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POIs a generic exception supported inside a catch?
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  1. POIs a generic exception supported inside a catch?
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    copied!<p><strong>The above block refering to another question+answer on SO does NOT contain a correct answer which applies here!</strong></p> <p>I have a method used for unit testing. The purpose of this method is to ensure that a piece of code (refered to by a delegate) will throw a specific exception. If that exception is thrown, the unit test succeeds. If no exception is thrown or another type exception is thrown, the unit test will fail.</p> <pre><code>/// &lt;summary&gt; /// Checks to make sure that the action throws a exception of type TException. /// &lt;/summary&gt; /// &lt;typeparam name="TException"&gt;The type of exception expected.&lt;/typeparam&gt; /// &lt;param name="action"&gt;The code to execute which is expected to generate the exception.&lt;/param&gt; public static void Throws&lt;TException&gt;(Action action) where TException : Exception { try { action(); } catch (TException) { return; } catch (Exception ex) { Assert.Fail("Wrong exception was thrown. Exception of type " + ex.GetType() + " was thrown, exception of type " + typeof(TException) + " was expected."); } Assert.Fail("No exception was thrown. Exception of type " + typeof(TException) + " was expected."); } </code></pre> <p>The next call should succeed, but it fails:</p> <pre><code>int result = 0; Throws&lt;DivideByZeroException&gt;(() =&gt; result = result / result); </code></pre> <p>When the expected exception of type <code>TException</code> is thrown, it is always caught by the second catch, not by the first catch. Why is this? </p> <p>Of course I can use a workarround with one catch and test if <code>ex</code> is of type <code>TException</code>. By I simply want to know/understand why this code compiles but simple (never?) works.</p> <p><strong>EDIT</strong></p> <p>At request a "working" demo:</p> <pre><code>using System; namespace GenericExceptionDemo { class Program { static void Main(string[] args) { int n = 0; Catch&lt;DivideByZeroException&gt;(() =&gt; n = n / n); } static public void Catch&lt;TException&gt;(Action action) where TException: Exception { try { action(); Console.WriteLine("No exception thrown. !!!Fail!!!"); } catch (TException) { Console.WriteLine("Expected exception thrown. PASS!"); } catch(Exception ex) { Console.WriteLine("An unexpected exception of type " + ex.GetType() + " thrown. !!!FAIL!!!"); } } } } </code></pre>
 

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