Parse string to poco class enum in linq query MVC 2.0 - entity-framework

Question

I have the following enum and POCO class
public enum Gender
{
Male,
Female,
Unknown
}
public class Person
{
public int PersonId { get; set; }
public string LastName { get; set; }
public string FirstName { get; set; }
public Gender? Gender { get; set; }
}
I would like to perform a "get all people" query in my repository such that it would look something like this:
return from p in _db.People
select new Model.Person
{
PersonId = p.PersonId,
LastName = p.LastName,
FirstName = p.FirstName,
Gender = p.Gender,
};
Unfortunately I get an error "Cannot implicitly convert type 'string' to 'Model.Gender'"
I would like to convert the string which is being queried from the entity framework to my Gender enum and assign it to my POCO class.

Answer 1

Enums are not supported in Entity Framework. There is a workaround by Alex James, but it's quite involved.
Instead, i prefer to do this:
public enum Gender : byte
{
Male = 1,
Female,
Unknown
}
public class Person
{
public int PersonId { get; set; }
public string LastName { get; set; }
public string FirstName { get; set; }
public byte Gender { get; set; } // this is the EF model property
public Gender GenderType // this is an additional custom property
{
get { return (Gender) Gender; }
set { Gender = (byte)value; }
}
}
It's basically a hook/wrapper for the actual value. In your database, store Gender as a tinyint (which maps to byte on the conceptual side).
Then you can use a byte enum to map to and from the model property:
return from p in _db.People
select new Model.Person
{
PersonId = p.PersonId,
LastName = p.LastName,
FirstName = p.FirstName,
Gender = p.Gender, // sets byte
};
But then if you access that ViewModel, because your setting the byte field for Gender, you will also have access to the enum property GenderType.
Does that solve your problem?

Answer 2

The Entity Framework that I am familiar with does not provide support for enums. EF uses your query expression to create an SQL statement that it then sends to the server, if it cannot create the SQL equivalent of some operation it will throw a NotSupportedException for that operation. If you are expecting to return a small set of data you can separate from the Entity Framework by creating an object in memory using the ToArray method.
var myEntities = (from entity in _db.Entities
where /* condition */
select entity)
.ToArray();
This will create a sequence of entities in memory. Any further query statements will then be in the realm of LINQ to Objects which allows parsing of strings into enums:
return from myEntity in myEntities
select new MyDataContract
{
ID = myEntity.ID,
Gender g = (Gender)Enum.Parse(typeof(Gender), myEntity.Gender, true)
};
Or you could even break it out into a foreach loop:
List<MyDataContract> myDataContracts = new List<MyDataContract>();
foreach (var myEntity in myEntities)
{
var dataContract = new MyDataContract { ID = myEntity.ID };
if (Enum.IsDefined(typeof(Gender), myEntity.Gender))
dataContract.Gender = (Gender)Enum.Parse(typeof(Gender), myEntity.Gender, true);
myDataContracts.Add(dataContract);
}
return myDataContracts.AsEnumerable();

Answer 3

if (Enum.IsDefined(typeof(Gender), genderstring))
Gender g = (Gender) Enum.Parse(typeof(Gender), genderstring, true);
else
//Deal with invalid string.

Answer 4

try
Gender = p.Gender != null ? (Gender)Enum.Parse(typeof(Gender), p.Gender) : (Gender?)null;
To parse the string as one of the enums
here's a workaround but it means changing your nice and clean POCO
http://blogs.msdn.com/b/alexj/archive/2009/06/05/tip-23-how-to-fake-enums-in-ef-4.aspx