dotnet/efcore · error · InvalidOperationException

The entity type '{entityType}' is sharing the container '{co

Error message

The entity type '{entityType}' is sharing the container '{container}' with other types, but does not have a discriminator value configured. Configure a unique discriminator value for this entity type.

What it means

A discriminator property exists but no unique discriminator value is assigned for an entity type sharing a container. CosmosModelValidator throws InvalidOperationException via CosmosStrings.NoDiscriminatorValue when GetDiscriminatorValue() returns null for an instantiable type in a shared container.

Source

Thrown at src/EFCore.Cosmos/Infrastructure/Internal/CosmosModelValidator.cs:207

            }

            if (mappedTypes.Count == 1)
            {
                break;
            }

            if (entityType.ClrType.IsInstantiable()
                && entityType.GetContainingPropertyName() == null)
            {
                if (entityType.FindDiscriminatorProperty() == null)
                {
                    throw new InvalidOperationException(CosmosStrings.NoDiscriminatorProperty(entityType.DisplayName(), container));
                }

                var discriminatorValue = entityType.GetDiscriminatorValue();
                if (discriminatorValue == null)
                {
                    throw new InvalidOperationException(CosmosStrings.NoDiscriminatorValue(entityType.DisplayName(), container));
                }

                if (discriminatorValues.TryGetValue(discriminatorValue, out var duplicateEntityType))
                {
                    throw new InvalidOperationException(
                        CosmosStrings.DuplicateDiscriminatorValue(
                            entityType.DisplayName(),
                            discriminatorValue,
                            duplicateEntityType.DisplayName(),
                            container));
                }

                discriminatorValues[discriminatorValue] = entityType;

                var currentIsDiscriminatorMappingComplete = entityType.GetIsDiscriminatorMappingComplete();
                if (isDiscriminatorMappingComplete == null)
                {
                    isDiscriminatorMappingComplete = currentIsDiscriminatorMappingComplete;

View on GitHub (pinned to dbf9771522)

Solutions

Assign a unique discriminator value via .HasValue(...) for each concrete entity type sharing the container.
Verify GetDiscriminatorValue() is non-null for every instantiable type in the shared container.

Example fix

// before (Dog has no discriminator value)
modelBuilder.Entity<Cat>(b => { b.ToContainer("pets"); b.HasDiscriminator(c => c.Kind).HasValue("cat"); });
modelBuilder.Entity<Dog>(b => { b.ToContainer("pets"); b.HasDiscriminator(d => d.Kind); });

// after (value assigned)
modelBuilder.Entity<Dog>(b => { b.ToContainer("pets"); b.HasDiscriminator(d => d.Kind).HasValue("dog"); });

Defensive patterns

Strategy: validation

Validate before calling

// Every instantiable type in a shared container needs a discriminator value.
var byContainer = modelBuilder.Model.GetEntityTypes()
    .Where(e => e.FindPrimaryKey() != null && e.GetContainer() != null && e.ClrType.IsInstantiable())
    .GroupBy(e => e.GetContainer());
foreach (var g in byContainer.Where(g => g.Count() > 1))
foreach (var et in g.Where(e => e.GetDiscriminatorValue() == null))
    throw new InvalidOperationException($"{et.Name} needs a discriminator value.");

Try / catch

try { context.Database.EnsureCreated(); }
catch (InvalidOperationException ex) when (ex.Message.Contains("discriminator value"))
{ throw new InvalidOperationException("Assign a HasValue(...) discriminator to each type.", ex); }

Prevention

After HasDiscriminator, always set HasValue on each concrete type.
Add a build-time test asserting GetDiscriminatorValue() is non-null per shared container.
When adding a new type, set its discriminator value immediately.

When it happens

Trigger: Configuring HasDiscriminator but omitting .HasValue(...) for one of the concrete entity types in a shared container.

Common situations: Adding a new derived/root type and forgetting its discriminator value; relying on convention that did not fill the value; abstract base configured but concrete types missing values.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/9c65571506323535. Report an issue: GitHub.