When to Use Delegates Instead of Interfaces

c#, delegates, interface

Solution

A class may need more than one implementation of the method.

public delegate int PerformCalculation(int x, int y);

void SomeMethod()
{
    PerformCalculation PerformCalculation_1 = myDelegateFun_1;
    PerformCalculation PerformCalculation_2 = myDelegateFun_2;
    PerformCalculation_1(5, 3);
    PerformCalculation_2(5, 3);      
}

private int myDelegateFun_1(int x, int y)
{
    return x + y;
}
private int myDelegateFun_2(int x, int y)
{
    return x + y;
}

In the above example PerformCalculation_1, PerformCalculation_2 are multiple implementation of PerformCalculation

A class only needs one implementation of the method

interface IDimensions 
{
   float Length();
}

class Box : IDimensions 
{
   float Length() 
   {
       return lengthInches;
   }
}

In the above example only single implementation of method exposed by interface.

Problem

According to this article, it says: Use a delegate in the following circumstances: - A class may need more than one implementation of the method. Use an interface in the following circumstances: - A class only needs one implementation of the method. Can someone explain this to me?

Original source