使用Func <>而不是lambda时，超时已过期

Question

Given:

public EntityAddress ReadSingle(Func<EntityAddress, bool> predicate)
{
    //var result = Context.CV3Address.FirstOrDefault(a => a.GUID == 1100222);
    var result = Context.CV3Address.FirstOrDefault(predicate);
    return result;
}

FirstOrDefault(a => a.GUID == 1100222); returns a result immediately.

FirstOrDefault(predicate); results in a timeout exception. Note that predicate = the lambda expression

My suspicion is that the latter method attempts to pull down all records which is not gonna happen with a table this large.

Why does this happen?

Answer 1

It happens because of the type of the predicate, which should have been instead

Expression<Func<CV3Address, bool>>

If the predicate is an expression tree (as above) and Context.CV3Address is an IQueryable<CV3Address> then EF can translate the expression tree to SQL and directly get the results from the database.

On the other hand, if the predicate is a Func<CV3Address, bool> (a delegate; a pointer to compiled code) then this cannot be translated to SQL. Therefore LINQ has no other option that to treat your repository as an IEnumerable<CV3Address> , which is a sequence that can be filtered in-memory. That has the side effect of needing to pull all records from the database in order to filter them.

If you hardcode the predicate then the compiler can treat it either as an expression tree or a delegate, and due to the type of Context.CV3Address it treats it as an expression tree.

Answer 2

FirstOrDefault(a => a.GUID == 1100222) creates an expression tree that uses LINQ to Entities to run the query on the DB server.

FirstOrDefault(predicate) downloads the entire table and runs the filter locally.

You need to change your method to take an expression tree:

Expression<Func<CV3Address, bool>>