How to get the Primary Key(s) in Entity Framework 4.1, i.e. using DbContext

Question

The answers I'm seeing here are for ObjectContext. Is there a property to determine an entity's primary key names when using DbContext?

Ah.. one of those times that I wish Entity Framework is open source! I can glean this primary key name information from .Find method :-)

Answer 1

You cannot use DbContext for that - DbContext API is just dumb wrapper with only most needed functionality. For everything more complex you must convert DbContext back to ObjectContext and use it. Try something like this:

Extract key names:

public static string[] GetEntityKeyNames<TEntity>(this DbContext context) where TEntity : class
{
  if (context == null)
    throw new ArgumentNullException("context");

  var set = ((IObjectContextAdapter)context).ObjectContext.CreateObjectSet<TEntity>();
  var entitySet = set.EntitySet;
  return entitySet.ElementType.KeyMembers.Select(k => k.Name).ToArray();
}

Here's a method that will extract the key values of an entity:

public static IEnumerable<object> GetEntityKeys<TEntity>(this DbContext context, TEntity entity)
  where TEntity : class
{
  if (context == null)
    throw new NullReferenceException("context");

  var type = typeof(TEntity);

  var set = ((IObjectContextAdapter)context).ObjectContext.CreateObjectSet<TEntity>();
  var entitySet = set.EntitySet;
  var keys = entitySet.ElementType.KeyMembers;
  var props = keys.Select(k => type.GetProperty(k.Name));
  return props.Select(p => p.GetValue(entity));
}

Answer 2

The solution proposed by Ladislav Mrnka won't work for derived entities as You can't create an object set for a derived type. You'll see this error :

ArgumentException: There are no EntitySets defined for the specified entity type ... If ... is a derived type, use the base type instead. Parameter name: TEntity

Here is my solution avoiding creating an object set :

public string[] GetEntityKeyNames<TEntity>(DbContext context)
{
    if (context == null)
    {
        throw new ArgumentNullException("context");
    }
    var objectContext = ((IObjectContextAdapter)Context).ObjectContext;
    //We must use the namespace of the context and the type name of the entity
    string entityTypeName = context.GetType().Namespace + '.' + typeof(TEntity).Name;
    var entityType = objectContext.MetadataWorkspace.GetItem<EntityType>(entityTypeName, DataSpace.CSpace);
    return entityType.KeyProperties.Select(k => k.Name).ToArray();
}