I just got this error after doing major code refactoring. The scenario is There was AgentBooking Entity. Now the business decide to add OnlineBooking, Then I need to refactor AgentBooking & OnlineBooking to inherit from Abstract Booking Entity and rename some columns.

This error appear, if I remove all previous migrations & DbContextModelSnapshot, then recreate one it will be successful. But there is no way to run this migration in production environment.

Is there anyway to debug migrations tools? I need to find out where is the problem. Key: System.Object Item [System.String] is kinda unhelpful at all.

This just hit us on a large project that has recently been upgraded to .net7 and EF Core 7.01. We are refactoring to remove all OwnedEntities. We removed one, successfully added a migration, then spent hours factoring out dozens of other owned entites. Then we tried to add another migration and this error hit.

The error is so vague it gives no clue how to proceed.

System.ArgumentException: An item with the same key has already been added. Key: System.Object Item [System.String]
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedProperties(IEntityType entityType, ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedColumns(ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(ITable target, DiffContext diffContext)+MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Sort(IEnumerable`1 operations, DiffContext diffContext)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetDifferences(IRelationalModel source, IRelationalModel target)
   at Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.ScaffoldMigration(String migrationName, String rootNamespace, String subNamespace, String language)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

Ah sorry, missed that. The version of the CLI tool doesn’t usually matter. Make sure your project is using the latest daily build packages.

@AndriySvyryd I just tested the latest dotnet ef cli, that I think should contain the fixe https://github.com/dotnet/efcore/pull/29981, and I still get the error.

PS C:\Users\apaquette> dotnet tool uninstall -g dotnet-ef
L'outil 'dotnet-ef' (version '7.0.2') a été désinstallé.
PS C:\Users\apaquette> dotnet tool install -g dotnet-ef --version 8.0.0-* --add-source https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json
Vous pouvez appeler l'outil à l'aide de la commande suivante : dotnet-ef
L'outil 'dotnet-ef' (version '8.0.0-alpha.1.23066.4') a été installé correctement.

I have a table that contains owned types and this table is not registered with a DbSet<> but only by a IEntityTypeConfiguration<>. When removing the configuration I get the same error as everybody.

Was it supposed to work or is it the same message for an other error? Thank you!

My error, and how I resolved it

I encountered the same error message when using Entity Framework Core .NET Command-line Tools 7.0.1 in a project that depended on versions 6.0.9 of each of the various Entity Framework Core packages. This is an oversimplification of my model, but it shows all the parts that were relevant to solving my error.

Code

DbContext snippet

public DbSet<Foo> Foos { get; set; } = null!;

protected override void OnModelCreating(ModelBuilder mb)
{
    mb.Entity<Foo>().OwnsOne(f => f.Prop1);
    mb.Entity<Foo>().OwnsOne(f => f.Prop2);
}

Foo snippet

[Key]
public int FooId { get; set; }
public Bar Prop1 { get; set; } = new();
public Bar Prop2 { get; set; } = new();

Bar snippet

public string Baz { get; set; } = "";
public string Qux { get; set; } = "";

The problem

Removing the Foo class and any references to it and then running dotnet ef migrations add MigrationName resulted in the same error message as reported by others in this thread.

How I resolved it

1. remove the navigation properties Prop1 and Prop2 from Foo
2. add a new migration
3. delete the DbSet<Foo> property from the DbContext
4. add a new migration

Conclusion

I am relatively new to Entity Framework Core, so I cannot diagnose what initially caused this error, but I assume it has something to do with deleting entities that have owned entities attached.

The one line diff I posted above should avoid the exception. But a better fix would sort columns without using PropertyInfo as a dictionary key.

For my own project I replaced the model differ service to test the one line code change. However, overriding the implementation of MigrationsModelDiffer.GetSortedProperties involved copying huge slabs of internal code. Not really viable as a workaround for production code. Though you only need this fix when generating migrations, you don’t need to deploy any of this code to production.

Seeing the same in 6.0.1 caused by removing an entity (and the associated DbSet on the DbContext) that had an owned entity.

I was able to work around this by removing the representation of said entity in the snapshot, scaffolding an empty migration and populating the respective Up and Down from the initial migration that was already present.

System.ArgumentException: An item with the same key has already been added. Key: System.Object Item [System.String]
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedProperties(IEntityType entityType, ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedProperties(IEntityType entityType, ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedColumns(ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(ITable target, DiffContext diffContext)+MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Sort(IEnumerable`1 operations, DiffContext diffContext)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetDifferences(IRelationalModel source, IRelationalModel target)
   at Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.ScaffoldMigration(String migrationName, String rootNamespace, String subNamespace, String language)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
An item with the same key has already been added. Key: System.Object Item [System.String]

I got the same error, I’m using entities with owned entities. If I remove the DbSets the generated migrations are empty. If I remove the DbSets and fluent-maps, it throw the error.

Got the same when adding a migration that removes two completely independent DbSets (flat tables no relationships etc).

Hello, I am getting same error, when I try to create migration after removing entity with owned entities. Version of all EFCore related nugets + tools: 6.0.7 In this example project EfMigrationIssueOwnedEntities.zip you can get error by using commands: When I try to add migration either by dotnet ef migrations add DroFoo or Add-Migration DropFoo in package manager console, i am getting same error:

Build started...
Build succeeded.
System.ArgumentException: An item with the same key has already been added. Key: System.Object Item [System.String]
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedProperties(IEntityType entityType, ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedColumns(ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(ITable target, DiffContext diffContext)+MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Sort(IEnumerable`1 operations, DiffContext diffContext)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetDifferences(IRelationalModel source, IRelationalModel target)
   at Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.ScaffoldMigration(String migrationName, String rootNamespace, String subNamespace, String language)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.De
[EfMigrationIssueOwnedEntities.zip](https://github.com/dotnet/efcore/files/9182842/EfMigrationIssueOwnedEntities.zip)
[EfMigrationIssueOwnedEntities.zip](https://github.com/dotnet/efcore/files/9182856/EfMigrationIssueOwnedEntities.zip)
sign.OperationExecutor.OperationBase.Execute(Action action)
An item with the same key has already been added. Key: System.Object Item [System.String]

There is workaround:

1. Flatten entity: replace owned entities with direct properties. Column names and types remains same
2. Create intermediate migration - it should be empty
3. Remove entity and entity configuration
4. Create migration - it should contain drop table command
5. You can remove intermediate migration

@srmagura upgrading to 6.0.2 as per @bricelam above, fixed issue for me

Is it fixed on 6.0.2? I’m still getting it on 6.0.4

   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedProperties(IEntityType entityType, ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetSortedColumns(ITable table)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(ITable target, DiffContext diffContext)+MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Sort(IEnumerable`1 operations, DiffContext diffContext)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetDifferences(IRelationalModel source, IRelationalModel target)
   at Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.ScaffoldMigration(String migrationName, String rootNamespace, String subNamespace, String language)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
An item with the same key has already been added. Key: System.Object Item [System.String]

Entity Framework Core .NET Command-line Tools
6.0.4

@srmagura upgrading to 6.0.2 as per @bricelam above, fixed issue for me

Sounds good, I will work on it. It may be about 2 weeks — I’ll comment here when it is ready.