如何解决C＃中泛型类型约束的局限性？

Question

Okay I'm looking for some input, I'm pretty sure this is not currently supported in .NET 3.5 but here goes.

I want to require a generic type passed into my class to have a constructor like this:

new(IDictionary<string,object>)

so the class would look like this

public MyClass<T>  where T : new(IDictionary<string,object>)
{
  T CreateObject(IDictionary<string,object> values)
  {
    return new T(values);
  }
}

But the compiler doesn't support this, it doesn't really know what I'm asking.

Some of you might ask, why do you want to do this? Well I'm working on a pet project of an ORM so I get values from the DB and then create the object and load the values.

I thought it would be cleaner to allow the object just create itself with the values I give it. As far as I can tell I have two options:

1) Use reflection(which I'm trying to avoid) to grab the PropertyInfo[] array and then use that to load the values.

2) require T to support an interface like so:

public interface ILoadValues { void LoadValues(IDictionary values); }

and then do this

public MyClass<T> where T:new(),ILoadValues
{
  T CreateObject(IDictionary<string,object> values)
  {
    T obj = new T();
    obj.LoadValues(values);
    return obj;
  }
}

The problem I have with the interface I guess is philosophical, I don't really want to expose a public method for people to load the values. Using the constructor the idea was that if I had an object like this

namespace DataSource.Data
{
  public class User
  {
    protected internal User(IDictionary<string,object> values)
    {
      //Initialize
    }
  }
}

As long as the MyClass<T> was in the same assembly the constructor would be available. I personally think that the Type constraint in my opinion should ask (Do I have access to this constructor? I do, great!)

Anyways any input is welcome.

Answer 1

As stakx has said, you can't do this with a generic constraint. A workaround I've used in the past is to have the generic class constructor take a factory method that it can use to construct the T:

public class MyClass<T>
{
  public delegate T Factory(IDictionary<string, object> values);

  private readonly Factory _factory;

  public MyClass(Factory factory)
  {
    _factory = factory;
  }

  public T CreateObject(IDictionary<string, object> values)
  {
    return _factory(values);
  }
}

Used as follows:

MyClass<Bob> instance = new MyClass<Bob>(dict => new Bob(dict));
Bob bob = instance.CreateObject(someDictionary);

This gives you compile time type safety, at the expense of a slightly more convoluted construction pattern, and the possibility that someone could pass you a delegate which doesn't actually create a new object (which may or may not be a major issue depending on how strict you want the semantics of CreateObject to be).

Answer 2

If you can create common base class for all of T ojects that you are going to pass to MyClass as type parameters than you can do following:

internal interface ILoadValues
{
    void LoadValues<TKey, TValue>(IDictionary<TKey, TValue> values);
}

public class Base : ILoadValues
{
    void ILoadValues.LoadValues<TKey, TValue>(IDictionary<TKey, TValue> values)
    {
        // Load values.
    }
}

public class MyClass<T>
    where T : Base, new()
{
    public T CreateObject(IDictionary<string,object> values)
    {
        ILoadValues obj = new T();
        obj.LoadValues(values);
        return (T)obj;
    }
}

If you cannot have common base class than I think you should go with solution proposed by itowlson.

Answer 3

You cannot do that. new (constructor) constraints are only for parameter-less constructors. You cannot have a constraint for a constructor with specific parameters.

I've come across the same problem, though, and I've finally settled on doing the "injection" part through a method that is provided in one of the interfaces that is listed as a constraint (as demonstrated in your code).

(I hope someone here has found a more elegant answer to this problem!)

Answer 4

I'm legitimately curious at how you would load the values of a class without using reflection unless you had methods hardcoded to accomplish it. I'm sure there's another answer, but I'm not too ashamed to say I do not have experience in it. As for something I wrote to auto-load data, I have two base data classes I work from: a single object and then a list. In the single object (BaseDataClass), I have this method.

    public virtual void InitializeClass(DataRow dr)
    {
        Type type = this.GetType();
        PropertyInfo[] propInfos = type.GetProperties();

        for (int i = 0; i < dr.ItemArray.GetLength(0); i++)
        {
            if (dr[i].GetType() != typeof(DBNull))
            {
                string field = dr.Table.Columns[i].ColumnName;
                foreach (PropertyInfo propInfo in propInfos)
                {
                    if (field.ToLower() == propInfo.Name.ToLower())
                    {
                        // get data value, set property, break
                        object o = dr[i];
                        propInfo.SetValue(this, o, null);
                        break;
                    }
                }
            }
        }
    }

And then in the data list

public abstract class GenericDataList<T> : List<T> where T : BaseDataClass
{
    protected void InitializeList(string sql)
    {
        DataHandler dh = new DataHandler(); // my general database class
        DataTable dt = dh.RetrieveData(sql); 
        if (dt != null)
        {
            this.InitializeList(dt);
            dt.Dispose();
        }
        dt = null;
        dh = null;
    }

    protected void InitializeList(DataTable dt)
    {
        if (dt != null)
        {
            Type type = typeof(T);
            MethodInfo methodInfo = type.GetMethod("InitializeClass");

            foreach (DataRow dr in dt.Rows)
            {
                T t = Activator.CreateInstance<T>();
                if (methodInfo != null)
                {
                    object[] paramArray = new object[1];
                    paramArray[0] = dr;
                    methodInfo.Invoke(t, paramArray);
                }

                this.Add(t);
            }
        }
    }
}

I'm open to criticism, because no one has ever reviewed this code before. I am the sole programmer where I work, so I do not have others to bounce ideas off of. Thankfully, now I've come across this website!

Edit: You know what? Looking at it now, I don't see why I shouldn't just rewrite that last method as

        protected void InitializeList(DataTable dt)
        {
            if (dt != null)
            {
                Type type = typeof(T);

                foreach (DataRow dr in dt.Rows)
                {
                    T t = Activator.CreateInstance<T>();
                    (t as BaseDataClass).InitializeClass(dr);

                    this.Add(t);
                }
            }
        }

I assume that works, although I haven't tested it. No need to use reflection on that part.