Linq: Fixes .NET 10 MemoryExtensions.Contains breaking change in LINQ queries

[adityasa]

Summary

Fixes #5518 - .NET 10 (C# 14) changed array.Contains(x) to use MemoryExtensions.Contains with ReadOnlySpan<T> instead of Enumerable.Contains. This breaks the LINQ-to-SQL translator.

Architecture / Code Flow

flowchart TD
    A["<b>User LINQ Query</b><br/>.Where(x => array.Contains(x))"] --> B["<b>C# Compiler</b>"]
    
    B -->|".NET 6-9"| C["Enumerable.Contains&lt;T&gt;(array, x)"]
    B -->|".NET 10+"| D["MemoryExtensions.Contains&lt;T&gt;(<br/>ReadOnlySpan&lt;T&gt;.op_Implicit(array), x)"]
    
    C --> E["<b>SQLTranslator.TranslateExpression()</b>"]
    D --> E
    
    E --> F["<b>Step 1: ConstantEvaluator.PartialEval</b><br/>Collapse evaluable sub-expressions to constants"]
    F --> G["<b>Step 2: ExpressionToSql</b><br/>Translate expression tree to SQL AST"]
    
    subgraph step1["Step 1: ConstantEvaluator (Nominator - bottom-up)"]
        direction TB
        F1["Visit op_Implicit(array)<br/><i>child node, visited first</i>"]
        F2{"DeclaringType is<br/>ReadOnlySpan or Span?"}
        F3["Block: return false<br/><i>ref struct cannot be boxed</i>"]
        F4["Propagates up:<br/>parent Contains also<br/>not nominated"]
        F1 --> F2
        F2 -->|"Yes"| F3
        F3 --> F4
    end
    
    F --> step1
    step1 --> G
    
    subgraph step2["Step 2: BuiltinFunctionVisitor (routing)"]
        direction TB
        G1{"IsMemoryExtensionsMethod?<br/>DeclaringType == typeof MemoryExtensions<br/>AND Name == Contains"}
        G2["Route to<br/>ArrayBuiltinFunctions"]
        G3["Not matched<br/>(IndexOf, SequenceEqual, etc.)"]
        G4["Throws DocumentQueryException<br/><i>Method X not supported</i>"]
        G1 -->|"Yes"| G2
        G1 -->|"No"| G3
        G3 --> G4
    end
    
    G --> step2
    
    subgraph step3["Step 3: ArrayBuiltinFunctions.VisitImplicit"]
        direction TB
        H1["UnwrapSpanImplicitConversion<br/>Detects op_Implicit on Span types"]
        H2["Extracts original array constant<br/>from args[0].Arguments[0]"]
        H3["Translates to SQL IN clause"]
        H1 --> H2
        H2 --> H3
    end
    
    G2 --> step3
    step3 --> I["<b>Output SQL</b><br/>SELECT * FROM c WHERE c.Name IN ('a', 'b', 'c')"]

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Without this fix (on .NET 10)

The ConstantEvaluator attempts to evaluate op_Implicit(array) which invokes ReadOnlySpan<T>.op_Implicit(T[]) and tries to box the ref struct result into Expression.Constant. This throws NotSupportedException which surfaces as a "Method not supported" error to the user.

End-to-End Verification (.NET 10)

Verified with a standalone .NET 10 (net10.0, LangVersion=preview) console app that references the SDK via ProjectReference. This app could not be checked in because CI pipelines do not yet support .NET 9/10, but the verification was run locally against the Cosmos DB Emulator.

All 8 scenarios pass:

#	Scenario	Generated SQL
1	string[].Contains(x.Field)	root["Name"] IN ("Alice", "Bob")
2	int[].Contains(x.Field)	root["Score"] IN (42, 99)
3	Guid[].Contains(x.Field)	root["Tag"] IN ("aaaa...")
4	Full round-trip (insert + query + read)	1 item returned
5	x.Children.Contains("value") (array field on doc)	ARRAY_CONTAINS(root["Children"], "child1")
6	x.Children.Contains round-trip	1 item returned
7	Customer exact repro: array.Contains(x) on simple type	root IN ("a")
8	enumerable.Contains(x) still works	root IN ("a")

Verification app code (click to expand)

// Temporary .NET 10 verification app for issue #5518 fix
// Target: net10.0, LangVersion=preview
// References Microsoft.Azure.Cosmos.csproj via ProjectReference

using Microsoft.Azure.Cosmos;
using Microsoft.Azure.Cosmos.Linq;

const string EndpointUrl = "https://localhost:443";
const string PrimaryKey = "<emulator-key>";
const string DatabaseId = "VerifyLinqNet10";
const string ContainerId = "Items";

int passed = 0, failed = 0;

using CosmosClient client = new(
    accountEndpoint: EndpointUrl,
    authKeyOrResourceToken: PrimaryKey,
    new CosmosClientOptions { ConnectionMode = ConnectionMode.Gateway });

Database database = await client.CreateDatabaseIfNotExistsAsync(DatabaseId);
Container container = await database.CreateContainerIfNotExistsAsync(ContainerId, "/pk");

var item = new TestItem
{
    id = "test-1", pk = "p1", Name = "Alice",
    Score = 42, Tag = Guid.Parse("aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee"),
    Children = ["child1", "child2", "child3"]
};
await container.UpsertItemAsync(item, new PartitionKey("p1"));

// Test 1: string[].Contains  variable-side, triggers MemoryExtensions path on .NET 10
string[] names = ["Alice", "Bob"];
var q1 = container.GetItemLinqQueryable<TestItem>().Where(x => names.Contains(x.Name));
Console.WriteLine(q1.ToQueryDefinition().QueryText);
// Output: SELECT VALUE root FROM root WHERE (root["Name"] IN ("Alice", "Bob"))

// Test 5: item.Children.Contains  document-field-side
var q5 = container.GetItemLinqQueryable<TestItem>().Where(x => x.Children.Contains("child1"));
Console.WriteLine(q5.ToQueryDefinition().QueryText);
// Output: SELECT VALUE root FROM root WHERE ARRAY_CONTAINS(root["Children"], "child1")

// Test 7: Customer exact repro from issue #5518
var array = new[] { "a" };
var q7 = container.GetItemLinqQueryable<string>().Where(x => array.Contains(x));
Console.WriteLine(q7.ToQueryDefinition().QueryText);
// Output: SELECT VALUE root FROM root WHERE (root IN ("a"))

// Test 8: Enumerable.Contains still works (regression check)
var enumerable = new[] { "a" }.AsEnumerable();
var q8 = container.GetItemLinqQueryable<string>().Where(x => enumerable.Contains(x));
Console.WriteLine(q8.ToQueryDefinition().QueryText);
// Output: SELECT VALUE root FROM root WHERE (root IN ("a"))

await database.DeleteAsync();

class TestItem
{
    public string id { get; set; } = "";
    public string pk { get; set; } = "";
    public string Name { get; set; } = "";
    public int Score { get; set; }
    public Guid Tag { get; set; }
    public string[] Children { get; set; } = [];
}

Changes

• ConstantEvaluator.cs: Exclude op_Implicit on ReadOnlySpan<>/Span<> types from partial evaluation (ref structs cannot be boxed)
• ArrayBuiltinFunctions.cs: UnwrapSpanImplicitConversion() unwraps op_Implicit to extract the underlying array before translating to SQL
• BuiltinFunctionVisitor.cs: Route MemoryExtensions.Contains to the array translator (scoped to Contains only; other methods produce a clear "method not supported" error)
• LinqMemoryExtensionsTests.cs: 12 unit tests covering:
  • ConstantEvaluator exclusions (op_Implicit on Span types blocked, non-Span still evaluable)
  • Supported translations (string/int/Guid arrays, Enumerable.Contains regression)
  • Unsupported methods produce clear errors (IndexOf, SequenceEqual)

Supersedes

This PR supersedes both #5585 and #5477 with a more targeted fix.

.NET Breaking Change Context

This issue stems from a by-design breaking change in C# 14 / .NET 10 where overload resolution now prefers Span/ReadOnlySpan extension methods ("first-class span" support). The .NET runtime team's mitigation is [OverloadResolutionPriority] on Span overloads so the compiler prefers IEnumerable versions. However, LINQ providers (EF Core, Cosmos DB, etc.) still need to handle the case where the compiler emits the Span-based call.

Related discussions:

• dotnet/runtime#109757 "First class span's break EFC" (main discussion)
• dotnet/runtime#109549 API Proposal: Apply [OverloadResolutionPriority] to Span-based overloads
• dotnet/efcore#35547 "Some usages of Contains no longer compile"
• dotnet/docs#43952 Breaking change documentation
• Official docs C# 14 overload resolution with span parameters

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[NaluTripician]

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[xinlian12]

✅ Review complete (38:00)

Posted 2 inline comment(s).

_{Steps: ✓ context, correctness, cross-sdk, design, history, past-prs, synthesis, test-coverage}

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