Treat ToString as casting to string in the database

[fingers10]

I'm trying to do a contains search on enum property in my DbSet and EF Core 3.1 throws the below error

The LINQ expression 'DbSet .Where(d =>
d.Position.ToString().Contains("acc"))' could not be translated.
Either rewrite the query in a form that can be translated, or switch
to client evaluation explicitly by inserting a call to either
AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync()

Entity:

public class DemoEntity
{
    [Key]
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public Position Position { get; set; }
}

Enum - Position:

public enum Position
{
    [Display(Name = "Accountant")]
    Accountant,
    [Display(Name = "Chief Executive Officer (CEO)")]
    ChiefExecutiveOfficer,
    [Display(Name = "Integration Specialist")]
    IntegrationSpecialist,
    [Display(Name = "Junior Technical Author")]
    JuniorTechnicalAuthor,
    [Display(Name = "Pre Sales Support")]
    PreSalesSupport,
    [Display(Name = "Sales Assistant")]
    SalesAssistant,
    [Display(Name = "Senior Javascript Developer")]
    SeniorJavascriptDeveloper,
    [Display(Name = "Software Engineer")]
    SoftwareEngineer
}

DbContext:

public class DemoDbContext : DbContext
{
    public DemoDbContext(DbContextOptions options)
        : base(options) { }

    public DbSet<DemoEntity> Demos { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder
            .Entity<DemoEntity>()
            .Property(e => e.Position)
            .HasConversion<string>();
    }
}

When I query the table as follows I'm getting the error

try
{
    var test = await _context.Demos.Where(x => x.Position.ToString().Contains("acc")).ToListAsync();
}
catch (System.Exception e)
{
    //throw;
}

The Position is of type NVARCHAR(MAX) in my database.

This is not possible? If so please can you help me with explanation?

[ajcvickers]

@fingers10 This should work:

.Where(x => ((string)(object)x.Position).Contains("acc"))

EF Core then interprets this as a cast to string and generates:

SELECT [d].[Id], [d].[FirstName], [d].[LastName], [d].[Position]
FROM [DemoEntity] AS [d]
WHERE CHARINDEX(N'acc', CAST([d].[Position] AS nvarchar(max))) > 0

However, we discussed that it would be useful to allow ToString to do the same thing--it would avoid the non-obvious trick of casting to object. There is some concern that the output of ToString on on the client could be very different to what is generated on the database, but given the weak semantics of ToString anyway, that's probably okay.

[smitpatel]

Also see #14205
Convert a different ToString represent can give incorrect results in query.

[fingers10]

@ajcvickers Thanks for the workaround. It works good and expected.

[Danevandy99]

I'm attempting to fix this issue. My goal right now is to have the p.EnumProperty.ToString().Contains("value") translate to a similar query to ((string)(object)p.EnumProperty).Contains("value") when the database type is a string, but build a case expression for database types that are not strings. The SQL for the second case (where the enum is stored as a number), would look something like this with Sqlite:

SELECT "p"."PostId", "p"."BlogId", "p"."Content", "p"."Title", "p"."Visibility"
FROM "Posts" AS "p"
WHERE 'pr' = '' OR instr(CASE
    WHEN "p"."Visibility" = 0 THEN 'Public'
    WHEN "p"."Visibility" = 1 THEN 'Private'
    ELSE NULL
END, 'pr') > 0
LIMIT 1

[roji]

@Danevandy99 please separate general translation of ToString() to a database-side cast (CAST(x AS nvarchar(max)) - which is what's tracked by this issue and what's already possible via (string)(object) - from any sort of enum-specific string conversion such as the CASE/WHEN you propose above. The latter is orthogonal to the former and requires careful thought (if you'd like to pursue this, please open a separate issue).

[Danevandy99]

@roji Sounds good! @ajcvickers mentioned this in his comment about the casting workaround above about how translating .Tostring() to do a database-side cast could cause unexpected results when compared to a client-side evalutation, but I can see how the CASE/WHEN conversion will require more thought and changes, so I'll make a separate issue for that.