| Quality.cs:26:19:26:26 | access to indexer | Call without target $@. | Quality.cs:26:19:26:26 | access to indexer | access to indexer |
| Quality.cs:29:21:29:27 | access to indexer | Call without target $@. | Quality.cs:29:21:29:27 | access to indexer | access to indexer |
| Quality.cs:38:16:38:26 | access to property MyProperty2 | Call without target $@. | Quality.cs:38:16:38:26 | access to property MyProperty2 | access to property MyProperty2 |
| Quality.cs:50:20:50:26 | object creation of type T | Call without target $@. | Quality.cs:50:20:50:26 | object creation of type T | object creation of type T |
4 changes: 2 additions & 2 deletions csharp/ql/test/query-tests/Telemetry/DatabaseQuality/Quality.cs
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Expand Up @@ -23,10 +23,10 @@ public Test()
Event1.Invoke(this, 5);

var str = "abcd";
var sub = str[..3]; // TODO: this is not an indexer call, but rather a `str.Substring(0, 3)` call.
var sub = str[..3];

Span<int> sp = null;
var slice = sp[..3]; // TODO: this is not an indexer call, but rather a `sp.Slice(0, 3)` call.
var slice = sp[..3];

Span<byte> guidBytes = stackalloc byte[16];
guidBytes[08] = 1;
Expand Down
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