dotnet/efcore · error · InvalidOperationException

The DbFunction '{function}' has an invalid return type '{typ

Error message

The DbFunction '{function}' has an invalid return type '{type}'. Ensure that the return type can be mapped by the current provider.

What it means

ValidateDbFunction (RelationalModelValidator.cs:363-371) throws for a SCALAR DbFunction whose TypeMapping is null - meaning the current provider has no relational type mapping for the function's return type. A scalar DbFunction must return a value the provider can map (int, string, DateTime, etc.); an unmappable return type cannot be translated to a SQL scalar, so the function registration is rejected.

Source

Thrown at src/EFCore.Relational/Infrastructure/RelationalModelValidator.cs:367

        IDiagnosticsLogger<DbLoggerCategory.Model.Validation> logger)
    {
    }

    /// <summary>
    ///     Validates a single database function.
    /// </summary>
    /// <param name="dbFunction">The database function to validate.</param>
    /// <param name="logger">The logger to use.</param>
    protected virtual void ValidateDbFunction(
        IDbFunction dbFunction,
        IDiagnosticsLogger<DbLoggerCategory.Model.Validation> logger)
    {
        var model = dbFunction.Model;
        if (dbFunction.IsScalar)
        {
            if (dbFunction.TypeMapping == null)
            {
                throw new InvalidOperationException(
                    RelationalStrings.DbFunctionInvalidReturnType(
                        dbFunction.ModelName,
                        dbFunction.ReturnType.ShortDisplayName()));
            }
        }
        else
        {
            var elementType = dbFunction.ReturnType.GetGenericArguments()[0];
            var entityType = model.FindEntityType(elementType);

            if (entityType?.IsOwned() == true
                || ((IConventionModel)model).IsOwned(elementType)
                || (entityType == null && model.GetEntityTypes().Any(e => e.ClrType == elementType)))
            {
                throw new InvalidOperationException(
                    RelationalStrings.DbFunctionInvalidIQueryableOwnedReturnType(
                        dbFunction.ModelName, elementType.ShortDisplayName()));
            }

View on GitHub (pinned to dbf9771522)

Solutions

Change the DbFunction return type to one the provider maps natively (int, string, DateTime, decimal, etc.).
If the type is custom/an enum, configure a value converter so it maps to a primitive the provider understands.
Register a custom relational type mapping for the return type via the provider's plugin if you control the provider.

Example fix

// before
public MyCustomStruct ComputeScore(int id) => default;
modelBuilder.HasDbFunction(() => ComputeScore(default)); // unmappable return -> throws

// after - return a mapped primitive (or convert the custom type)
public int ComputeScore(int id) => 0;
modelBuilder.HasDbFunction(() => ComputeScore(default));

Defensive patterns

Strategy: validation

Validate before calling

// Pre-validate scalar DbFunction return types are provider-mapped.
foreach (var fn in context.Model.GetDbFunctions())
{
    if (fn.IsScalar && fn.TypeMapping == null)
    {
        // will throw - change the return type or add a value converter/mapping.
    }
}

Prevention

Use provider-mappable primitive return types for scalar DbFunctions.
Configure value conversions for enum/custom return types.
Check provider type-coverage tables before adding exotic return types.

When it happens

Trigger: Registering a DbFunction via HasDbFunction() whose CLR return type is not supported by the provider (e.g. a custom struct/enum without a mapping, or an unsupported type like IntPtr/Guid on a limited provider). Thrown at model validation.

Common situations: Custom value types as DbFunction returns; enums not configured with HasConversion; using a type the provider doesn't map (e.g. certain providers and char/TimeSpan variants); porting DbFunctions between providers with different type coverage.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/bff5dd2f20ee7cb6. Report an issue: GitHub.