When to use keyword "implicit" or "explicit" when overloading conversion operators?

Question

I'm learning C# now. The tutorial doesn't make it clear when to use keyword implicit or explicit when overloading conversion operators.

The example it provides is like this:

When Class1 contains a field of type int and Class2 contains a field of type double, we should define an explicit conversion from Class2 to Class1, and an implicit conversion from Class1 to Class2.

The tutorial doesn't say what will happen if I use the wrong keyword.

But if Class1 contains a complex subclass and Class2 contains a different subclass, which keyword should I use between implicit and explicit? Can anyone gives a clear explanation? Thanks a lot.

Answer 1

Implicit conversions: No special syntax is required because the conversion is type safe and no data will be lost. Examples include conversions from smaller to larger integral types, and conversions from derived classes to base classes.

Explicit conversions (casts):

Explicit conversions require a cast operator. Casting is required when information might be lost in the conversion, or when the conversion might not succeed for other reasons. Typical examples include numeric conversion to a type that has less precision or a smaller range, and conversion of a base-class instance to a derived class.

Check the bold texts in this explanation. Here is the detail article in MSDN

Here is little code example:

// Create a new derived type.
Giraffe g = new Giraffe();

// Implicit conversion to base type is safe.
Animal a = g;

// Explicit conversion is required to cast back
// to derived type. Note: This will compile but will
// throw an exception at run time if the right-side
// object is not in fact a Giraffe.
Giraffe g2 = (Giraffe) a;