Question

What is the difference between

try { ... }
catch{ throw }

and

try{ ... }
catch(Exception e) {throw new Exception(e.message) }

regardless that the second shows a message.

Answer 1

throw; rethrows the original exception and preserves its original stack trace.

throw ex; throws the original exception but resets the stack trace, destroying all stack trace information until your catch block.

NEVER write throw ex;

throw new Exception(ex.Message); is even worse. It creates a brand new Exception instance, losing the original stack trace of the exception, as well as its type. (eg, IOException).
In addition, some exceptions hold additional information (eg, ArgumentException.ParamName).
throw new Exception(ex.Message); will destroy this information too.

In certain cases, you may want to wrap all exceptions in a custom exception object, so that you can provide additional information about what the code was doing when the exception was thrown.

To do this, define a new class that inherits Exception, add all four exception constructors, and optionally an additional constructor that takes an InnerException as well as additional information, and throw your new exception class, passing ex as the InnerException parameter. By passing the original InnerException, you preserve all of the original exception’s properties, including the stack trace.