Why does the C# compiler authorize "throw ex" in catch, and is there a case where "throw ex" is useful?

Question

In C#, younger developers use often "throw ex" instead of "throw" to throw exception to parent method.

Example :

try
{
    // do stuff that can fail
}
catch (Exception ex)
{
    // do stuff
    throw ex;
}

"throw ex" is a bad practise because the stack trace is truncated below the method that failed. So it's more difficult to debug code. So the code must be :

try
{
    // do stuff that can fail
}
catch (Exception ex)
{
    // do stuff
    throw;
}

My question is why compilator authorize this (or doesn't display a warning message ?) Is there a case which "throw ex" is useful ?

Answer 1

Is there a case which "throw ex" is useful ?

Sure - sometimes you want to truncate the stack trace - to avoid exposing implementation details, etc. Other times you may want to throw a new exception, which would mean the compiler would have to distinguish from just re-throwing the caught exception and throwing a new exception.

So why would you want the compiler to prevent you form doing something that 1) is not illegal and 2) could be useful?