Question

This is a Two (2) Part Question about Generics

I’ve got to create several similar classes to model similarly designed database tables.

All tables contain an ID int and a Text nvarchar(50) field. One or two may contain a few other fields.

I rarely use generics, but I see examples of it on here quite frequently. This is my largest attempt to create a generic class that is used in another generic class.

My basic construct is as follows, and I will point out with a comment what does not work and the error message Visual Studio 2010 is displaying:

public class IdText {

  public IdText(int id, string text) {
    ID = id;
    Text = text;
  }

  public int ID { get; private set; }

  public string Text { get; private set; }

}

public class TCollection<T> : IEnumerable<T> where T : IdText {

  private List<T> list;

  public TCollection() {
    list = new List<T>();
  }

  public void Add(int id, string text) {
    foreach (var item in list) {
      if (item.ID == id) {
        return;
      }
    }
    list.Add(new T(id, text)); // STOP HERE
    // Cannot create an instance of the variable type 'T'
    // because it does not have the new() constraint
  }

  public T this[int index] {
    get {
      if ((-1 < 0) && (index < list.Count)) {
        return list[index];
      }
      return null;
    }
  }

  public T Pull(int id) {
    foreach (var item in list) {
      if (item.ID == id) {
        return item;
      }
    }
    return null;
  }

  public T Pull(string status) {
    foreach (var item in list) {
      if (item.Text == status) {
        return item;
      }
    }
    return null;
  }

  #region IEnumerable<T> Members

  public IEnumerator<T> GetEnumerator() {
    foreach (var item in list) yield return item;
  }

  #endregion

  #region IEnumerable Members

  System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() {
    return list.GetEnumerator();
  }

  #endregion

}

Visual Studio’s IntelliSence wants me to add list.Add(T item), but I need to create this first.

I have attempted to re-write the offending line list.Add(new T(id, text)); as list.Add(new IdText(id, text));, but then I am reprimanded with the message “cannot convert from IdText to T“.

How exactly do I get around this?

Next: When I go in to actually create a version of this IdText class later, I am not sure how exactly I can use this new class in the TCollection class I have designed for it.

For example, given this derived class:

public class ManufacturedPart : IdText {

  public ManufacturedPart(int id, string partNum, string description)
    : base(int id, string partNum) {
    Description = description;
  }

  public string Description { get; private set; }

}

…would I need to also derive a special version of TCollection to accompany it, like so?

public class ManufacturedParts<T> : IEnumerable<T> where T : ManufacturedPart {

  // OK, now I'm lost! Surely this can't be right!

}

Answer 1

1) You could use the new() constraint, make your properties public and add a parameterless constructor:

public class IdText
{
    public IdText()
    {
    }

    public IdText(int id, string text)
    {
        ID = id;
        Text = text;
    }

    public int ID { get; set; }

    public string Text { get; set; }

}

public class TCollection<T> : IEnumerable<T> where T : IdText, new()
{

    private List<T> list;

    public TCollection()
    {
        list = new List<T>();
    }

    public void Add(int id, string text)
    {
        foreach (var item in list)
        {
            if (item.ID == id)
            {
                return;
            }
        }
        list.Add(new T { ID = id, Text = text }); 
    }
}

2) You have multiple options:

If you want your collection to store any IdText (ManufacturedPart or anything else that derived from IdText):

TCollection<IdText> ss = new TCollection<IdText>();

The above, for now, can only store IdText as you instantiate objects in the Add(int, string) method, but if you provide a Add(T object) method, it could store any IdText instance.

If you want your collection to only contains ManufacturedParts:

public class ManufacturedParts<T> : TCollection<T> where T : ManufacturedPart, new()
{
     // Provide here some specific implementation related to ManufacturedParts
     // if you want. For example, a TotalPrice property if ManufacturedPart
     // has a Price property.
}

TCollection<ManufacturedPart> ss2 = new ManufacturedParts<ManufacturedPart>();

or even simpler, if your collection doesn’t provide any additional method depending on the type of the stored objects:

TCollection<ManufacturedPart> ss2 = new TCollection<ManufacturedPart>();

Even simpler, if your goal is to only store objects, a custom collection isn’t needed:

List<IdText> ss2 = new List<IdText>();  // Uses the built-in generic List<T> type