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Fix false positive in MissedSelectOpportunity when foreach body uses await #21708

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Merged
hvitved merged 3 commits into from
Apr 15, 2026
Merged

Fix false positive in MissedSelectOpportunity when foreach body uses await #21708

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0e66555
Initial plan
Copilot Apr 14, 2026
3483050
Fix false positive in MissedSelectOpportunity for async/await loops
Copilot Apr 14, 2026
878cfd7
C#: Use inline test expectations
hvitved Apr 14, 2026
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8 changes: 5 additions & 3 deletions csharp/ql/lib/Linq/Helpers.qll
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Expand Up @@ -121,15 +121,17 @@ predicate missedOfTypeOpportunity(ForeachStmtEnumerable fes, LocalVariableDeclSt
/**
* Holds if `foreach` statement `fes` can be converted to a `.Select()` call.
* That is, the loop variable is accessed only in the first statement of the
* block, the access is not a cast, and the first statement is a
* local variable declaration statement `s`.
* block, the access is not a cast, the first statement is a
* local variable declaration statement `s`, and the initializer does not
* contain an `await` expression (since `Select` does not support async lambdas).
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*/
predicate missedSelectOpportunity(ForeachStmtGenericEnumerable fes, LocalVariableDeclStmt s) {
s = firstStmt(fes) and
forex(VariableAccess va | va = fes.getVariable().getAnAccess() |
va = s.getAVariableDeclExpr().getAChildExpr*()
) and
not s.getAVariableDeclExpr().getInitializer() instanceof Cast
not s.getAVariableDeclExpr().getInitializer() instanceof Cast and
not s.getAVariableDeclExpr().getInitializer().getAChildExpr*() instanceof AwaitExpr
}

/**
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32 changes: 32 additions & 0 deletions csharp/ql/test/query-tests/Linq/MissedSelectOpportunity/MissedSelectOpportunity.cs
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@@ -0,0 +1,32 @@
using System;
using System.Linq;
using System.Collections.Generic;
using System.Threading.Tasks;

class MissedSelectOpportunity
{
public void M1(List<int> lst)
{
// BAD: Can be replaced with lst.Select(i => i * i)
foreach (int i in lst)
{
int j = i * i;
Console.WriteLine(j);
}
}

public async Task M2(IEnumerable<ICounter> counters)
{
// GOOD: Cannot use Select because the initializer contains an await expression
foreach (var counter in counters)
{
var count = await counter.CountAsync();
Console.WriteLine(count);
}
}

public interface ICounter
{
Task<int> CountAsync();
}
}
1 change: 1 addition & 0 deletions csharp/ql/test/query-tests/Linq/MissedSelectOpportunity/MissedSelectOpportunity.expected
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@@ -0,0 +1 @@
| MissedSelectOpportunity.cs:11:9:15:9 | foreach (... ... in ...) ... | This foreach loop immediately $@ - consider mapping the sequence explicitly using '.Select(...)'. | MissedSelectOpportunity.cs:13:13:13:26 | ... ...; | maps its iteration variable to another variable |
1 change: 1 addition & 0 deletions csharp/ql/test/query-tests/Linq/MissedSelectOpportunity/MissedSelectOpportunity.qlref
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@@ -0,0 +1 @@
Linq/MissedSelectOpportunity.ql

Check warning

Code scanning / CodeQL

Query test without inline test expectations Warning test

Query test does not use inline test expectations.
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@hvitved hvitved Apr 14, 2026

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Should use inline test expectations, i.e.

postprocess:
 - utils/test/InlineExpectationsTestQuery.ql

This also means adding // $ Alert marker comment in the test file.

2 changes: 2 additions & 0 deletions csharp/ql/test/query-tests/Linq/MissedSelectOpportunity/options
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@@ -0,0 +1,2 @@
semmle-extractor-options: /nostdlib /noconfig
semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj
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