使用表达式<func>在表达式内<func></func></func>

Question

The below code works fine (throws no translation error):

string text = "someString";
string refId = "refId";
bool forward = true;
IQueryable<Tin> q = queryable.Where(o => forward ? text.IsGreaterThan(refId) : text.IsLessThan(refId));
var x = await q.ToArrayAsync();

IsGreaterThan() and IsLessThan() are methods that have been registered in OnModelCreating(ModelBuilder builder) from dbContext, as explained in this answer . So the problem is not with those methods.

The below code doesn't work:

string refId = "refId";
Expression<Func<Tin, string>> key = obj => obj.Id;
bool forward = true;
IQueryable<Tin> q = queryable.Where(o => forward ? key.Compile()(o).IsGreaterThan(refId) : key.Compile()(o).IsLessThan(refId));
var x = await q.ToArrayAsync();

It throws the below error.

System.InvalidOperationException: The LINQ expression 'DbSet<Follow>
    .Where(f => f.FollowerId == __userId_0)
    .Where(f => Invoke(__Compile_1, f[Follow])
        .IsGreaterThan(__refId_2))' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync().

I understand why it fails. Expression<Func<>>.Compile() returns a Func<> and those cannot be used in Linq to Sql. So I've been looking for workarounds.

The most promising I found is from this answer . It worked as expected in the fiddle he provided, but for some reason, it didn't work for me. My code after implementing his answer:

string refId = "refId";
Expression<Func<Tin, string>> key = obj => obj.Id;
bool forward = true;
Func<Tin, string> keyFunc = key.Compile();
Expression<Func<Tin, bool>> where = o => forward ? keyFunc(o).IsGreaterThan(refId) : keyFunc(o).IsLessThan(refId);
where.Replace(() => keyFunc, key);
IQueryable<Tin> q = queryable.Where(where);
var x = await q.ToArrayAsync();

Is there some other way to nest Expression<Func<>> s?

EDIT : As mentioned by @pinkfloydx33:

where.Replace(() => keyFunc, key);

should have been:

where = where.Replace(() => keyFunc, key);

Funny enough, I make this same mistake with string.Replace() a loooot. It works fine now after this.

Answer 1

This feature is not supported by EF, and i don't know why;) You have to install LINQKit and use AsExpandable() adn Invoke instead of Compile :

string refId = "refId";
Expression<Func<Tin, string>> key = obj => obj.Id;
bool forward = true;
IQueryable<Tin> q = queryable
  .AsExpandable()
  .Where(o => forward ? key.Invoke(o).IsGreaterThan(refId) : key.Invoke(o).IsLessThan(refId));
var x = await q.ToArrayAsync();