How do you call a Base Class Constructor from Derived Class Constructor in .NET?

During inheritance, constructors are not inherited. But .NET provides you an option to explicitly call a base class constructor from the derived class constructor using the keyword “base”.

class baseClass {
int member1;
public baseClass() {
member1=1;
}
public baseClass(int data) {
member1 = data;
}
}
class derivedClass:baseClass {
int member2;
public derivedClass() : base() {
member2 = 2;
}
public derivedClass(int data1, int data2) : base(data1) {
member2 = data2;
}
public void displayMsg() {
Console.WriteLine(“Member1 = {0}, Member2 = {1}”, member1, member2);
}
}
class testClass {
public static void Main() {
derivedClass obj = new derivedClass(10, 20);
obj.displayMsg();
derivedClass obj1 = new derivedClass();
obj1.displayMsg();
}
}

Output of this code will be:

Member1 = 10, Member2 = 20
Member1 = 1, Member2 = 2

In this example, you have called the no-argument constructor and one-argument constructor of base class explicitly from the derived class constructor.

Note that calling a base class constructor from derived class constructor will execute only the matching constructor of the immediate base class. It does not trigger matching constructors in all base classes of the derived class in the hierarchy. Consider the following example:

class baseClass {
public baseClass { }
public baseClass(string msg) {
Console.WriteLine(“Passed the following message to baseClass:” + msg);
}
}
class derivedClass1:baseClass {
public derivedClass1(string msg) {
Console.WriteLine(“Calling derivedClass1 constructor…”);
}
}
class derivedClass2:derivedClass1 {
public derivedClass2(string msg):base(msg) {
Console.WriteLine(“Calling derivedClass2 constructor..”);
}
}
class testClass {
public static void Main() {
derivedClass2 obj = new derivedClass2(“Hello”);
}
}

Output of this code will be:

Calling derivedClass1 constructor…
Calling derivedClass2 constructor…

You have called the base class constructor from derivedClass2 using the keyword base. It will trigger only its immediate base class derivedClass1 and therefore you get the above shown output. If you want the next higher level of base class named baseClass to be triggered, then you have to modify the derivedClass1 constructor as shown below:
public derivedClass1(string msg):base(msg) {
Console.WriteLine(“Calling derivedClass1 constructor…”);
}

Now you will get the following output if you execute the whole program:

Passed the following message to baseClass:Hello
Calling derivedClass1 constructor…
Calling derivedClass2 constructor…

Also note that when you call a base class constructor, you have to ensure that the constructor with the specified signature already exists. If not, then you will end up in compilation error. This is demonstrated in the code shown below:

class baseClass {
public baseClass() {
Console.WriteLine(‘Calling baseClass constructor…”);
}
}
class derivedClass1:baseClass {
public derivedClass1(string msg):base(msg) {
Console.WriteLine("Calling derivedClass1 constructor…");
}
}
class testClass {
public static void Main() {
derivedClass1 obj = new derivedClass1("Hello");
}
}

In this example, derivedClass1 constructor calls the baseClass constructor with one argument. This doesn’t exist in baseClass. Hence during compilation, you will end up in the following error:
No overload for method ‘baseClass’ takes ‘1’ arguments

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