通用EF6存储库方法无法生成正确的SQL

Question

I have this method in my repository exposing EF6 DbContext.

public IList<T> GetEntities<T>(Func<T, bool> predicate) where T : class
{
    return db.Set<T>().Where(predicate).ToList<T>();
}

When I watch this method execute in SQL Profiler, the predicate is executed in memory. The SQL statement contains no where clause.

Any ideas?

Answer 1

.Where accepts one of two things, either Func<T, bool> or Expression<Func<T, bool>> . If you pass in Expression<Func<T, bool>> , then your EF query should work properly.

public IList<T> GetEntities<T>(Expression<Func<T, bool>> predicate) where T : class

You'd call it the same way:

GetEntities(x => x.Id == 34)

When you pass in Func<T, bool> , the IEnumerable<T> implementation executes, which uses Linq-to-Objects rather than Linq-to-Entities.

Answer 2

Your predicate should be an Expression so that Entity Framework can actually use it to generate SQL instead of just executing it. If you pass in a Func you are actually calling the Enumerable.Where method instead of Queryable.Where :

public IList<T> GetEntities<T>(Expression<Func<T, bool>> predicate) where T : class
{
    return db.Set<T>().Where(predicate).ToList<T>();
}