如何使用 EF 传递 Lambda 表达式作为方法参数

Question

How do I pass an EF expression as a method parameter?

To illustrate my question I have created a pseudo code example:

The first example is my method today. The example utilizes EF and a Fancy Retry Logic. What I need to do is to encapsulate the Fancy Retry Logic so that it becomes more generic and does not duplicate.

In the second example is how I want it to be, with a helper method that accepts the EF expression as a parameter.

This would be a trivial thing to do with SQL, but I want to do it with EF so that I can benefit from the strongly typed objects.

First Example:

public static User GetUser(String userEmail)
{
    using (MyEntities dataModel = new MyEntities ())
    {
        var query =  FancyRetryLogic(() =>
        {
            (dataModel.Users.FirstOrDefault<User>(x => x.UserEmail == userEmail)));
        });

        return query;
    }
}

Second Example:

T RetryHelper<T>(Expression<Func<T, TValue>> expression)
{
    using (MyEntities dataModel = new (MyEntities ())
    {

        var query = FancyRetryLogic(() =>
                    {
                        return dataModel.expression
                    });
    }
}

public User GetUser(String userEmail)
{
    return RetryHelper<User>(<User>.FirstOrDefault<User>(x => x.UserEmail == userEmail))
}

Answer 1

Maybe something like this?

public TValue RetryHelper<T, TValue>(Func<ObjectSet<T>, TValue> func)
    where T : class
{
    using (MyEntities dataModel = new MyEntities())
    {
        var entitySet = dataModel.CreateObjectSet<T>();
        return FancyRetryLogic(() =>
               {
                   return func(entitySet);
               });
    }
}

public User GetUser(String userEmail)
{
    return RetryHelper<User, User>(u => u.FirstOrDefault(x => x.UserEmail == userEmail));
}

Answer 2

Just to post what we discussed already...

Here is a link that might help you, I think it has something similar to what you need.

Using AsQueryable With Linq To Objects And Linq To SQL

How do I cache an IQueryable object?

I've seen better examples but I don't have them handy, basically as I mentioned you can use that to keep your query in a form so that you can further filter, change until the very last moment when you know all is done and can actually realize and enumerate the query.

hope it helps