In a previous tip, you learned how to redefine the function of simple operators like plus (+) and minus (-) using overloaded operators. In this tip, you'll learn how to overload the constructor of a class, which is the method called when you instantiate an object.

In classes that I create, I often define a constructor to ensure that the object is initially populated with all the information it needs to operate properly. As an example, one of my databound objects can take a database connection or can take both a database connection and an object representing the unique ID for the record. Each of these methods for instantiating the object is a separate constructor method.

To show how this works, I'll create a simple class that has three public variables on it. One constructor will accept values for each of these three variables, and the other constructor will accept a DataRow that has values for each of the three variables.

public class DataObject
{
   public string Value1;
   public string Value2;
   public string Value3;
   public DataObject(string value1, string value2, string value3)
   {
      Value1 = value1;
      Value2 = value2;
      Value3 = value3;
   }
   public DataObject(DataRow inputRow)
   {
      Value1 = inputRow["value1"].ToString();
      Value2 = inputRow["value2"].ToString();
      Value3 = inputRow["value3"].ToString();
   }
}

One of the class's constructors will accept values for each of the three variables, and its other constructor will accept a DataRow that has values for each of the three variables. The code assumes that you have a database table somewhere that has fields named value1, value2, and value3, and that all three fields are strings.

To call this function, you can use one of the following methods, which will end up populating the DataObject class with the three values:

DataObject obj1 = new DataObject("test1", "test2", "test3");
DataTable dt = <code to retrieve a data table>
DataObject obj2 = new DataObject(dt.Rows[0]);

You also can use this method to overload any method on your class. As long as the parameter list is different, you can add an additional overload. You can't, for instance, have two overloads that each has three string parameters. You can have one overload that accepts one string and another that accepts two strings.