C# Throw Exception and catch in the same method why it's bad?

Question

Have some discussion about one implementation:

// Pseudocode
accessor type GetValue() 
{
    try
    {
        do some action with possible throw exception1
        do some action with possible throw exception2

        return value;
    }
    catch (Exception ex)
    {
        value = default;
        throw Wraped in Meaningfull Exception ex

    }
}

Can someone explain why it might be a bad design, to use try-catch like that (throw and catch at the same method) to safely do some actions and agregate different type of similar exception?

Answer 1

It's not rethrowing

 throw new WrapedException("MyNewMessage", ex);

that is wrong but catching all the exceptions

 catch (Exception ex) {
   ...
 }

is a bad design: it masks potentially dangerous behaviour. Let's see why. Suppose we use GetValue() like this:

   try {
     someValue = GetValue();
   }
   catch (WrapedException) {  
     // We failed to obtain someValue;
     // The reason - WrapedException - is innocent
     // Let's use default value then

     someValue = defaultSomeValue;
   }

And the actual picture is

   public GetValue() {
     try {
       do some action with possible throw exception1

       // Catastrophy here: AccessViolationException! System is in ruins!
       do some action with possible throw exception2

       return value;
     }
     catch (Exception ex) { // AccessViolationException will be caught...
       // ...and the disaster will have been masked as being just WrapedException
       throw new WrapedException("MyNewMessage", ex);
     }
   }

Your design is OK if you catch only expected exception types:

   public GetValue() {
     try {
       do some action with possible throw exception1
       do some action with possible throw exception2

       return value;
     }
     catch (FileNotFound ex) {
       // File not found, nothing special in the context of the routine
       throw new WrapedException("File not found and we can't load the CCalue", ex);
     }
   }