Allow foreach on typed null enumerables by Copilot · Pull Request #80783 · dotnet/roslyn

Copilot · 2025-10-16T22:01:50Z

Problem

When using a foreach loop on a constant null value that has been cast to an enumerable type (like IEnumerable<T>, arrays, or strings), the compiler was incorrectly reporting a compile-time error instead of allowing it to compile and fail at runtime.

For example, this code:

foreach (var i in (IEnumerable<int>)null) { }
foreach (int x in (int[])null) { }
foreach (int x in default(int[])) { }

Was incorrectly reporting:

error CS0186: Use of null is not valid in this context

This is semantically incorrect - if you explicitly cast null to a type, that cast is valid from a type-safety perspective, and the foreach should be allowed to compile (even though it will throw a NullReferenceException at runtime).

Solution

Removed the compile-time constant null checks for typed foreach collections in ForEachLoopBinder.cs:

Pattern-based enumeration (IEnumerable with GetEnumerator)
Interface-based enumeration (types implementing IEnumerable)
Special types (arrays, dynamic, strings)

The only case where we still report a compile-time error is when the collection expression has no type at all (e.g., bare null literal as a BoundLiteral).

Behavior Change

Before:

foreach (var x in null) → Error (still an error)
foreach (var x in (IEnumerable<int>)null) → Error (now allowed)
foreach (var x in default(int[])) → Error (now allowed)
foreach (var x in (int[])null) → Error (now allowed)

After:

foreach (var x in null) → Error (no type information)
foreach (var x in (IEnumerable<int>)null) → Compiles, runtime error
foreach (var x in default(int[])) → Compiles, runtime error
foreach (var x in (int[])null) → Compiles, runtime error

This matches the general C# principle that type casts are validated at compile time based on types, not values.

With #nullable enable, these scenarios correctly produce nullable reference type warnings (CS8602: Dereference of a possibly null reference) without blocking compilation.

Testing

Comprehensive test coverage including:

Tests for default(int[]), default(string), default(IEnumerable<T>)
Tests for (int[])null, (string)null, (IEnumerable<T>)null
Tests for (dynamic)null and (int[,])null (multidimensional arrays)
Tests for type parameter (T)null where T : class, IEnumerable<int>
Tests for untyped expressions: default, new(), []
Test for plain null (still reports error as expected)
Nullable-enabled versions of all tests verifying appropriate warnings
All tests use VerifyEmitDiagnostics where appropriate with complete type definitions

Fixes #45616

Original prompt

This section details on the original issue you should resolve

<issue_title>Undeserved error when foreach loop used on null enumerable</issue_title>
<issue_description>This program
using System;
using System.Collections.Generic;
public class C
{
    static void Main()
    {
        foreach (var i in (IEnumerable<int>)null)
        {
            Console.Write(i);
        }
    }
}
causes the compiler to produce

error CS0186: Use of null is not valid in this context

but that error is not justified by the language specification.
</issue_description>

<agent_instructions>@copilot This does seem inappropriate. Can you determine why this is happening?</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #45616

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

…r for IEnumerable types - Added check for constant null before reporting missing GetEnumerator error - Applied fix to both sync (IEnumerable) and async (IAsyncEnumerable) cases - Added tests to verify correct error reporting for null IEnumerable<T> and IEnumerable Co-authored-by: CyrusNajmabadi <[email protected]>

CyrusNajmabadi · 2025-10-16T22:35:28Z

@copilot i think you misunderstood. This code should be legal. It is legal to cast null to an IEnumerable<T> and it is legal to enumerate such an IEnumerable<T>. So the code shuld be legal (though if it has a Nullable Reference Type warning, that's appropriate).

Remove compile-time null checks for typed foreach collections. Casting null to IEnumerable<T>, arrays, or strings is now legal and will fail at runtime instead of compile-time. This matches C# semantics where type-safe casts are valid even if the value is null. - Removed null checks for pattern-based enumeration - Removed null checks for interface-based enumeration - Removed null checks for arrays, dynamic, and strings - Updated tests to verify the new behavior Co-authored-by: CyrusNajmabadi <[email protected]>

Copilot · 2025-10-16T22:48:57Z

@copilot i think you misunderstood. This code should be legal. It is legal to cast null to an IEnumerable<T> and it is legal to enumerate such an IEnumerable<T>. So the code shuld be legal (tho...

You're absolutely right - I misunderstood the issue. I've fixed it now in commit 0b49420.

The code now allows foreach on null-typed enumerables ((IEnumerable<T>)null, (int[])null, default(int[])) to compile. These will fail at runtime with NullReferenceException, but they're type-safe from a compilation perspective, so they should be allowed.

The only case that still reports a compile error is when there's no type information at all (e.g., bare null literal).

CyrusNajmabadi · 2025-10-16T23:02:05Z

src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs

+        }
+
+        [Fact]
+        public void TestNullCastToIEnumerable()


@copilot add WorkItem attributes to all tests. Use raw string literals for all tests. Add tests that validate what happens in a #nullable enabled file. Add an explicit test for default(int[]) default(string) default(IEnumerable<T>). And add an explicit test for just plain null. Remove the explanatory comments in tests. It is redundant with the calls to .VerifyDiagnostics()

Done in commit 570e4da. All tests now use raw string literals, have WorkItem attributes, include nullable enabled scenarios, and cover all the default/cast/null cases you requested.

- Added WorkItem attributes referencing issue #45616 - Converted all test code to raw string literals - Added explicit tests for default(int[]), default(string), default(IEnumerable<T>) - Added explicit test for plain null - Added tests for all scenarios with #nullable enable - Removed explanatory comments from tests - Fixed test names to follow consistent naming pattern Co-authored-by: CyrusNajmabadi <[email protected]>

CyrusNajmabadi · 2025-10-16T23:18:59Z

src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs

+        }
+
+        [WorkItem(45616, "https://github.com/dotnet/roslyn/issues/45616")]
+        [Fact]


@copilot merge all WorkItem and Fact attributes you added like so: [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/45616")]. Note that you don't need to repeat the issue number at the start of the WorkItem, it is already in the URL in the second arg.

Done in commit 987f561. All attributes now use the merged format [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/45616")].

Changed all test attributes from separate lines: [WorkItem(45616, "#45616")] [Fact] To merged format: [Fact, WorkItem("#45616")] Removed redundant issue number from WorkItem since it's already in the URL. Co-authored-by: CyrusNajmabadi <[email protected]>

CyrusNajmabadi · 2025-10-16T23:34:43Z

src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs

+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/45616")]
+        public void ForeachOnNullCastToStringWithNullableEnabled()


@copilot add a test where null is casted to (dynamic). And a case with null casted to a multidimensional array (like int[,]).

Done in commit 2f4f633. Added tests for (dynamic)null and (int[,])null, both with and without #nullable enable.

Added 4 new tests as requested: - ForeachOnNullCastToDynamic: Test (dynamic)null without nullable - ForeachOnNullCastToMultidimensionalArray: Test (int[,])null without nullable - ForeachOnNullCastToDynamicWithNullableEnabled: Test (dynamic)null with #nullable enable - ForeachOnNullCastToMultidimensionalArrayWithNullableEnabled: Test (int[,])null with #nullable enable All tests verify that these typed null casts compile successfully, with appropriate warnings in nullable-enabled contexts. Co-authored-by: CyrusNajmabadi <[email protected]>

CyrusNajmabadi · 2025-10-16T23:45:07Z

src/Compilers/CSharp/Test/Semantic/Semantics/ForEachTests.cs

+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/45616")]
+        public void ForeachOnNullCastToMultidimensionalArrayWithNullableEnabled()


@copilot add a test where null is being casted to a type parameter T where T is constrained to some IEnumerable<> type.

Done in commit 7f1c97c. Added tests for (T)null where T : class, IEnumerable<int>, both with and without #nullable enable. The class constraint is necessary to allow the null cast.

Added abstract class System.Enum that derives from ValueType to the systemSource in ForEach_StringNotIEnumerable test. This is required for emitting the AttributeTargets enum properly when using VerifyEmitDiagnostics. Co-authored-by: AlekseyTs <[email protected]>

AlekseyTs · 2025-10-19T01:18:45Z

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs


            var comp = CreateEmptyCompilation(new[] { source, systemSource }, options: WithNullableEnable());
-            comp.VerifyDiagnostics(
+            comp.VerifyEmitDiagnostics(


VerifyEmitDiagnostics

@copilot Please update baseline for this test #Closed

@copilot Please run the enclosing test and add a missing argument based on the output.

AlekseyTs · 2025-10-20T16:23:56Z

@CyrusNajmabadi It looks like this PR needs some attention. Copilot is unable to fixup baseline for a failing test, and an attempt to do that through a GitHub suggestion comment failed (presumably due to a size of the affected file). #Closed

AlekseyTs

LGTM (commit 20)

jcouv · 2025-10-21T17:26:36Z

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs

-                    //         await foreach (var i in (IAsyncEnumerable<int>)null)
-                    Diagnostic(ErrorCode.ERR_NullNotValid, "(IAsyncEnumerable<int>)null").WithLocation(9, 33)
-                    );
+                .VerifyEmitDiagnostics();


It would be even better to verify execution (throws NRE). Comment also applies to other tests that no longer have errors

@copilot can you figure out how tests validate that they throw at runtime, and update all the passing tests to then run and demonstrate that they do crash with a NullReferenceException as expected.

will consider in followup.

jcouv · 2025-10-21T17:30:23Z

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

+
+    public enum AttributeTargets
+    {
+        Assembly = 0x0001,


nit: do we need all these values for this test?

jcouv

LGTM Thanks (commit 20) with two suggestions to consider

* upstream/main: (123 commits) Fix SafeContext of Span-valued collection expressions to match specification (dotnet#80684) Improve detection of invalid references for implicitly typed expression variables declared within implicit object creation expressions. (dotnet#80546) Add test demonstrating behavior of ToMinimalDisplayString. (dotnet#80757) Only set DOTNET_HOST_PATH if something was installed (dotnet#80842) Clean up a Razor external access service (dotnet#80830) Remove unused statement (dotnet#80823) Allow foreach on typed null enumerables (dotnet#80783) Update documentation for DeclaringSyntaxReferences and Locations to clarify partial members behavior (dotnet#80704) Fix issue converting an auto prop to a full prop when 'field' and 'initializers' are involved Rename childIsSimple to innerExpressionHasPrimaryPrecedence and add more test cases Remove placeholder WorkItem attributes from new tests Fix RemoveUnnecessaryParentheses to detect simple expressions in bitwise operations [main] Update dependencies from dotnet/arcade (dotnet#80828) Fix ITypeSymbol.BaseType documentation for type parameters (dotnet#80770) soft-select select camelcase matched item if user might be typing an undefined type parameter (dotnet#80809) Allow semantic tokens in Razor to be better behaved (dotnet#80815) Rebase Remove using Update test Add fix all test ...

Initial plan

c83025b

Copilot AI assigned Copilot and CyrusNajmabadi Oct 16, 2025

Copilot started work on behalf of CyrusNajmabadi October 16, 2025 22:02 View session

Copilot AI changed the title ~~[WIP] Fix undeserved error for null enumerable in foreach loop~~ Fix incorrect error reporting for foreach on null IEnumerable Oct 16, 2025

Copilot AI requested a review from CyrusNajmabadi October 16, 2025 22:23

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Copilot AI changed the title ~~Fix incorrect error reporting for foreach on null IEnumerable~~ Allow foreach on typed null enumerables Oct 16, 2025

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Updaate baseline

74c24ea

AlekseyTs approved these changes Oct 21, 2025

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jcouv reviewed Oct 21, 2025

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jcouv self-assigned this Oct 21, 2025

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Copilot started work on behalf of CyrusNajmabadi October 21, 2025 17:56 View session

Copilot finished work on behalf of CyrusNajmabadi October 21, 2025 17:58

CyrusNajmabadi merged commit d46a551 into main Oct 21, 2025
25 checks passed

CyrusNajmabadi deleted the copilot/fix-null-enumerable-error branch October 21, 2025 18:00

dotnet-policy-service bot added this to the Next milestone Oct 21, 2025

Copilot AI mentioned this pull request Oct 21, 2025

Add runtime NullReferenceException validation to foreach null iteration tests #80839

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dotnet-maestro bot mentioned this pull request Oct 23, 2025

[main] Source code updates from dotnet/roslyn dotnet/dotnet#3061

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This was referenced Oct 25, 2025

[Automated] PRs inserted in VS build feature.polytype-11124.148 #80896

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[Automated] PRs inserted in VS build main-11124.32 #80897

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[Automated] PRs inserted in VS build feature.debugger.main-11127.165 #80926

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[main] Source code updates from dotnet/roslyn dotnet/dotnet#3147

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dotnet-maestro bot mentioned this pull request Dec 18, 2025

[main] Source code updates from dotnet/roslyn dotnet/dotnet#3957

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davidwengier modified the milestones: Next, 18.3 Jan 6, 2026

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