Implementing IDisposable correctly

Question

In my classes I implement IDisposable as follows:

public class User : IDisposable
{
    public int id { get; protected set; }
    public string name { get; protected set; }
    public string pass { get; protected set; }

    public User(int UserID)
    {
        id = UserID;
    }
    public User(string Username, string Password)
    {
        name = Username;
        pass = Password;
    }

    // Other functions go here...

    public void Dispose()
    {
        // Clear all property values that maybe have been set
        // when the class was instantiated
        id = 0;
        name = String.Empty;
        pass = String.Empty;
    }
}

In VS2012, my Code Analysis says to implement IDisposable correctly, but I'm not sure what I've done wrong here.
The exact text is as follows:

CA1063 Implement IDisposable correctly Provide an overridable implementation of Dispose(bool) on 'User' or mark the type as sealed. A call to Dispose(false) should only clean up native resources. A call to Dispose(true) should clean up both managed and native resources. stman User.cs 10

For reference: CA1063: Implement IDisposable correctly

I've read through this page, but I'm afraid I don't really understand what needs to be done here.

If anyone can explain in more layman's terms what the problem is and/or how IDisposable should be implemented, that will really help!

Solution 1

This would be the correct implementation, although I don't see anything you need to dispose in the code you posted. You only need to implement IDisposable when:

1. You have unmanaged resources
2. You're holding on to references of things that are themselves disposable.

Nothing in the code you posted needs to be disposed.

public class User : IDisposable
{
    public int id { get; protected set; }
    public string name { get; protected set; }
    public string pass { get; protected set; }

    public User(int userID)
    {
        id = userID;
    }
    public User(string Username, string Password)
    {
        name = Username;
        pass = Password;
    }

    // Other functions go here...

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing) 
        {
            // free managed resources
        }
        // free native resources if there are any.
    }
}

Solution 2

First of all, you don't need to "clean up" strings and ints - they will be taken care of automatically by the garbage collector. The only thing that needs to be cleaned up in Dispose are unmanaged resources or managed recources that implement IDisposable.

However, assuming this is just a learning exercise, the recommended way to implement IDisposable is to add a "safety catch" to ensure that any resources aren't disposed of twice:

public void Dispose()
{
    Dispose(true);

    // Use SupressFinalize in case a subclass 
    // of this type implements a finalizer.
    GC.SuppressFinalize(this);   
}
protected virtual void Dispose(bool disposing)
{
    if (!_disposed)
    {
        if (disposing) 
        {
            // Clear all property values that maybe have been set
            // when the class was instantiated
            id = 0;
            name = String.Empty;
            pass = String.Empty;
        }

        // Indicate that the instance has been disposed.
        _disposed = true;   
    }
}

Solution 3

The following example shows the general best practice to implement IDisposable interface. Reference

Keep in mind that you need a destructor(finalizer) only if you have unmanaged resources in your class. And if you add a destructor you should suppress Finalization in the Dispose, otherwise it will cause your objects resides in memory for two garbage cycles (Note: Read how Finalization works). Below example elaborate all above.

public class DisposeExample
{
    // A base class that implements IDisposable. 
    // By implementing IDisposable, you are announcing that 
    // instances of this type allocate scarce resources. 
    public class MyResource: IDisposable
    {
        // Pointer to an external unmanaged resource. 
        private IntPtr handle;
        // Other managed resource this class uses. 
        private Component component = new Component();
        // Track whether Dispose has been called. 
        private bool disposed = false;

        // The class constructor. 
        public MyResource(IntPtr handle)
        {
            this.handle = handle;
        }

        // Implement IDisposable. 
        // Do not make this method virtual. 
        // A derived class should not be able to override this method. 
        public void Dispose()
        {
            Dispose(true);
            // This object will be cleaned up by the Dispose method. 
            // Therefore, you should call GC.SupressFinalize to 
            // take this object off the finalization queue 
            // and prevent finalization code for this object 
            // from executing a second time.
            GC.SuppressFinalize(this);
        }

        // Dispose(bool disposing) executes in two distinct scenarios. 
        // If disposing equals true, the method has been called directly 
        // or indirectly by a user's code. Managed and unmanaged resources 
        // can be disposed. 
        // If disposing equals false, the method has been called by the 
        // runtime from inside the finalizer and you should not reference 
        // other objects. Only unmanaged resources can be disposed. 
        protected virtual void Dispose(bool disposing)
        {
            // Check to see if Dispose has already been called. 
            if(!this.disposed)
            {
                // If disposing equals true, dispose all managed 
                // and unmanaged resources. 
                if(disposing)
                {
                    // Dispose managed resources.
                    component.Dispose();
                }

                // Call the appropriate methods to clean up 
                // unmanaged resources here. 
                // If disposing is false, 
                // only the following code is executed.
                CloseHandle(handle);
                handle = IntPtr.Zero;

                // Note disposing has been done.
                disposed = true;

            }
        }

        // Use interop to call the method necessary 
        // to clean up the unmanaged resource.
        [System.Runtime.InteropServices.DllImport("Kernel32")]
        private extern static Boolean CloseHandle(IntPtr handle);

        // Use C# destructor syntax for finalization code. 
        // This destructor will run only if the Dispose method 
        // does not get called. 
        // It gives your base class the opportunity to finalize. 
        // Do not provide destructors in types derived from this class.
        ~MyResource()
        {
            // Do not re-create Dispose clean-up code here. 
            // Calling Dispose(false) is optimal in terms of 
            // readability and maintainability.
            Dispose(false);
        }
    }
    public static void Main()
    {
        // Insert code here to create 
        // and use the MyResource object.
    }
}

Solution 4

IDisposable exists to provide a means for you to clean up unmanaged resources that won't be cleaned up automatically by the Garbage Collector.

All of the resources that you are "cleaning up" are managed resources, and as such your Dispose method is accomplishing nothing. Your class shouldn't implement IDisposable at all. The Garbage Collector will take care of all of those fields just fine on its own.

Solution 5

You need to use the Disposable Pattern like this:

private bool _disposed = false;

protected virtual void Dispose(bool disposing)
{
    if (!_disposed)
    {
        if (disposing)
        {
            // Dispose any managed objects
            // ...
        }

        // Now disposed of any unmanaged objects
        // ...

        _disposed = true;
    }
}

public void Dispose()
{
    Dispose(true);
    GC.SuppressFinalize(this);  
}

// Destructor
~YourClassName()
{
    Dispose(false);
}

View more solutions

Comments

[Claudio Redi]

Is that all the code inside Dispose?

[Daniel Mann]

Did you look at the code sample provided in the link you posted?

[Belogix]

There is the IDisposable pattern that you should use / investigate. I am sure you will get lots of answers with details soon but basically it involves GC.SupressFinalize() and destructor etc.

[user1703401]

You should implement your Dispose() method to call the Dispose() method on any of the members of your class. None of those members have one. You should therefore not implement IDisposable. Resetting the property values is pointless.

[jason]

You only need to implement IDispoable if you have unmanaged resources to dispose of (this includes unmanaged resources that are wrapped (SqlConnection, FileStream, etc.). You do not and should not implement IDisposable if you only have managed resources such as here. This is, IMO, a major problem with code analysis. It's very good at checking silly little rules, but not good at checking conceptual errors.

[Keith Payne]

@Ortund there is already voluminous material on SO concerning the Disposable pattern. Even in the answers to this question there are subtle examples of misunderstanding the pattern. It is much better to point future questioners to the first related SO question (which has 309 upvotes).

[tjmoore]

So don't downvote, don't upvote, leave the post at zero and close the question with a helpful pointer.

[Ortund]

I was told when I started writing in C# that it's best to make use of using(){ } whenever possible, but to do that, you need to implement IDisposable, so in general, I prefer to access a class through usings, esp. if I only need the class in one or two functions

[Daniel Mann]

@Ortund You misunderstood. It's best to use a using block when the class implements IDisposable. If you don't need a class to be disposable, don't implement it. It serves no purpose.

[Chandramouleswaran Ravichandra]

Agree with this - there is a notion of disposing everything when it is actually not needed. Dispose should be used only if you have unmanaged resources to clean up!!

[Thomas]

@DanielMann The semantics of a using block do tend to be appealing beyond the IDisposable interface alone, though. I imagine there have been more than a few abuses of IDisposable just for scoping purposes.

[Thomas]

+1, having a flag to make sure the cleanup code is executed only once is way, way better than setting properties to null or whatever (especially since that interferes with readonly semantics)

[Matt Wilko]

Not strictly true, the Dispose method also allows you to dispose of "managed resources that implement IDisposable"

[Servy]

@MattWilko That makes it an indirect way of disposing of unmanaged resources, because it allows another resource to dispose of an unmanaged resource. Here there isn't even an indirect reference to an unmanaged resource through another managed resource.

[Sudhanshu Mishra]

wouldn't it be wiser to have a call to GC.SuppressFinalize(this) in the destructor as well? Otherwise the object itself would be reclaimed in the next GC

[schoetbi]

@dotnetguy: The objects destructor is called when the gc runs. So calling twice is not possible. See here: msdn.microsoft.com/en-us/library/ms244737.aspx

[Chel]

So now are we calling any piece of boilerplate code a "pattern"?

[Belogix]

@rdhs No we are not. MSDN states it IS a pattern "Dispose Pattern" here - msdn.microsoft.com/en-us/library/b1yfkh5e(v=vs.110).aspx so before down-voting maybe Google a little?

[mariozski]

Just as a side-note if you would have any unmanaged resources to be freed you should include Finalizer calling Dispose(false), that will allow GC to call Finalizer when doing garbage collection (in case Dispose was not called yet) and properly free unmanaged resources.

[Haymo Kutschbach]

Without a finalizer in your implementation calling GC.SuppressFinalize(this); is pointless. As @mariozski pointed out a finalizer would help to ensure that Dispose is called at all if the class is not used inside a using block.

[John Kurtz]

I think you would want to use a Dispose method to detach any events that you hooked up inside the object.

[phougatv]

This is the generic pattern of implementing IDisposable. I would like to know how someone actually disposes of the object(new HttpClientAdaptor())? Is this the right way to do it: stackoverflow.com/questions/18336856/…

[BatteryBackupUnit]

Neither Microsoft nor your post clearly state why the pattern should look like this. Generally, it's not even boilerplate, it's just superfluous - superseded by SafeHandle (and sub types). In the case of managed resources implementing proper disposal becomes much simpler; you can trim the code down to a simple implementation of the void Dispose() method.

[MikeJ]

Microsoft has done a lot of great things. The dispose pattern isn't one of them. Their pattern was created to handle too many scenarios and as such has a lot of problems associated with it. There are better ways to handle dispose

[Papa Smurf]

That's usually the case. But on the other hand, the using construct opens the possibility of writing something akin to C++ smart pointers, namely an object that will restore the previous state no matter how a using block is exited. The only way I've found of doing this is making such an object implement IDisposable. It does seem I can ignore the compiler's warning in such a marginal use case.

[Murphybro2]

+1 for using the users code (even though it will be cleaned up automatically) to make it clear what goes there. Also, for not being a salty sailor and hammering him for making a small mistake whilst learning like many others on here.

[panky sharma]

@MattWilko Dispose will automatically called in any Managed resource who implemented IDesposable

[Servy]

@pankysharma No, it will not. It needs to be manually called. That's the whole point of it. You can't assume it will automatically be called, you only know people are supposed to manually call it, but people do make mistakes and forget.

[Mike Rosoft]

It's the contract of IDisposable that calling Dispose more than once is harmless (it must not corrupt the object or throw exceptions). Remembering that the object is disposed serves mainly so that methods which can't be used on a disposed object can throw ObjectDisposedException early (rather than when calling method on a contained object, or even causing an unexpected different exception).