How to dispose managed resource in Dispose() method in C#?

Question

Hi,

I know Dispose() is intended for unmanaged resource, and the resource should be disposed when it is no longer needed without waiting for the garbage collector to finalize the object.

However, when disposing the object, it suppress finalization of the garbage collector (GC.SuppressFinalize(this); in the code below). This means that if the object includes managed resource, we will have to take care of that too because garbage collector will not clean this up.

In the example code below (from MSDN), "Component" is a managed resource, and we call dispose() for this resource (component.Dispose()). My question is, how do we implement this method for Component class which is managed resource? Should we use something like Collect() to poke garbage collector to clean this portion?

Any idea would be appreciated. Thanks.

Below is the code I'm looking at which is from MSDN:

using System;
using System.ComponentModel;

// The following example demonstrates how to create
// a resource class that implements the IDisposable interface
// and the IDisposable.Dispose method.

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.
    private 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.
}
}

Answer 1

Sorry if I've misunderstood your question!!, but if your class just has referenced to other managed classes, and those references to those objects don't need disposing then your class does not necessarily nee to implement IDisposable.

Answer 2

This means that if the object includes managed resource, we will have to take care of that too because garbage collector will not clean this up.

That is false. The garbage collector will still clean up your managed resources. Finalizers are also strictly for cleaning up unmanaged resources, and so the SuppressFinalize() call won't hurt you.

And since you are new to the IDisposable pattern I will anticipate your next point of confusion: writing finalizers. In C#, you should only write a finalizer when dealing with a completely new kind of unmanaged resource. So if you have, for example, a class that wraps the System.Data.SqlClient.SqlConnection type as part of a data access layer, you should not write a finalizer for that type because you're still dealing with the same kind of underlying unmanaged resource: sql server database connections. The finalizer for that resource is taken care of already by the base SqlConnection type.

On the other hand, if you're building an ADO.Net provider for a completely new kind of database engine you would need to implement a finalizer in your connection class because that has never been done before.

Answer 3

Maybe a little clearer. GC.SuppressFinalize(this) only affects the object referenced by the this pointer but not to any members of the object. That is to say that SuppressFinalize does not recursively apply to the object's members. When the Garbage Collector reclaims the memory for the Disposed object, it is likely that there will be no active references to the objects fields. Since you did not call GC.SuppressFinalize on all the fields of the object, then the Garbage Collector will call the finalize method on these objects if they exist. When it does this is completely up to the runtime and in general you should just let it do its thing.

Answer 4

That disposable pattern is confusing. Here's a better way to implement it:

Step 1. Create a disposable class to encapsulate each unmanaged resource that you have. This should be really rare, most people don't have unmanaged resources to clean up. This class only cares (pdf) about its unmanaged resource, and should have a finalizer. The implementation looks like this:

public class NativeDisposable : IDisposable {

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

  protected virtual void CleanUpNativeResource() {
    // ...
  }

  ~NativeDisposable() {
    CleanUpNativeResource();
  }

  // ...

  IntPtr _nativeResource;

}

Step 2. Create a disposable class when the class holds other disposable classes. That's simple to implement, you don't need a finalizer for it. In your Dispose method, just call Dispose on the other disposables. You don't care about unmanaged resources in this case:

public class ManagedDisposable : IDisposable {

  // ...

  public virtual void Dispose() {
    _otherDisposable.Dispose();
  }

  IDisposable _otherDisposable;

}

The "Component" in the example would be either one of these, depending on whether it encapsulates an unmanaged resource or if it is just composed of other disposable resources.

Also note that supressing finalization does not mean you're suppressing the garbage collector from cleaning up your instance; it just means that when the garbage collector runs in your instance, it will not call the finalizer that's defined for it.