Why does using throw an exception when it's supposed to behave like try/catch/finally?

Question

As far as I understand, using works like a try/catch/finally, so I would expect that if an exception occurs in a using statement it would get caught (which is kinda odd, because that would also mean that the exception is silently eaten). The using statement should catch the exception and call the Dispose method, however, that is not happening. I've devised a simple test to demonstrate the issue.

Here is where I force an exception to occur inside the using statement:

using (TcpClient client = new TcpClient())
{
    // Why does this throw when the using statement is supposed to be a try/catch/finally?
    client.Connect(null);
}

An exception is throw by client.Connect() (meaning that it was not caught by the using statement or that it was re-thrown):

System.ArgumentNullException: Value cannot be null.
Parameter name: remoteEP
   at System.Net.Sockets.TcpClient.Connect(IPEndPoint remoteEP)
   at DotNETSandbox.Program.Main(String[] args) in C:\path\to\Sandbox\Program.cs:line 42

According to a Microsoft article on the topic, the using statement might throw if the Dispose method throws.

However, when I'm following the using pattern, it is evident that the Dispose method does not throw:

TcpClient c2 = new TcpClient();
try
{
    c2.Connect(null);               
}
catch (Exception e)
{
    // We caught the null ref exception
    try
    {
        // Try to dispose: works fine, does not throw!
        ((IDisposable)c2).Dispose();
    }
    catch (Exception e2)
    {
        Console.WriteLine(e2.ToString());
    }
    Console.WriteLine(e.ToString());
}

I'm a little confused, since I was expecting using to behave like a try/catch. Could anybody explain why this is happening?

Answer 1

using is

try
{
}
finally
{
}

only !!!

there is no catch !

in .net you dont have to catch the exception..

its just a mechanism which calls DISPOSE method as soon as you leave the scope . thats all.

p.s. : " how will i know if my object can be used with using ?"

answer : don't worry - you will see error on compile time.