Default interface methods in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Default interface methods

Define interface with default method

↓

Implement interface in class

↓

Create instance of class

↓

Call default method via interface

↓

Execute default method code

↓

Output result

The flow shows how an interface with a default method is defined, implemented by a class, and how calling the method runs the default code if not overridden.

Execution Sample

C Sharp (C#)

interface IGreeter {
    void Greet() {
        Console.WriteLine("Hello from default method!");
    }
}

class Greeter : IGreeter {}

IGreeter g = new Greeter();
g.Greet();

This code defines an interface with a default method, implements it in a class without overriding, then calls the default method.

Execution Table

Step	Action	Evaluation	Result
1	Define interface IGreeter with default method Greet()	Interface created with default method	IGreeter has Greet() with default implementation
2	Define class Greeter implementing IGreeter	Class created without overriding Greet()	Greeter inherits default Greet()
3	Create instance g of Greeter as IGreeter	Instance created	g is Greeter instance
4	Call g.Greet()	No override found in Greeter	Default Greet() runs
5	Execute default Greet() method	Console.WriteLine runs	Output: Hello from default method!

💡 Method call completes after default method executes and prints output

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
g	null	Greeter instance	Greeter instance	Greeter instance

Key Moments - 2 Insights

Why does calling Greet() on Greeter instance run the default method?

Because Greeter does not override Greet(), the call uses the interface's default implementation as shown in execution_table step 4.

Can the class override the default interface method?

Yes, if the class defines its own Greet() method, it will run that instead of the default, replacing the behavior in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when g.Greet() is called?

A Compilation error

B "Hello from default method!"

C No output

D "Hello from Greeter!"

💡 Hint

See execution_table row 5 where the default method prints the output

At which step does the program create the instance of Greeter?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Check execution_table row 3 for instance creation

If Greeter class had its own Greet() method, what would change in the execution?

A Compilation error occurs

B The default method would still run

C The class method would run instead of default

D Interface method is removed

💡 Hint

Refer to key_moments about overriding default methods

Concept Snapshot

Default interface methods allow interfaces to provide method bodies.
Classes implementing the interface can use these defaults without overriding.
If a class overrides, its method runs instead.
This helps add new methods to interfaces without breaking existing code.
Call the method via interface reference to use default implementation.

Full Transcript

This example shows how default interface methods work in C#. First, an interface IGreeter is defined with a default method Greet() that prints a message. Then, a class Greeter implements IGreeter but does not override Greet(). When we create an instance of Greeter and call Greet() via the interface reference, the default method runs and prints the message. The execution table traces each step: defining interface and class, creating instance, calling method, and output. The variable tracker shows the instance variable g changes from null to a Greeter object. Key moments clarify why the default method runs and how overriding works. The quiz tests understanding of output, instance creation step, and effect of overriding. The snapshot summarizes the concept simply for quick review.