How to solve "Object must implement IConvertible."

you have to implement IConvertable on the object because Covert.ChangeType() expects IConvertable object. So Convert object knows how to convert given object to the type that you need - even it is the same type. you may want to research into Polymorphism in object-oriented programming for a better understanding of why the structure of method require IConvertable.

The main idea is that your object may have multiple forms of interpretation. For example, if you have and Apple object, the apple object can interpreted as a fruit (IFruit), eatable (IEatable), and round-shape(IShape). These identifications define your apple object in different way, though it is the same object. Now when a grocery store look at your apple, they will place in the fruit aisle (not caring which shape it would be). This is same when an graphic design look at the apple, they would only care how would render the shape of the apple.

So when you use Convert object. It does not really care what object it is as long as that object is convertible to another (by implementing IConvetable). And as you inherite IConvertable, you just have to implement method of conversion from given object to the new object.

Here is a reference to how to use IConvertable