C＃：在（哈希表）中存储对象的实例

Question

Hi I tried filling a Hashtable in the following way:

      ResearchCourse resCourse= new ResearchCourse();//Class Instance
      resCourse.CID="RC1000";
      resCourse.CName="Rocket Science";

      TaughtCourse tauCourse= new TaughtCourse();//Class Instance
      tauCourse.CID="TC1000";
      tauCourse.CName="Marketing";

      Hashtable catalog = new Hashtable();

        catalog.Add("1", "resCourse.CID");
        catalog.Add("2", "tauCourse.CID");


        foreach (DictionaryEntry de in catalog)
        {
            Console.WriteLine("{0}, {1}", de.Key, de.Value);
        }

Output Result to Console was:

1, resCourse.CID
2, tauCourse.CID

Expected Result to be:

1, RC1000
2, TC2000

What am I misunderstanding about Hashtables?
What is an easy way for the Hashtable to store the class instance and its values?

Answer 1

HashTable just maps keys to values. You map the string "1" to the string "resCourse.CID" so the results you get are completely normal. Do it like this way:

catalog.Add("1", resCourse.CID);
catalog.Add("2", tauCourse.CID);

Now you will map the real values you need, not the string "resCourse.CID" .

Answer 2

You're adding explicit strings to the hash table instead of actual IDs.

Replace the two catalog.Add lines with:

catalog.Add("1", resCourse.CID);
catalog.Add("2", tauCourse.CID);

No quotation marks around the values.

Answer 3

Remove the quotes around resCourse.CID and tauCourse.CID in your catalog.Add statements. You're adding the literal strings, not the value of the properties.

Answer 4

I'm assuming that you'll actually want to store the entire objects:

catalog.Add("1", resCourse);
catalog.Add("2", tauCourse);

Of course, once you go there you're going to have to have a common base class (or interface) for your two courses to derive from in order to access the values:

public abstract class CourseBase {
   public string CID { get; set; }
   public string CName{ get; set; }
}

public class ResearchCourse : CourseBase { }
public class TaughtCourse : CourseBase { }

Then you can store/access like this:

Hashtable catalog = new Hashtable();
catalog.Add("1", resCourse);
catalog.Add("2", tauCourse);

foreach (DictionaryEntry de in catalog) 
{
   Console.WriteLine("{0}, {1}", de.Key, ((CourseBase)de.Value).CID);
}

Or better yet, use a generic Dictionary:

    var resCourse = new ResearchCourse() { CID = "RC1000", CName = "Rocket Science" };
    var tauCourse = new ResearchCourse() { CID = "TC1000", CName = "Marketing" };

    var catalog = new Dictionary<string, CourseBase>();
    catalog.Add("1", resCourse);
    catalog.Add("2", tauCourse);

    foreach (string key in catalog.Keys)
    {
        Console.WriteLine("{0}, {1}", key, catalog[key].CID);
    }