从type参数的实例创建通用类型的实例

Question

I have a BL consisting of a generic Repository and some specific implementations for a couple of entity types (My DAL is a Entity Framework 5 model). The Repository uses for almost all it's functionality the method Set() of the System.Data.Entity.DbContext. I have some Expressions for things like getting siblings of a entity T, these won't work on the result of Set(Type entityType).

At one moment I have a instance of a non-generic DbEntityEntry. The entity is of type Object. When I use GetType() I get the approptiate Entity Type. I know want to get the related entity using my functions in Repository. Is there a way to do this from the non-generic DbEntityEntry.

I tried:

public Repository(T entity)
{
    \\ Construction here
}

But I get a Repository<Object> and the call to Set<Object> returns nothing as expected.

I've searched for a way to do this but came up empty.

Answer 1

The constructor of a generic type is part of the specific class generated for its generic type argument. For instance, the constructor of a List<int> is a different constructor than that of a List<string> . In other words, the code that "finds" the type cannot exist inside the generic class.

As far as I know these are your two options:

• convert the object to its actual type (a factory-like solution)
• use reflection to create the generic class instance

To create an instance using reflection, you can use:

Type genericType = typeof(List<>);
Type genericArgument = typeof(int);
Type specificType = genericType.MakeGenericType(genericArgument);
object instance = Activator.CreateInstance(specificType); // this is a List<int>

Answer 2

Take a look at this example posted by George Mauer in this thread. It requires .Net 4.0 or higher.

dynamic DynamicCast(object entity, Type to)
{
    var openCast = this.GetType().GetMethod("Cast", BindingFlags.Static | BindingFlags.NonPublic);
    var closeCast = openCast.MakeGenericMethod(to);
    return closeCast.Invoke(entity, new[] { entity });
}
static T Cast<T>(object entity) where T : object
{
    return entity as T;
}

You can use a slightly modified variant of this to create a matching Repository instance:

public static dynamic CreateRepository(object entiry, Type targetType)
{
    Type BaseType = typeof(Repository<>);   
    Type ConcreteType = BaseType.MakeGenericType(targetType);
    var Instance = Activator.CreateInstance(ConcreteType, entiry);

    return DynamicCast(Instance, ConcreteType);
}

Answer 3

If you can constraint your generic types to have a parameterless constructor, this problem can be solved pretty elegantly

public Repository(T entity) where T : new()
{
    T foo = new T(); //that's it
}

Answer 4

You can do:

T entity = default(T);

This works for both value types of T, as well as structs and classes. Where T is a class however it does require a parameterless constructor.