Throw for unions mapped as entity or complex types by Copilot · Pull Request #38398 · dotnet/efcore

Copilot · 2026-06-10T18:07:19Z

EF doesn't support C# union types yet. Mapping one as an entity type or complex type currently produces undefined/confusing behavior instead of a clear error.

Changes

Detection (SharedTypeExtensions): added Type.IsUnion(), which checks for System.Runtime.CompilerServices.UnionAttribute (matched by full name to avoid a compile-time dependency on the marker type).
Validation (ModelValidator): throw CoreStrings.UnionTypeNotSupported from ValidateEntityType (union entity types) and ValidatePropertyMapping(IComplexProperty) (union complex types; also covers complex collections and value-type unions).
Resource: new UnionTypeNotSupported string in CoreStrings.
Tests: ModelValidatorTest cases for union entity and complex types, simulated via the real [Union] attribute.

Message

The type '{type}' cannot be mapped because union types are not supported. See #36375 for more information.

Notes

Applies across all providers via the shared ModelValidator.
Detection keys off the canonical [Union] marker, so it covers both class and struct unions.

Co-authored-by: AndriySvyryd <6539701+AndriySvyryd@users.noreply.github.com>

Copilot

Pull request overview

This PR adds first-class model validation for upcoming C# union types by detecting [System.Runtime.CompilerServices.Union] / UnionAttribute on CLR types and throwing a clear, user-facing error when such types are mapped as EF entity or complex types (aligning behavior across all providers).

Changes:

Add Type.IsUnion() detection in SharedTypeExtensions (attribute full-name match to avoid hard dependency).
Throw CoreStrings.UnionTypeNotSupported(...) from ModelValidator when unions are encountered as entity or complex types.
Add the new resource string and unit tests covering both entity-type and complex-type mapping scenarios.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/EFCore.Tests/Infrastructure/ModelValidatorTest.cs	Adds regression tests asserting unions mapped as entity/complex types produce the new clear validation error.
src/Shared/SharedTypeExtensions.cs	Introduces `IsUnion()` extension method based on `UnionAttribute` full-name detection.
src/EFCore/Infrastructure/ModelValidator.cs	Enforces the new validation rule by throwing `UnionTypeNotSupported` for union entity/complex types.
src/EFCore/Properties/CoreStrings.resx	Adds the user-facing error message resource for unsupported union mapping.
src/EFCore/Properties/CoreStrings.Designer.cs	Adds the generated accessor for `UnionTypeNotSupported`.

Files not reviewed (1)

src/EFCore/Properties/CoreStrings.Designer.cs: Language not supported

Co-authored-by: AndriySvyryd <6539701+AndriySvyryd@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated no new comments.

Files not reviewed (1)

src/EFCore/Properties/CoreStrings.Designer.cs: Language not supported

github-actions · 2026-06-17T02:53:16Z

API review baseline changes for `src/EFCore/EFCore.baseline.json`

Show diff

The diff below was generated by ApiChief between the base and the PR.

  static class Microsoft.EntityFrameworkCore.Diagnostics.CoreStrings
+ static string UnionTypeNotSupported(object? type);

The C# preview "union" language feature now compiles union declarations (e.g. union UnionEntity(int);) as value types (structs implementing System.Runtime.CompilerServices.IUnion), rather than reference types. This broke the existing detection of union types mapped as entity or complex types added in #38398: - SharedTypeExtensions.IsValidEntityType required IsClass: true, so EntityType's constructor now threw a generic ArgumentException ("must be a non-interface reference type") as soon as a union type was added to the model, before ModelValidator ever got a chance to detect it and produce the specific UnionTypeNotSupported message. - ConstructorBindingConvention (an IModelFinalizingConvention that runs during FinalizeModel, before ModelValidator.Validate) attempted to bind a constructor for union types used as complex properties, throwing "No suitable constructor was found" before the model validator's union check was reached. Fix: - IsValidEntityType now also accepts union types (mirroring IsValidComplexType, which already allowed them), so union entity types can still be added to the model and are rejected later, with the intended friendly error message, by ModelValidator. - ConstructorBindingConvention now skips constructor binding for union entity/complex types, since they can never have a meaningful constructor binding, allowing model finalization to proceed to ModelValidator.Validate where the specific union error is thrown. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Backflow from https://github.com/dotnet/dotnet / 7bcb900 build 324583 Diff: https://github.com/dotnet/dotnet/compare/dadfb24164945e42d8fda33324ddf87b99b7686d..7bcb9009c40399c9073e7db938250486229cc85d From: dotnet/dotnet@dadfb24 To: dotnet/dotnet@7bcb900 [[ commit created by automation ]] * Update dependencies from build 324583 Updated Dependencies: Microsoft.DotNet.Arcade.Sdk, Microsoft.DotNet.Build.Tasks.Templating, Microsoft.DotNet.Helix.Sdk (Version 11.0.0-beta.26365.101 -> 11.0.0-beta.26376.106) Microsoft.Extensions.Caching.Memory, Microsoft.Extensions.Configuration, Microsoft.Extensions.Configuration.EnvironmentVariables, Microsoft.Extensions.Configuration.Json, Microsoft.Extensions.DependencyInjection, Microsoft.Extensions.DependencyModel, Microsoft.Extensions.HostFactoryResolver.Sources, Microsoft.Extensions.Logging, Microsoft.NETCore.App.Ref, Microsoft.NETCore.Platforms, System.Formats.Asn1, System.Runtime.Caching, System.Text.Encodings.Web, System.Text.Json (Version 11.0.0-preview.7.26365.101 -> 11.0.0-preview.7.26376.106) [[ commit created by automation ]] * Fix union model validator tests for new union shape Co-authored-by: wtgodbe <14283640+wtgodbe@users.noreply.github.com> * Update union model validator tests for .NET 11 preview behavior Co-authored-by: wtgodbe <14283640+wtgodbe@users.noreply.github.com> * Update dependencies from build 324662 Updated Dependencies: Microsoft.DotNet.Arcade.Sdk, Microsoft.DotNet.Build.Tasks.Templating, Microsoft.DotNet.Helix.Sdk (Version 11.0.0-beta.26376.106 -> 11.0.0-beta.26377.110) Microsoft.Extensions.Caching.Memory, Microsoft.Extensions.Configuration, Microsoft.Extensions.Configuration.EnvironmentVariables, Microsoft.Extensions.Configuration.Json, Microsoft.Extensions.DependencyInjection, Microsoft.Extensions.DependencyModel, Microsoft.Extensions.HostFactoryResolver.Sources, Microsoft.Extensions.Logging, Microsoft.NETCore.App.Ref, Microsoft.NETCore.Platforms, System.Formats.Asn1, System.Runtime.Caching, System.Text.Encodings.Web, System.Text.Json (Version 11.0.0-preview.7.26376.106 -> 11.0.0-preview.7.26377.110) [[ commit created by automation ]] * Update dependencies from build 324774 Updated Dependencies: Microsoft.DotNet.Arcade.Sdk, Microsoft.DotNet.Build.Tasks.Templating, Microsoft.DotNet.Helix.Sdk (Version 11.0.0-beta.26377.110 -> 11.0.0-beta.26378.106) Microsoft.Extensions.Caching.Memory, Microsoft.Extensions.Configuration, Microsoft.Extensions.Configuration.EnvironmentVariables, Microsoft.Extensions.Configuration.Json, Microsoft.Extensions.DependencyInjection, Microsoft.Extensions.DependencyModel, Microsoft.Extensions.HostFactoryResolver.Sources, Microsoft.Extensions.Logging, Microsoft.NETCore.App.Ref, Microsoft.NETCore.Platforms, System.Formats.Asn1, System.Runtime.Caching, System.Text.Encodings.Web, System.Text.Json (Version 11.0.0-preview.7.26377.110 -> 11.0.0-preview.7.26378.106) [[ commit created by automation ]] * Revert "Update union model validator tests for .NET 11 preview behavior" This reverts commit 22b6fc6. * Revert "Fix union model validator tests for new union shape" This reverts commit 26b6058. * Fix union type detection after C# union types became value types The C# preview "union" language feature now compiles union declarations (e.g. union UnionEntity(int);) as value types (structs implementing System.Runtime.CompilerServices.IUnion), rather than reference types. This broke the existing detection of union types mapped as entity or complex types added in #38398: - SharedTypeExtensions.IsValidEntityType required IsClass: true, so EntityType's constructor now threw a generic ArgumentException ("must be a non-interface reference type") as soon as a union type was added to the model, before ModelValidator ever got a chance to detect it and produce the specific UnionTypeNotSupported message. - ConstructorBindingConvention (an IModelFinalizingConvention that runs during FinalizeModel, before ModelValidator.Validate) attempted to bind a constructor for union types used as complex properties, throwing "No suitable constructor was found" before the model validator's union check was reached. Fix: - IsValidEntityType now also accepts union types (mirroring IsValidComplexType, which already allowed them), so union entity types can still be added to the model and are rejected later, with the intended friendly error message, by ModelValidator. - ConstructorBindingConvention now skips constructor binding for union entity/complex types, since they can never have a meaningful constructor binding, allowing model finalization to proceed to ModelValidator.Validate where the specific union error is thrown. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: dotnet-maestro[bot] <dotnet-maestro[bot]@users.noreply.github.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: wtgodbe <14283640+wtgodbe@users.noreply.github.com> Co-authored-by: Will Godbe <willgodbe@Wills-MacBook-Pro.local> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Initial plan

c3f2c8d

Copilot AI assigned Copilot and AndriySvyryd Jun 10, 2026

Copilot started work on behalf of AndriySvyryd June 10, 2026 18:07 View session

Copilot AI linked an issue Jun 10, 2026 that may be closed by this pull request

Throw for unions #38376

Closed

Copilot AI and others added 2 commits June 10, 2026 18:17

Throw for unions mapped as entity or complex types

ed467bd

Co-authored-by: AndriySvyryd <6539701+AndriySvyryd@users.noreply.github.com>

Add tests for throwing on union entity and complex types

6b4c926

Co-authored-by: AndriySvyryd <6539701+AndriySvyryd@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Throw error for unions as entity types in EF~~ Throw for unions mapped as entity or complex types Jun 10, 2026

Copilot finished work on behalf of AndriySvyryd June 10, 2026 18:23

Copilot AI requested a review from AndriySvyryd June 10, 2026 18:23

AndriySvyryd requested a review from Copilot June 10, 2026 19:07

Copilot started reviewing on behalf of AndriySvyryd June 10, 2026 19:07 View session

Copilot AI reviewed Jun 10, 2026

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AndriySvyryd reviewed Jun 10, 2026

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Comment thread src/EFCore/Properties/CoreStrings.Designer.cs

Copilot started work on behalf of AndriySvyryd June 10, 2026 21:12 View session

Update EFCore API baseline with UnionTypeNotSupported

f0921ad

Co-authored-by: AndriySvyryd <6539701+AndriySvyryd@users.noreply.github.com>

Copilot finished work on behalf of AndriySvyryd June 10, 2026 21:19

Copilot AI requested a review from AndriySvyryd June 10, 2026 21:19

AndriySvyryd unassigned AndriySvyryd and Copilot Jun 10, 2026

AndriySvyryd marked this pull request as ready for review June 10, 2026 22:05

AndriySvyryd requested a review from a team as a code owner June 10, 2026 22:05

Copilot AI review requested due to automatic review settings June 10, 2026 22:05

Copilot started reviewing on behalf of AndriySvyryd June 10, 2026 22:05 View session

AndriySvyryd requested a review from cincuranet June 10, 2026 22:07

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cincuranet approved these changes Jun 16, 2026

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AndriySvyryd merged commit a18fcc4 into main Jun 17, 2026
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AndriySvyryd deleted the copilot/throw-for-unions branch June 17, 2026 02:52

github-actions Bot added the api-review This PR or issue is introducing public API changes that need to be reviewed label Jun 17, 2026

AndriySvyryd removed the api-review This PR or issue is introducing public API changes that need to be reviewed label Jun 17, 2026

dotnet-maestro Bot mentioned this pull request Jun 18, 2026

[main] Source code updates from dotnet/efcore dotnet/dotnet#7266

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dotnet-milestone-bot Bot added this to the 11.0-preview6 milestone Jun 23, 2026

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Throw for unions mapped as entity or complex types - #38398

Throw for unions mapped as entity or complex types#38398
AndriySvyryd merged 4 commits into
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Copilot AI commented Jun 10, 2026 •

edited by AndriySvyryd

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github-actions Bot commented Jun 17, 2026

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github-actions Bot commented Jun 17, 2026

API review baseline changes for src/EFCore/EFCore.baseline.json

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Copilot AI commented Jun 10, 2026 •

edited by AndriySvyryd

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API review baseline changes for `src/EFCore/EFCore.baseline.json`