Why calling Dispose() on BinaryReader results in compile error?

Question

I have a following class witch uses BinaryReader internally and implements IDisposable.

class DisposableClass : IDisposable
    {
        private BinaryReader reader;
        public DisposableClass(Stream stream)
        {
            reader = new BinaryReader(stream);
        }

        protected virtual void Dispose(bool disposing)
        {
            if (disposing)
            {
                ((IDisposable)reader).Dispose();
//                reader.Dispose();// this won't compile
            }
        }

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

I have already figured out that I need to cast BinaryReader to IDisposable to be able to call Dispose on it, but I don't understand why I can't just call the Dispose() method directly without casting to IDisposable?

Answer 1

It won't work because the Dispose method on BinaryReader has been explicitly implemented.

Instead of being implicitly implemented, as in:

public void Dispose()
{
}

...it has been explicitly implemented, as in:

void IDisposable.Dispose()
{
}

...which means it can only be accessed via the IDisposable interface. Therefore, you have to cast the instance to IDisposable first.

Answer 2

Expanding on my comments here, the BinaryReader class does not properly implement the Dispose pattern.

Looking at this class in Reflector, it looks like this (for .NET 3.5):

public class BinaryReader : IDisposable
{
    public virtual void Close()
    {
       this.Dispose(true);
    }
    protected virtual void Dispose(bool disposing)
    {
       if (disposing)
       {
          Stream stream = this.m_stream;
          this.m_stream = null;
          if (stream != null)
          {
             stream.Close();
          }
       }
       this.m_stream = null;
       this.m_buffer = null;
       this.m_decoder = null;
       this.m_charBytes = null;
       this.m_singleChar = null;
       this.m_charBuffer = null;
   }
   void IDisposable.Dispose()
   {
      this.Dispose(true);
   }
}

The problem here is that by making IDisposable.Dispose() an explicit interface implementaiton it forces a developer to call Close() instead of Dispose().

In this context, we have a case of imbalanced semantics. There was never a call to "Open" the reader so it is not intuitive to "Close" the reader.

Going one step further, in order to call Dispose() you must then explicitly cast to IDisposable, which is not something you ordinarily need to do. You do have the option of calling Dispose(bool) directly, but how do you know what the boolean parameter should be?

To properly follow the pattern, it should have been implmented as:

public class BinaryReader : IDisposable
{
    public virtual void Close()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }
    protected virtual void Dispose(bool disposing)
    {
       if (disposing)
       {
          Stream stream = this.m_stream;
          this.m_stream = null;
          if (stream != null)
          {
             stream.Close();
          }
       }
       this.m_stream = null;
       this.m_buffer = null;
       this.m_decoder = null;
       this.m_charBytes = null;
       this.m_singleChar = null;
       this.m_charBuffer = null;
   }
   public void Dispose()
   {
      this.Close();
   }
}

This would allow you to call either Close() or Dispose(), in which case either call continues to result in calling Dispose(true). (This is the same flow as the actual implementation by calling Close() or ((IDisposable)reader).Dispose()).

Fortunately (or unfortunately, depending on which way you choose to look at it) because BinaryReader does implement the IDisposable interface it is allowed in a using statement:

using (BinaryReader reader = new BinaryReader(...))
{
}

Answer 3

Actually they have chosen to use Close() instead of Dispose() Dispose has been explicitly implemented. Which is why you can't see it.

However Close does the same thing as dispose and this is the method they want you to use. Reflector gives the following disassembly for the Close method

public virtual void Close()
{
    this.Dispose(true);
}

Close() is used because it is a better choice of words in the context of a binary reader.