We Keep Coding

What is the resulting behavior when an IDisposable is passed into a parent IDisposable

Yesterday, after running Visual Studio code analysis on our codebase, the following code was highlighted as an issue:
using (var stringReader = new StringReader(someString))
{
using (var reader = XmlReader.Create(stringReader)) {
// Code
}
}
The warning that was returned was
Warning CA2202 Object 'stringReader' can be disposed more than once in
method '(method name)'. To avoid generating a
System.ObjectDisposedException you should not call Dispose more than
one time on an object.
After searching stack overflow, I've come to the general understanding that if I were to create a custom class containing IDisposable members, it should implement IDisposable itself, and call the dispose() method of the member.
My two questions are
In all cases where object X takes a reference to an IDisposable object Y as a parameter during creation, is it correct to assume that the object X will take ownership of Y and from that point onwards, calling X.dispose() will always result in calling Y.dispose()
This is an old piece of code and exceptions as described in the warning message have never been reported (to the best of my knowledge). If the above point is assumed, why does the double using block not result in calling stringReader.dispose() twice and therefore throwing an exception?

is it correct to assume that the object X will take ownership of Y and from that point onwards, calling X.dispose() will always result in calling Y.dispose()
No, it is never save to assume that. Let's check this specific case: XmlReader.Create(Stream).
After going to quite some code in the Reference Source, I have found that the Dispose method calls the Close method. That is quite obvious. Then notice this piece of code:
public override void Close() {
Close( closeInput );
}
So whether the backing stream will be closed and disposed depends on the value of the setting closeInput, which you can set through the XmlReaderSettings.CloseInput setting.
So the answer here is a definite no: you can't be sure it is disposed. You should always make sure yourself it is.

No, you cannot assume that another object will call Dispose() while disposing itself. The object having the disposable object as a reference might not even be using the disposable resource.
This warning is known to be somewhat weird. Look here to see some complaints about the warning. You should design your class so that it must be safe to call Dispose() more than once.
By the way, MSDN says:
A method implementation contains code paths that could cause multiple calls to IDisposable.Dispose or a Dispose equivalent, such as a Close() method on some types, on the same object.
So a path to the Close() method call can generate this warning too, which is why you see this warning in your case.

In all cases where object X takes a reference to an IDisposable object Y as a parameter during creation, is it correct to assume that the object X will take ownership of Y and from that point onwards, calling X.dispose() will always result in calling Y.dispose()
I think not and I will try to explain why.
There is a pattern called IDisposablePattern which looks something like that:
public class SimpleClass : IDisposable
{
// managed resources SqlConnection implements IDisposable as well.
private SqlConnection _connection;
private bool _disposed;
// implementing IDisposable
public void Dispose()
{
// Here in original Dispose method we call protected method with parameter true,
// saying that this object is being disposed.
this.Dispose(true);
// Then we "tell" garbage collector to suppress finalizer for this object because we are releasing
// its memory and doesnt need to be finalized. Calling finalizer(destructor) of a given type is expensive
// and tweaks like this help us improve performance of the application.
GC.SuppressFinalize(this);
}
// Following the best practices we should create another method in the class
// with parameter saying whether or not the object is being disposed.
// Its really important that this method DOES NOT throw exceptions thus allowing to be called multiple times
protected virtual void Dispose(bool disposing)
{
// another thing we may add is flag that tells us if object is disposed already
// and use it here
if (_disposed) { return; }
if (_connection != null)
{
_connection.Dispose();
_connection = null;
}
_disposed = true;
// call base Dispose(flag) method if we are using hierarchy.
}
}
Note that this can be extended to new level when your class uses unmanaged resources like this one:
public class SimpleClass2: IDisposable
{
// managed resources
private SqlConnection _connection;
private bool _disposed;
// unmanaged resources
private IntPtr _unmanagedResources;
// simple method for the demo
public string GetDate()
{
// One good practice that .NET Framework implies is that when object is being disposed
// trying to work with its resources should throw ObjectDisposedException so..
if(_disposed) { throw new ObjectDisposedException(this.GetType().Name);}
if (_connection == null)
{
_connection = new SqlConnection("Server=.\\SQLEXPRESS;Database=master;Integrated Security=SSPI;App=IDisposablePattern");
_connection.Open();
}
// allocation of unmanaged resources for the sake of demo.
if (_unmanagedResources == IntPtr.Zero)
{
_unmanagedResources = Marshal.AllocHGlobal(100 * 1024 * 1024);
}
using (var command = _connection.CreateCommand())
{
command.CommandText = "SELECT getdate()";
return command.ExecuteScalar().ToString();
}
}
public void Dispose()
{
// Here in original Dispose method we call protected method with parameter true,
// saying that this object is being disposed.
this.Dispose(true);
// Then we "tell" garbage collector to suppress finalizer for this object because we are releasing
// its memory and doesnt need to be finalized. Calling finalizer(destructor) of a given type is expensive
// and tweaks like this help us improve performance of the application.
// This is only when your class doesnt have unmanaged resources!!!
// Since this is just made to be a demo I will leave it there, but this contradicts with our defined finalizer.
GC.SuppressFinalize(this);
}
// Following the best practices we should create another method in the class
// with parameter saying wether or not the object is being disposed.
// Its really important that this method DOES NOT throw exceptions thus allowing to be called multiple times
protected virtual void Dispose(bool disposing)
{
// another thing we may add is flag that tells us if object is disposed already
// and use it here
if (_disposed) { return; }
// Thus Dispose method CAN NOT release UNMANAGED resources such as IntPtr structure,
// flag is also helping us know whether we are disposing managed or unmanaged resources
if (disposing)
{
if (_connection != null)
{
_connection.Dispose();
_connection = null;
}
_disposed = true;
}
// Why do we need to do that?
// If consumer of this class forgets to call its Dispose method ( simply by not using the object in "using" statement
// Nevertheless garbage collector will fire eventually and it will invoke Dispose method whats the problem with that is if we didn't
// have the following code unmanaged resources wouldnt be disposed , because as we know GC cant release unmanaged code.
// So thats why we need destructor(finalizer).
if (_unmanagedResources != IntPtr.Zero)
{
Marshal.FreeHGlobal(_unmanagedResources);
_unmanagedResources = IntPtr.Zero;;
}
// call base Dispose(flag) method if we are using hierarchy.
}
~DatabaseStateImpr()
{
// At this point GC called our finalizer method , meaning
// that we don't know what state our managed resources are (collected or not) because
// our consumer may not used our object properly(not in using statement) so thats why
// we skip unmanaged resources as they may have been finalized themselves and we cant guarantee that we can
// access them - Remember? No exceptions in Dispose methods.
Dispose(false);
}
}

Should IDisposable objects stored in collections be disposed manually?

I have a stack of WindowsImpersonationContext objects, which are IDisposable type. Now consider these two operations:
Stack<WindowsImpersonationContext> stack = new Stack<WindowsImpersonationContext>();
public void BeginOperation()
{
// Push object to stack
var context = WindowsIdentity.Impersonate(...);
stack.Push(context);
}
public void EndOperation()
{
// Pop object from stack.
if (stack.Count != 0)
{
var context = stack.Pop();
... do various stuff with 'context' ...
context.Dispose(); // should this be called once the object is not needed anymore?
}
}
Should I call Dispose() manually if I don't want to wait for the GC to do it? Is it safe to do it like this if I'm 100% no other code will try to use the context object referenced in the stack? To be precise, if I don't explicitly call context.Dispose(), I assume that even after the execution returns from EndOperation(), I'm at the mercy of GC in regard to disposing resources occupied by the 'context' object, despite it not being referenced by the stack anymore? Is my reasoning correct?
Also, using the above code, is it correct to claim that after executing the following set of operations, the stack will end up empty in terms of resources occupied (i.e. all objects it used to contain are disposed at the end, and I don't need to worry about disposing anything elsewhere anymore):
BeginOperation();
BeginOperation();
EndOperation();
EndOperation();
EDIT: I realise that using can/should be used for safety purposes, but mainly I'm concerned with what's going on in the background in terms of collections/GC, regardless of the syntax ...

If you don't need it anymore and it implements IDisposable use the using-statement.
public void EndOperation()
{
// Pop object from stack.
if (stack.Count != 0)
{
using(var context = stack.Pop())
{
// ... do various stuff with 'context' ...
}
}
}
Apart from that Stack.Pop is implemented in this way(ILSPy, .NET 4):
// System.Collections.Stack
public virtual object Pop()
{
if (this._size == 0)
{
throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EmptyStack"));
}
this._version++;
object result = this._array[--this._size];
this._array[this._size] = null;
return result;
}
So you see that there's no reference anymore in the stack after Pop.
In general you should use using (or try-finally) whenever you use unmanaged resources. On this way it is ensured that they are disposed even on error.

If you are sure that nothing other than the stack had a reference to the object in question, then yes: You should dispose it yourself manually.
I'd change your code to look more like this:
public void EndOperation()
{
// Pop object from stack.
if (stack.Count != 0)
{
using (var context = stack.Pop())
{
... do various stuff with 'context' ...
}
}
}
That way it will be disposed even if an exception occurs in the "do various stuff" block.
If you don't dispose it yourself, then you rely on a finalizer being implemented for the class being disposed - and even if a finalizer is implemented, it may be called too late (imagine if it was an open file handle).

You can use using statement but before you should implements IDisposable.

Is this a valid object disposal code pattern in method with return?

I noticed the following object disposal code pattern in a C# project and I was wondering if it's acceptable (although it works).
public object GetData()
{
object obj;
try
{
obj = new Object();
// code to populate SortedList
return obj;
}
catch
{
return null;
}
finally
{
if (obj != null)
{
obj.Dispose();
obj = null;
}
}
}
For this example, I'm using a general 'object' instead of the actual IDisposable class in the project.
I know that the 'finally' block will be executed every time, even when the value is returned, but would it affect the return value (or would it be a new object instance) in any way since the object is being set to null (for what seems like object disposal and GC purposes).
Update 1:
I tried the following snippet and the return object is non-null, although the local object is set to null, so it works, which is a bit strange considering some of the comments below:
public StringBuilder TestDate()
{
StringBuilder sb;
try
{
sb = new StringBuilder();
sb.Append(DateTime.UtcNow.ToString());
return sb;
}
catch
{
return null;
}
finally
{
sb = null;
}
}
Btw, I'm using C# 4.0.
P.S. I'm just reviewing this project code. I'm not the original author.
Update 2:
Found the answer to this mystery [1]. The finally statement is executed, but the return value isn't affected (if set/reset in the finally block).
[1] What really happens in a try { return x; } finally { x = null; } statement?

This code will compile fine (assuming that you are not actually using an Object but something that implements IDisposable), but it probably won't do what you want it to do. In C#, you don't get a new object without a new; this code will return a reference to an object that has already been disposed, and depending on the object and what Dispose() actually does, trying to use a disposed object may or may not crash your program.
I assume the idea is to create an object, do some stuff with it, then return the object if successful or null (and dispose the object) on failure. If so, what you should do is:
try {
obj = new MyClass();
// ... do some stuff with obj
return obj;
}
catch {
if(obj != null) obj.Dispose();
return null;
}

Simply using the using statement achieves the same result as that, and is the standard practice
public int A()
{
using(IDisposable obj = new MyClass())
{
//...
return something;
}
}
I would, however, advise against returning your IDisposable object.
When you dispose of an object, it is supposed to be considered "unusable". And so, why return it?
If the object's lifetime needs to be longer than the method A's lifetime, consider having the calling method B instantiate the object, and pass it as a parameter to method A.
In this case, method Bwould be the one using the using statement, inside which it would call A.

If you are returning an IDisposable object, then it is the responsibility of your caller to dispose of it:
public IDisposable MakeDisposableObject()
{
return new SqlConnection(""); // or whatever
}
caller:
using (var obj = MakeDisposableObject())
{
}
It makes less than no sense for your method to dispose of an object and then return it. The disposed object will be of no value to the caller. In general, referencing a disposable object which has been disposed should produce an ObjectDisposedException.

A few observations.
That code wouldn't compile because object doesn't have a .Dispose() method.
Why wouldn't you use IDisposable?
Why would you dispose of an object that is being returned, since returning you would return an object for the purpose of some other code to use it. The concept of "disposing" of something is to give it a chance to clean up after itself and its used, un-managed resources. If you are returning an object that is supposed to be used elsewhere, but has unmanaged resources that you want to clean up before the object gets used anywhere else, then you shuld really have 2 separate objects. One to load some data that would be disposable, and another object that would contain the usable loaded content that you want to pass around. An example of this would be something like stream readers in the .NET framework. You would normally new a stream reader, read it into a byte[] or some other data object, .Dispose() the stream reader, then return the byte[]. The "loader" that has some resources to dispose of in a timely fashion is separate from the object containing the "loaded" data that can be used without needing to be disposed.

return the variable used for using inside the using C#

I am returning the variable I am creating in a using statement inside the using statement (sounds funny):
public DataTable foo ()
{
using (DataTable properties = new DataTable())
{
// do something
return properties;
}
}
Will this Dispose the properties variable??
After doing this am still getting this Warning:
Warning 34 CA2000 : Microsoft.Reliability : In method 'test.test', call System.IDisposable.Dispose on object 'properties' before all references to it are out of scope.
Any Ideas?
Thanks

If you want to return it, you can't wrap it in a using statement, because once you leave the braces, it goes out of scope and gets disposed.
You will have to instantiate it like this:
public DataTable Foo()
{
DataTable properties = new DataTable();
return properties;
}
and call Dispose() on it later.

Yes, it will dispose it - and then return it. This is almost always a bad thing to do.
In fact for DataTable, Dispose almost never does anything (the exception being if it's remoted somewhere, IIRC) but it's still a generally bad idea. Normally you should regard disposed objects as being unusable.

Supposedly, this is the pattern for a factory method that creates a disposable object. But, I've still seen Code Analysis complain about this, too:
Wrapper tempWrapper = null;
Wrapper wrapper = null;
try
{
tempWrapper = new Wrapper(callback);
Initialize(tempWrapper);
wrapper = tempWrapper;
tempWrapper = null;
}
finally
{
if (tempWrapper != null)
tempWrapper.Dispose();
}
return wrapper;
This should guarantee that if the initialization fails, the object is properly disposed, but if everything succeeds, an undisposed instance is returned from the method.
MSDN Article: CA2000: Dispose objects before losing scope.

Yes. Why are you using the using keyword on something you don't want disposed at the end of the code block?
The purpose of the using keyword is to dispose of the object.
http://msdn.microsoft.com/en-us/library/yh598w02.aspx

The point of a using block is to create an artificial scope for a value/object. When the using block completes, the object is cleaned up because it is no longer needed. If you really want to return the object you are creating, than it is not a case where you want to use using.
This will work just fine.
public DataTable foo ()
{
DataTable properties = new DataTable();
// do something
return properties;
}

Your code using the using keyword expands to:
{
DataTable properties = new DataTable();
try
{
//do something
return properties;
}
finally
{
if(properties != null)
{
((IDisposable)properties).Dispose();
}
}
}
Your variable is being disposed by nature of how using works. If you want to be able to return properties, don't wrap it in a using block.

The other responses are correct: as soon as you exit the using block, your object is disposed. The using block is great for making sure that an object gets disposed in a timely manner, so if you don't want to rely on the consumers of your function to remember to dispose the object later, you can try something like this:
public void UsingDataContext (Action<DataContext> action)
{
using (DataContext ctx = new DataContext())
{
action(ctx)
}
}
This way you can say something like:
var user = GetNewUserInfo();
UsingDataContext(c => c.UserSet.Add(user));

Can an object be declared above a using statement instead of in the brackets

Most of the examples of the using statement in C# declare the object inside the brackets like this:
using (SqlCommand cmd = new SqlCommand("SELECT * FROM Customers", connection))
{
// Code goes here
}
What happens if I use the using statement in the following way with the object declared outside the using statement:
SqlCommand cmd = new SqlCommand("SELECT * FROM Customers", connection);
using (cmd)
{
// Code goes here
}
Is it a bad idea to use the using statement in the way I have in the second example and why?

Declaring the variable inside the using statement's control expression limits the scope of the variable to inside the using statement. In your second example the variable cmd can continue to be used after the using statement (when it will have been disposed).
Generally it is recommended to only use a variable for one purpose, limiting its scope allows another command with the same name later in scope (maybe in another using expression). Perhaps more importantly it tells a reader of your code (and maintenance takes more effort than initial writing) that cmd is not used beyond the using statement: your code is a little bit more understandable.

Yes, that is valid - the object will still be disposed in the same manner, ie, at the end and if execution flow tries to leave the block (return / exception).
However if you try to use it again after the using, it will have been disposed, so you cannot know if that instance is safe to continue using as dispose doesn't have to reset the object state. Also if an exception occurs during construction, it will not have hit the using block.
I'd declare and initialize the variable inside the statement to define its scope. Chances are very good you won't need it outside the scope if you are using a using anyway.
MemoryStream ms = new MemoryStream(); // Initialisation not compiled into the using.
using (ms) { }
int i = ms.ReadByte(); // Will fail on closed stream.
Below is valid, but somewhat unnecessary in most cases:
MemoryStream ms = null;
using (ms = new MemoryStream())
{ }
// Do not continue to use ms unless re-initializing.

I wrote a little code along with some unit tests. I like it when I can validate statements about the question at hand. My findings:
Whether an object is created before or in the using statement doesn't matter. It must implement IDisposable and Dispose() will be called upon leaving the using statement block (closing brace).
If the constructor throws an exception when invoked in the using statement Dispose() will not be invoked. This is reasonable as the object has not been successfully constructed when an exception is thrown in the constructor. Therefore no instance exists at that point and calling instance members (non-static members) on the object doesn't make sense. This includes Dispose().
To reproduce my findings, please refer to the source code below.
So bottom line you can - as pointed out by others - instantiate an object ahead of the using statement and then use it inside the using statement. I also agree, however, moving the construction outside the using statement leads to code that is less readable.
One more item that you may want to be aware of is the fact that some classes can throw an exception in the Dispose() implementation. Although the guideline is not to do that, even Microsoft has cases of this, e.g. as discussed here.
So here is my source code include a (lengthy) test:
public class Bar : IDisposable {
public Bar() {
DisposeCalled = false;
}
public void Blah() {
if (DisposeCalled) {
// object was disposed you shouldn't use it anymore
throw new ObjectDisposedException("Object was already disposed.");
}
}
public void Dispose() {
// give back / free up resources that were used by the Bar object
DisposeCalled = true;
}
public bool DisposeCalled { get; private set; }
}
public class ConstructorThrows : IDisposable {
public ConstructorThrows(int argument) {
throw new ArgumentException("argument");
}
public void Dispose() {
Log.Info("Constructor.Dispose() called.");
}
}
[Test]
public void Foo() {
var bar = new Bar();
using (bar) {
bar.Blah(); // ok to call
}// Upon hitting this closing brace Dispose() will be invoked on bar.
try {
bar.Blah(); // Throws ObjectDisposedException
Assert.Fail();
}
catch(ObjectDisposedException) {
// This exception is expected here
}
using (bar = new Bar()) { // can reuse the variable, though
bar.Blah(); // Fine to call as this is a second instance.
}
// The following code demonstrates that Dispose() won't be called if
// the constructor throws an exception:
using (var throws = new ConstructorThrows(35)) {
}
}

The idea behind using is to define a scope, outside of which an object or objects will be disposed.
If you declare the object you are about to use inside using in advance, there's no point to use the using statement at all.

It has been answered and the answer is: Yes, it's possible.However, from a programmers viewpoint, don't do it! It will confuse any programmer who will be working on this code and who doesn't expect such a construction. Basically, if you give the code to someone else to work on, that other person could end up being very confused if they use the "cmd" variable after the using. This becomes even worse if there's even more lines of code between the creation of the object and the "using" part.