如何调用空引用对象的属性

Question

Consider the following class which contains a property Height

internal class TreeNode
{
  public int value;
  public TreeNode Left;
  public TreeNode Right;

  public int Height{get;private set}
}

a TreeNode type variable node is initialized (ie node==null returns false) and TreeNode.Left =null

Trying node.Left.Height will throw a NullRefrenceException, Is there any direct way so that i can access the property Height from a null reference object so that node.Left.Height returns -1.

I know one method is to use a function with a signature int getHeight(TreeNode node) where i can check the node for nullity. but i want to use node.Left.Height kind of expression to return the desired value.

Answer 1

You're asking how to get the height of an undefined object? That is simply not possible. Like you can't measure the height of an building if it is not there.

Stick to the method based approach. Or create a property in your class like this:

public int LeftHeight 
{ 
    get 
    { 
        return Left == null ? -1 : Left.Height;
    }
}

Answer 2

You could implement properties for Left and Right that never return null.

private TreeNode _left;
private TreeNode _right;
public TreeNode Left
{
    get
    {
        return _left ?? new TreeNode() { Height = -1 };
    }
    set
    {
        _left = value;
    }
}
public TreeNode Right
{
    get
    {
        return _right ?? new TreeNode() { Height = -1 };
    }
    set
    {
        _right = value;
    }
}

Answer 3

Actually when u are dealing with c# 6 there is a null propagation operator.

Sample:

int? length = text?.Length;

Null Propagation OPerator

Answer 4

Some people have made similar suggestions, but you could do something like

text?.Length ?? -1

If text is null it'll give the default value -1. There is no way to "directly" do text.Length though - that wouldn't make sense because there is no object to get the length of .

As an analogy, if I were to ask you what the height of the unicorn living in Milwaukee, WI is, how would you answer? Clearly, there is no unicorn in Milwaukee, WI to get the height of, so there's no way to give a meaningful answer other than "there is no such unicorn."