Entity Framework Core: Client-side Evaluation

My post a few weeks ago on Entity Framework Core: String Filter Tips may have come across as too harsh on client-side evaluation based on what I saw on Reddit. My point wasn’t that you shouldn’t use client-side evaluation when you need too, but that you should be making a choice when to use client-side evaluation and it is very easy to miss when a query is running client-side.

TimeRemove did a great job explaining ways to detect and deal with client-side evaluation which this post will review. The sample application is the same one used in  Entity Framework Core: String Filter Tips as defined in the Sample Application section.

Sample Client-side Query

The following is the query we will be using to trigger client-side evaluation.

var contacts = 
 await _context
       .Contacts
       .Where(c => String.Compare(c.FirstName, "D", StringComparison.Ordinal) > 0)
       .ToListAsync();

No Changes

Running this query with no change to the application will actually give you the following warning in the Debug Window.

Microsoft.EntityFrameworkCore.Query:Warning: The LINQ expression ‘where (Compare([c].FirstName, “D”, Ordinal) > 0)’ could not be translated and will be evaluated locally.

I’m not sure how long this warning has been there, but this is the first time I have noticed it. Good reminder to make sure and check the debug window more often.

Throw Exception for Client-side Evaluation

Making the following change to the DbContext will cause Entity Framework Core to throw an exception when it does client-side evaluation instead of just showing a warning. In this example, we are overriding the OnConfiguring function of our DbContext and changing to throw an exception if the application is built in debug mode.

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
#if DEBUG
    optionsBuilder.ConfigureWarnings(w => 
          w.Throw(RelationalEventId.QueryClientEvaluationWarning));
#endif

    base.OnConfiguring(optionsBuilder);
}

Now running the sample query results in the following exception.

InvalidOperationException: Error generated for warning ‘Microsoft.EntityFrameworkCore.Query.QueryClientEvaluationWarning: The LINQ expression ‘where (Compare([c].FirstName, “D”, Ordinal) > 0)’ could not be translated and will be evaluated locally.’. This exception can be suppressed or logged by passing event ID ‘RelationalEventId.QueryClientEvaluationWarning’ to the ‘ConfigureWarnings’ method in ‘DbContext.OnConfiguring’ or ‘AddDbContext’.

There are times when a client-side evaluation is what you want. How do you deal with that need and leave the exception enabled so you get a notification when a client-side evaluation is happening? To accomplish this you will need to split your query into multiple parts running the server evaluated parts and then in a different statement running the client evaluated parts. The following is our sample query broken up so that no exception is thrown.

//Server evaluation
var contacts = await _context.Contacts.ToListAsync();

//Client evaluation
contacts = 
    contacts
        .Where(c => String.Compare(c.FirstName, "D", StringComparison.Ordinal) > 0)
        .ToList();

Wrapping Up

If I introduced any confusion I hope this post helps clear it up. Just to be 100% clear client-side evaluation isn’t bad, but I feel it is important for it to be a choice that the developer has made. I hope the above will give you the tools to be clear on when you are choosing client-side evaluation.

Thanks again to TimeRemove for the clarifications to my original post.

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