Fix PendingModelChangesWarning for Unicode diacritics (NFC/NFD) on Linux by TomasMoralesBarr · Pull Request #38197 · dotnet/efcore

TomasMoralesBarr · 2026-04-30T05:50:01Z

Problem

On Linux/WSL2, string values containing diacritic characters (e.g. Café)
can be saved in NFD Unicode normalization form, while on Windows they are
typically NFC. EF Core's MigrationsModelDiffer used ordinal comparison
which treated NFC and NFD as different values, causing a false
PendingModelChangesWarning.

Fix

Normalize string values to NFC before comparison in DiffData().

Test

Added Seed_data_with_NFD_vs_NFC_unicode_string_is_considered_unchanged to
MigrationsModelDifferTest which confirms NFC and NFD strings representing
the same character produce no migration diff.

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Commit messages follow this format:

        Summary of the changes
        - Detail 1
        - Detail 2

        Fixes #bugnumber

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Fixes dotnet#38191 On Linux/WSL2, string values with diacritic characters (e.g. 'Café') can be stored in NFD Unicode form, while on Windows they are typically NFC. EF Core's migration differ used ordinal string comparison which considered NFC and NFD as different values, causing a false PendingModelChangesWarning. Fix by normalizing string seed data values to NFC before comparison in DiffData."

Copilot

Pull request overview

Fixes false PendingModelChangesWarning on Linux/WSL2 caused by NFC vs NFD Unicode normalization differences when diffing seed data in migrations.

Changes:

Normalize string seed-data values prior to comparison during MigrationsModelDiffer.DiffData().
Add a regression test asserting that NFC and NFD equivalents in seed data produce no migration operations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs	Normalizes string values before comparing seed-data column modifications during diffing.
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds a test covering NFC vs NFD equivalence for seed-data string values.

Use NormalizationForm.FormC explicitly instead of relying on the default to make the intent clear to future readers of the code.

Copilot

Pull request overview

Addresses false PendingModelChangesWarning on Linux/WSL2 when seed data strings differ only by Unicode normalization form (NFC vs NFD), by normalizing string values prior to comparison in the seed data diff logic.

Changes:

Normalize string seed values to NFC before equality comparison in MigrationsModelDiffer.DiffData().
Add a regression test asserting that canonically equivalent NFC/NFD seed strings produce no migration operations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs	Normalizes string values (NFC) before comparing source/target seed data values.
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds a regression test covering NFC vs NFD equivalence for seeded string values.

AndriySvyryd

I agree with @roji that a more robust way of fixing this is by using StringComparison.InvariantCulture in a new comparer returned from

efcore/src/EFCore/ChangeTracking/ValueComparer.cs

Line 258 in d86b520

public static ValueComparer CreateDefault(

That way it will also benefit the update pipeline.

…alence - Add DefaultStringValueComparer in ValueComparer using StringComparison.InvariantCulture - Route string type through DefaultStringValueComparer in CreateDefault<T>() - Remove NFC normalization workaround from MigrationsModelDiffer.DiffData() - Restore regression test for NFC/NFD seed data in MigrationsModelDifferTest - Add unit tests for DefaultStringValueComparer in ValueComparerTest Fixes dotnet#38191"

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

+        // produce the same hash code.
+        public override int GetHashCode(string instance)
+            => StringComparer.InvariantCulture.GetHashCode(instance);
+


+        if (nonNullableType == typeof(string))
+        {
+            return new DefaultStringValueComparer(favorStructuralComparisons);
+        }


+        var comparer = ValueComparer.CreateDefault<string>(false);
+
+        Assert.True(comparer.Equals(nfcString, nfdString));
+        Assert.Equal(comparer.GetHashCode(nfcString), comparer.GetHashCode(nfdString));


…ueComparer - Update ValueComparerExtensions.IsDefault() to walk the base type chain, so subclasses of DefaultValueComparer<T> (e.g. DefaultStringValueComparer) are correctly recognized as default comparers - Update CosmosTypeMappingSourceTest assertions from DefaultValueComparer<string> to DefaultStringValueComparer - Regenerate compiled model baselines to reflect new string equality expression (string.Equals with InvariantCulture instead of ==)"

Replace 'v1 == v2' with 'string.Equals(v1, v2, StringComparison.InvariantCulture)' in all Scaffolding/Baselines across SqlServer and Cosmos test projects, reflecting the new DefaultStringValueComparer expression tree output."

Copilot

Pull request overview

Copilot reviewed 57 out of 62 changed files in this pull request and generated 1 comment.

+#pragma warning disable CA1309 // Use ordinal StringComparison - InvariantCulture is intentional to handle Unicode canonical equivalence (NFC/NFD)
+    internal sealed class DefaultStringValueComparer(bool favorStructuralComparisons)
+        : DefaultValueComparer<string>((v1, v2) => string.Equals(v1, v2, StringComparison.InvariantCulture), favorStructuralComparisons)
+    {
+        private static readonly MethodInfo StringEqualsMethodInfo
+            = typeof(string).GetMethod(nameof(string.Equals), [typeof(string), typeof(string), typeof(StringComparison)])!;
+
+        // String canonical equivalence (e.g. NFC vs NFD Unicode normalization) is handled by InvariantCulture comparison.
+        // Override hash code to be consistent with InvariantCulture equality so that canonically equivalent strings
+        // produce the same hash code.
+        public override int GetHashCode(string instance)
+            => StringComparer.InvariantCulture.GetHashCode(instance);
+
+        public override Expression ExtractEqualsBody(Expression leftExpression, Expression rightExpression)
+            => Expression.Call(StringEqualsMethodInfo, leftExpression, rightExpression, Expression.Constant(StringComparison.InvariantCulture));
+    }


Copilot AI review requested due to automatic review settings April 30, 2026 05:50

TomasMoralesBarr requested a review from a team as a code owner April 30, 2026 05:50

github-actions Bot added the community-contribution label Apr 30, 2026

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TomasMoralesBarr added 2 commits April 30, 2026 02:05

Make Unicode normalization form explicit (NormalizationForm.FormC)

b94608c

Use NormalizationForm.FormC explicitly instead of relying on the default to make the intent clear to future readers of the code.

Add missing using directive for System.Text in MigrationsModelDiffer

a5fe96e

Copilot AI review requested due to automatic review settings April 30, 2026 06:14

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roji mentioned this pull request Apr 30, 2026

PendingModelChangesWarning Diacritic Café #38191

Open

roji assigned AndriySvyryd Apr 30, 2026

AndriySvyryd requested changes Apr 30, 2026

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TomasMoralesBarr and others added 2 commits May 2, 2026 19:02

Merge branch 'dotnet:main' into fix/unicode-normalization-seed-diff

c582302

Copilot AI review requested due to automatic review settings May 4, 2026 03:42

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TomasMoralesBarr added 2 commits May 4, 2026 00:57

Copilot AI review requested due to automatic review settings May 4, 2026 06:28

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Fix PendingModelChangesWarning for Unicode diacritics (NFC/NFD) on Linux#38197

Fix PendingModelChangesWarning for Unicode diacritics (NFC/NFD) on Linux#38197
TomasMoralesBarr wants to merge 7 commits intodotnet:mainfrom
TomasMoralesBarr:fix/unicode-normalization-seed-diff

TomasMoralesBarr commented Apr 30, 2026 •

edited

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TomasMoralesBarr commented Apr 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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TomasMoralesBarr commented Apr 30, 2026 •

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