To make Dependency Injection concept simple to understand. Let's take an example of switch button to toggle(on/off) a bulb.

Without Dependency Injection

Switch needs to know beforehand which bulb I am connected to (compile time) so,

Switch(){
PermanentBulb = new Bulb();
PermanentBulb.Toggle();
}

With Dependency Injection

Switch only knows I need to turn on/off whichever Bulb is passed to me.

Switch(AnyBulb){ //pass it whichever bulb you like
AnyBulb.Toggle();
}

Modifying James Example for Switch and Bulb:

public class SwitchTest { 
  TestToggleBulb() { 
    MockBulb mockbulb = new MockBulb(); 

    // MockBulb is a subclass of Bulb, so we can 
    // "inject" it here: 
    Switch switch = new Switch(mockBulb); 

    switch.ToggleBulb(); 
    mockBulb.AssertToggleWasCalled(); 
  } 
}

public class Switch { 
  private Bulb myBulb; 

  public Switch() { 
    myBulb = new Bulb(); 
  } 

  public Switch(Bulb useThisBulbInstead) { 
    myBulb = useThisBulbInstead; 
  } 

  public void ToggleBulb() { 
    ... 
    myBulb.Toggle(); 
    ... 
  } 
}`

To make Dependency Injection concept simple to understand. Let's take an example of switch button to toggle(on/off) a bulb.

Without Dependency Injection

Switch needs to know beforehand which bulb I am connected to (hard-coded dependency). So,

Switch -> PermanentBulb //switch is directly connected to permanent bulb, testing not possible easily

Switch(){
PermanentBulb = new Bulb();
PermanentBulb.Toggle();
}

With Dependency Injection

Switch only knows I need to turn on/off whichever Bulb is passed to me. So,

Switch -> Bulb1 OR Bulb2 OR NightBulb (injected dependency)

Switch(AnyBulb){ //pass it whichever bulb you like
AnyBulb.Toggle();
}

Modifying James Example for Switch and Bulb:

public class SwitchTest { 
  TestToggleBulb() { 
    MockBulb mockbulb = new MockBulb(); 

    // MockBulb is a subclass of Bulb, so we can 
    // "inject" it here: 
    Switch switch = new Switch(mockBulb); 

    switch.ToggleBulb(); 
    mockBulb.AssertToggleWasCalled(); 
  } 
}

public class Switch { 
  private Bulb myBulb; 

  public Switch() { 
    myBulb = new Bulb(); 
  } 

  public Switch(Bulb useThisBulbInstead) { 
    myBulb = useThisBulbInstead; 
  } 

  public void ToggleBulb() { 
    ... 
    myBulb.Toggle(); 
    ... 
  } 
}`