How do I dispose an IDisposable object if the using statement throws an exception?

Question

How can I make sure in the following code snippet that IDataReader is disposed of if ExecuteReader throws an exception?

using (IDataReader rdr = cmd.ExecuteReader())
{
    // use it
}

It makes sense to me that the using syntatic sugar does not call Dispose (since there is no instance to call it on). However, how can I be sure that the scarce resources that are normally allocated by classes that implement IDisposable will be releases?

Answer 1

I thought that the using statement was translated to the IL that is similar to:

try
{
}
finally
{
    disposableUsedObject.Dispose();
}

So, I would think that in normal circumstances, the Dispose should be called ?

Also, you should not throw exceptions inside the constructor , as users will not expect that an exception will be thrown when they instantiate an object. I would move the initialization logic that can throw an exception to another method (Initialize) that has to be called after instantiating the object.

Answer 2

If ExecuteReader, in your example, throws an exception it never returns anything. It is then up to the implementation of ExecuteReader to dispose of anything created before the exception.

Answer 3

If the constructor of an object fails to run then you do not have an object that needs to be disposed.

If you are writing a constructor which could throw an exception, you had better make sure you clean up anything you need to by using using or try-catch block.

In your IDataReader example, it is sufficient to simply dispose of the command object if the cmd.ExecuteReader() method call fails.

Answer 4

How about moving the code that does the initialization outside the constructor into a separate function. In essence you'll have

using (var tc = new TestClass())
{
    tc.Initialize();  // setup the object. acquire expensive resources here, etc.

    ..more code ..
}