Class Node

public abstract class Node : INode
{
    public virtual void DoSomething()
    {
    }
    protected int _someField;
    private string _anotherField;
    public Node(string anotherField)
    {
    }
}

protected Node Node(NodeType nodeType)
   {
       this.NodeType = nodeType;
       this.ChildNodes = new List<Node>();
       this.ParentNode = null;
   }

public Document Document
   {
      get
      {
         return _document;
      }
   }
In this case, I've added braces to enclose the getter and made sure that there is a consistent spacing between properties, methods, braces, and operators. This is in line with standard C# conventions for readability and indentation.

public virtual bool IsComposite
{
    get;
}

public INode NextSibling
   {
      get
      {
      }
   }

public NodeType
    {
        get
        {
            return _nodeType;
        }
    }
    private readonly NodeType _nodeType;

public ICompositeNode ParentNode
   {
      get;
   }

public INode PreviousSibling
   {
      get
      {
         return _next?.Previous;
      }
   }

public abstract void Accept(DocumentVisitor visitor)
   {
   }