c# - Combining expression trees

Question

I have the following expression:

public Expression<Func<T, bool>> UserAccessCheckExpression<T>(int userId) where T : class
{
    return x => (IsAdmin || userId == CurrentUserId || userId == 0);
}

Then I want to apply this filter to several collections (IQueryable) like this one:

return tasks
  .Where(t => t.TaskUsers
     .Any(x => UserAccessCheckExpression<TaskUser>(x.User) && x.SomeBool == true));

I'm getting the following error while doing so:

Error 40 Cannot implicitly convert type System.Linq.Expressions.Expression<System.Func<TaskUser,bool>> to bool

I can't use workaround with interface inheritance (like TaskUser inherits interface with int UserId property (where T : IHasUserId)) since I want to combine logic.

Answer 1

The problem is that your UserAccessCheckExpression() method is returning an Expression while the Any() method is expecting a boolean.

Now, you can get your code to compile by compiling the Expression and invoking the method (using UserAccessCheckExpression<TaskUser>(x.User).Compile().Invoke(x.User)) but that would obviously fail on runtime because Linq-to-Entities wouldn't be able to translate your Any() to a store query as it no longer contains an Expression.

LinqKit is aiming to solve this problem using its own Invoke extension method that while letting your code compile, will make sure your Expression will get replaced back to its original form using another extension method named AsExpandable() that is extending the entity set.

Try this:

using LinqKit.Extensions;


return tasks
      .AsExpandable()
      .Where(t => t.TaskUsers.Any(
                       x => UserAccessCheckExpression<TaskUser>(x.User).Invoke(x)
                            && x.SomeBool == true));

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