problem user defined conversion must convert to or from enclosing type

I have a simple problem that is :

must declare a body because it is not marked abstract,extern,or partial user-defined conversion must convert to olr from the enclosing type

and my code is :

 public static explicit operator byte[] (SqlBinary x);

i want to convert a sqlbinary to byte[] and i use above code and get me that error please tell me what is the problem