C#

public void AddImageRequestToQueue(string isbn)
{
    Contract.Requires(!string.IsNullOrEmpty(isbn));

    lock (_queuePadLock)
    {
        _imageRequests.Enqueue(isbn);
    }
}

private void ProcessImageQueue()
{
    do
    {
        if (_imageRequests.Count > 0)
        {
            string isbn = string.Empty;
            lock (_queuePadLock)
            {
                isbn = _imageRequests.Dequeue();
            }

            IImageResult imageResult = GetImage(isbn, ImageSize.Small);

            OnProcessImageCompleted(new ProcessImageCompletedEventArgs(imageResult.Image, imageResult.ISBN));

            Thread.Sleep(SleepTime);
        }
    }
    while (true);
}

public partial class frmMain : Form
    {
    ...
    }

public partial class frmMain : Form, ICloneable, IComparable, IDisposable
    {
    ...
    }

Data obj = new Data();
ICheck check = obj as ICheck;
if (check != null)
  check.Fun();

ICheck obj = MyIocContainer.Resolve(/* some reference to the Data class);
obj.Fun();

Pete O'Hanlon wrote:

Now, if Data doesn't implement this interface, the code still compiles and behaves as you'd expect.

public class Data : IData {
 private IDataChecker mDataChecker;

 //...

 public Data(IDataChecker pDataChecker) {
  mDataChecker = pDataChecker;
 }

 //...
}