You can use this() to call an overloaded constructor in the same class by passing into this() the correct parameters. Instead of a separate this() statement, in C#, the this keyword is used like an extension of the constructor declaration as shown in the example below.
1: using System; 2: 3: public class Test{ 4: public static void Main(){ 5: Test t = new Test(0); 6: } 7: 8: public Test(int i) :this("a string") { 9: Console.WriteLine("constructor with int param"); 10: } 11: 12: public Test(string s){ 13: Console.WriteLine("constructor with string param"); 14: } 15: }
Output:
c:\expt>test constructor with string param constructor with int param
The this keyword is used on line 8 so that when this constructor is invoked, it first invokes the overloaded constructor which takes in a string parameter (on lines 12 “ 14).
You can use base() to call a specific constructor in the superclass by passing into base() the correct parameters which match one of the constructors in the superclass. Like this() , base() is an extension of the constructor's method declaration. Examine this example.
1: using System; 2: 3: public class Child:Parent{ 4: public static void Main(){ 5: Child c = new Child(0); 6: } 7: 8: public Child(int i): base("hello!") { 9: Console.WriteLine("Child with int param"); 10: } 11: } 12: 13: public class Parent{ 14: public Parent(string s){ 15: Console.WriteLine("Parent with string param"); 16: } 17: public Parent(){ 18: Console.WriteLine("Parent with no param"); 19: } 20: }
Output:
c:\expt>test Parent with string param Child with int param
Constructor chaining up the inheritance hierarchy, as seen in Java, still happens in C#. If none of the superclass's constructor is explicitly invoked via the base() extension of the method declaration, the constructor of a subclass will always invoke the default constructor [8] of the immediate superclass before executing the rest of its statements, as this example shows:
[8] The default constructor is the constructor with no parameters.
1: using System; 2: 3: public class Child:Parent{ 4: public static void Main(){ 5: Child c = new Child(0); 6: } 7: 8: public Child(int i){ // invoked last 9: Console.WriteLine("Child with 1 param"); 10: } 11: } 12: 13: public class Parent:GrandParent{ 14: public Parent(){ // invoked 2nd 15: Console.WriteLine("Parent"); 16: } 17: } 18: 19: public class GrandParent{ 20: public GrandParent(){ // invoked 1st 21: Console.WriteLine("GrandParent"); 22: } 23: }