实体框架和扩展方法中的委托

Question

Recently I faced a problem with using delegates as predicates in LINQ extensions with EF. Here is a simple example.

Func<LmsIdentityUser, bool> predicate = u => u.FullName.Contains(val) || u.EmployeeId.Contains(val);
var result = _ctx.Users.Where(predicate).ToList();
var result2 = _ctx.Users.Where(u => u.FullName.Contains(val) || u.EmployeeId.Contains(val)).ToList();

I expected that both queries should produce identical SQL query because predicate used in Where() is the same. The only difference is that in the first case it is defined using a delegate. But here are actual SQL queries generated by EF for these cases.

Case 1 (using delegate)

SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[EmployeeId] AS [EmployeeId], 
[Extent1].[FullName] AS [FullName]    
FROM [dbo].[AspNetUsers] AS [Extent1]

Case 2 (using lambda directly)

SELECT TOP (15) 
[Extent1].[Id] AS [Id], 
[Extent1].[EmployeeId] AS [EmployeeId], 
[Extent1].[FullName] AS [FullName]
FROM [dbo].[AspNetUsers] AS [Extent1]
WHERE ([Extent1].[FullName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[EmployeeId] LIKE @p__linq__1 ESCAPE N'~')

So the question is what is wrong with delegates? What should I do to use them in LINQ extensions?

Answer 1

Delegates are of the form Func<Input, Output> whereas LINQ queries are Expression<Func<Input, Output>> . LINQ parses the expression to generate a query whereas the delegate acts on the data returned by the SELECT query. Note there's no filtering at all for the delegate version.

I believe you could change your predicate to be an Expression<Func<>> to get the behaviour you're looking for though.