无法翻译 LINQ 表达式，将在本地计算

Question

Im getting this WARNING in EntityFramework Core what is wrong?

I already set MSSQL Datebase to Case Sensitive.

Latin1_General_100_CS_AS

var test = await _context.Students
                .FirstOrDefaultAsync(m => m.LastName.Equals("ALEXANDER", StringComparison.InvariantCultureIgnoreCase));

Microsoft.EntityFrameworkCore.Query:Warning: The LINQ expression 'where [m].LastName.Equals("ALEXANDER", InvariantCultureIgnoreCase)' could not be translated and will be evaluated locally.

Answer 1

You have to be aware of the difference between IEnumerable and Iqueryable .

An IEnumerable object represents a sequence of objects. It holds everything to enumerate over this sequence: you can ask for the first element of the sequence, and once you've got an element you can ask for the next one, as long as there is a next one.

An IQueryable object seems like an IEnumerable, however, it does not represent an enumerable sequence, it represents the potential to get an IEnumerable sequence.

The IQueryable object holds an Expression and a Provider . The Expression is a generic description expressing what must be queried. The Provider knows who will execute the query (usually a database management system) and what language is used to communicate with this DBMS (usually SQL).

If you start enumerating an IQueryable, either explicitly using GetEnumerator and MoveNext , or implicitly by calling foreach, ToList, Max, FirstOrDefault, etc, which will deep inside call GetEnumerator and MoveNext, the Expression is sent to the Provider, who will translate it into SQL and fetch the data from the DBMS. The fetched data is returned as an IEnumerable, of which the GetEnumerator and MoveNext are called.

So the query is not executed before you call GetEnumerator and MoveNext.

What does this have to do with my question?

Entity framework can only convert classes and methods to SQL that it knows about. Entity Framework does not know your own functions. In fact, there are several LINQ function that are not supported by entity framework. See Supported and Unsupported LINQ methods

One of the unsupported methods is String.Equals(string, StringComparison) . If you use this function, the compiler can't complain, because the compiler does not know what functions are supported by your version of entity framework. Therefore you won't see this error at compile time, you'll see it at runtime.

The error tells you that the data will first be fetched before the function is called. This might lead to inefficient behaviour.

Your LINQ statement is equal to (leave out the async-await, not part of the problem)

var test = dbContext.Students
    .Where(student => student.LastName.Equals("ALEXANDER", StringComparison.InvariantCultureIgnoreCase))
    .FirstOrDefault();
    

Since Equals can't be used, the warning says that the data is fetched locally before the Where is executed. So it might be that several items that will not pass the Where will be transferred from the DBMS to your local process.

If your database can ignore case sensitivity, consider changing your code to:

var test = dbContext.Students
    .Where(student => student.LastName == "ALEXANDER")
    .FirstOrDefault();

This will result in a SQL statement similar to:

SELECT TOP 1 * from myDatabase.Students where LastName = "ALEXANDER"

(not sure if this is correct SQL, since I use entity framework my SQL is a bit rusty. I guess you'll get the gist)

Answer 2

EntityFramework 无法将Equals("ALEXANDER", StringComparison.InvariantCultureIgnoreCase)转换为 SQL，因此它会将所有 Student 表加载到内存中，然后搜索满足等式的第一个条目。

Answer 3

I got this error by querying my LINQ methods in the wrong order before calling a ToListAsync call at the end.

Here was the code that threw this error for me:

var sideeffects = await Context.SideEffects.Include(x => x.Language)
.Select(x => new SideEffect() { Name = x.Name, Language = x.Language, CleanName = x.CleanName, Id = x.Id, SideEffects = x.SideEffects, Related = x.Related, DrugSideEffects = x.DrugSideEffects })
.Where(x => !x.IsDeleted).ToListAsync();

Calling the where at the end after the select and include screwed up the query and created this error. When I moved the where clause to the beginning, it cleared the error and allowed the query to be performed successfully with all required fields:

var sideeffects = await Context.SideEffects.Where(x => !x.IsDeleted)
.Include(x => x.Language)
.Select(x => new SideEffect() { Name = x.Name, Language = x.Language, CleanName = x.CleanName, Id = x.Id, SideEffects = x.SideEffects, Related = x.Related, DrugSideEffects = x.DrugSideEffects }).ToListAsync();

Be mindful of the order of your LINQ queries when using entity framework - especially when using a Select or an Include.