What is the difference between throw and throw ex in C# exception handling?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Catch the exception

   Enter a catch block where an exception object is in flight, e.g. 'catch (Exception ex)'.

2. Step 2

   Decide whether to rethrow

   If you cannot handle it here, you rethrow so an outer handler can deal with it.

3. Step 3

   Use 'throw;' to preserve the trace

   Write 'throw;' with no operand to rethrow the current exception keeping its original stack trace.

4. Step 4

   Avoid 'throw ex;'

   Writing 'throw ex;' resets the stack trace to this line, discarding the true origin.

5. Step 5

   Add context safely if needed

   Wrap it: 'throw new DataException("context", ex);' so the original is kept as InnerException.

Code Example

try
{
    ProcessOrder();
}
catch (Exception ex)
{
    _logger.LogError(ex, "Order failed");

    throw;            // GOOD: keeps original stack trace
    // throw ex;      // BAD: resets stack trace to this line
}

// Adding context while preserving the original:
catch (SqlException ex)
{
    throw new DataException("Failed loading order", ex); // ex becomes InnerException
}

Follow-up Questions

• How do you add context to an exception without losing the original trace?
• What is an InnerException and when do you use it?
• What happens if you swallow an exception in an empty catch?
• How does ExceptionDispatchInfo help rethrow across async boundaries?
• When should you catch a specific exception type versus the base Exception?