How to convert DbSet in Entity framework to ObjectQuery

问题

I am using CodeFirst approach and struck with an issue where I need to convert DbSet to ObjectQuery. This is what I did for conversion.

ObjectContext objectContext = ((IObjectContextAdapter)db).ObjectContext;
ObjectSet<Request> objectSet = objectContext.CreateObjectSet<Request>();

where db is the context inheriting from DbContext and Request is class.

So, when I try to call the method that expects ObjectQuery as ObjectQueryMethod(objectSet), it throws the following error.

"Type of conditional expression cannot be determined because there is no implicit conversion between 'System.Data.Entity.DbSet<>' and 'System.Data.Objects.ObjectQuery<>'"

Any help is greatly appreciated!

回答1:

I found the answer. Of course, it is possible to convert DbSet in Entity framework to ObjectQuery using the below lines of code.

ObjectContext objectContext = ((IObjectContextAdapter)db).ObjectContext;  
ObjectSet<Request> objectSet = objectContext.CreateObjectSet<Request>("Requests");

where,

• db - Context class inherting from DbContext.
• Requests - DbSet<Request> defined in Context class.
• objectSet - Can now be passed as ObjectQuery.

回答2:

Thanks for the correct answer 'inspiringmyself'; this is just to elaborate on your answer. I managed to do this with a generic type, so just to share it:

private List<T> GetByCustomCriteria<T>(string criteria) where T: class
{
  var objectContext = ((IObjectContextAdapter)_myModelEntities).ObjectContext;
  //note: _myModelEntities is a DbContext in EF6.
  var objectSet = objectContext.CreateObjectSet<T>();
  return new List<T>(objectSet.Where(criteria));
 }

And I thought the above post could do with an example of what criteria to send so here's an example:

 //sample criteria for int field:
 var myClientById = GetByCustomCriteria<Client>("it.Id == 1");`

//sample criteria for string field, note the single quotes
var myClientByName = GetByCustomCriteria<Client>("it.Surname == 'Simpson'"); 

来源：https://stackoverflow.com/questions/11252096/how-to-convert-dbset-in-entity-framework-to-objectquery