How to catch all exceptions in c# using try and catch?

Both approaches will catch all exceptions. There is no significant difference between your two code examples except that the first will generate a compiler warning because ex is declared but not used.

But note that some exceptions are special and will be rethrown automatically.

ThreadAbortException is a special exception that can be caught, but it will automatically be raised again at the end of the catch block.

http://msdn.microsoft.com/en-us/library/system.threading.threadabortexception.aspx

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As mentioned in the comments, it is usually a very bad idea to catch and ignore all exceptions. Usually you want to do one of the following instead:

• Catch and ignore a specific exception that you know is not fatal.

  catch (SomeSpecificException)
  {
      // Ignore this exception.
  }
  

• Catch and log all exceptions.

  catch (Exception e)
  {
      // Something unexpected went wrong.
      Log(e);
      // Maybe it is also necessary to terminate / restart the application.
  }
  

• Catch all exceptions, do some cleanup, then rethrow the exception.

  catch
  {
      SomeCleanUp();
      throw;
  }
  

Note that in the last case the exception is rethrown using throw; and not throw ex;.

Note that besides all other comments there is a small difference, which should be mentioned here for completeness!

With the empty catch clause you can catch non-CLSCompliant Exceptions when the assembly is marked with "RuntimeCompatibility(WrapNonExceptionThrows = false)" (which is true by default since CLR2). [1][2][3]

[1] http://msdn.microsoft.com/en-us/library/bb264489.aspx

[2] http://blogs.msdn.com/b/pedram/archive/2007/01/07/non-cls-exceptions.aspx

[3] Will CLR handle both CLS-Complaint and non-CLS complaint exceptions?

    static void Main(string[] args)
    {
        AppDomain.CurrentDomain.UnhandledException += CurrentDomain_UnhandledException;
    }

    static void CurrentDomain_UnhandledException(object sender, UnhandledExceptionEventArgs e)
    {
        throw new NotImplementedException();
    }