There are a few restrictions to conversion operators : Conversion Operator Overload « Operator Overload « C# / CSharp Tutorial

1. Either the target type or the source type of the conversion must be a class that you create.
2. You cannot redefine the conversion from double to int.
3. You cannot define a conversion to or from Object.
4. You cannot define both an implicit and an explicit conversion for the same source and target types.
5. You cannot define a conversion from a base class to a derived class.
6. You cannot define a conversion from or to an interface.

(Quote from C# The Complete Reference, Publisher: Osborne/McGraw-Hill, March 8, 2002, Language: English ISBN-10: 0072134852 ISBN-13: 978-0072134858)

1. Here are the other operators that cannot be overloaded.

2. &&
3. ||
4. []
5. ()
6. new
7. is
8. sizeof
9. typeof
10. ?
11. ->
12. .
13. =