如何使用反射创建具有已定义参数的通用对象

Question

I am trying to build a generic base class for an Entity Framework Repository and UnitOfWork class. I have defined by base repository class as follows:

public class EntityRepository<CContext, TEntity>
    where TEntity : class
    where CContext : DbContext
{

     protected readonly CContext _DbContext;
     protected readonly DbSet<TEntity> _DbSet;

     public EntityRepository(CContext context)
     {
         this._DbContext = context;
         this._DbSet = context.Set<TEntity>();
     }

     // Implementation details go here
}

I can create typed instance of the repository as follows:

var ctx = new ProductContext();
var db = new EntityRepository<ProductContext, Product>(ctx);

Although, in practice, a type specific repository would be created for each class in the data context, for example...

public class ProductDb
    : EntityRepository<ProductContext, Product>
{
    public ProductDb(ProductContext context)
        : base(context)
    {
    }

}

This all seems to work as expected. My problem lies in my generic UnitOfWork base class. Previously, I have manually defined each repository property in the UnitOfWork implementation, but I would like to automate this by using a hashtable and reflection (similar to that described in Long Le's Blog )

public class UnitOfWork<CContext>
    where CContext : DbContext
{
    protected readonly CContext _Context;
    protected readonly Hashtable _Repositories;

    public UnitOfWork(CContext context)
    {
       this._Context = context;
       this._Repositories = new Hashtable();
    }

    public EntityRepository<CContext, TEntity> Repository<TEntity>()
        where TEntity : class
    {
        var type = typeof(TEntity).Name;

        if (!_Repositories.ContainsKey(type))
        {
            var repositoryType = typeof(EntityRepository<CContext, TEntity>);

            var repositoryInstance = Activator.CreateInstance(
                repositoryType.MakeGenericType(typeof(TEntity))
             );

            _Repositories.Add(type, repositoryInstance);
        }

        return (EntityRepository<CContext, TEntity>)_Repositories[type];
    }
}

When I run this code it fails with the error message:

Product is not a GenericTypeDefinition.MakeGenericType

Can anyone point me in the right direction to resolve this problem, or to put the question another way, how can I create a new instance of my EntityType using reflection?

Thanks.

Answer 1

Thats because you are getting the wrong type. This is wrong because it gets the type of a generic instance.

var repositoryType = typeof(EntityRepository<CContext, TEntity>);

Use this instead to get the type of the generic.

var repositoryType = typeof(EntityRepository<,>);

Also you need to change this

var repositoryInstance = Activator.CreateInstance(
                repositoryType.MakeGenericType(typeof(CContext>,typeof(TEntity))
             );