C# Using Equals method in a generic list fails

问题

I have a project where I have class State which uses templates. I have a class Cell, and I use it as State, so State holds a Cell as genericState. Now I have a generic function which checks if two instances are equal. Problem is, it never leaves the State Equals method to Cell Equals method.

public class State<T>
{
    public T genericState;  //in my case T is a cell
    public State(T cellState) // CTOR
    {
        this.genericState = cellState;  
    }

    public override bool Equals(object obj)
    {            
        return genericState.Equals((obj as State<T>).genericState); 
    } //never leaves
}

and code of Class Cell, in which it never gets:

public class Cell
{
    public int row, col;
    public bool visited;
    public char value;
    public bool Equals(Cell other)   //never gets here
    {            
       return other != null && other.row == row && other.col == col;    
    }
 }

I don't understand why it never gets to Equal method of Cell. What could be wrong with the code?

回答1:

The problem is that your code does not know that T has a special method

bool Equals<T>(T other)

It thinks that it should be calling Cell's override of Equals(object), which your code does not override.

Fixing this is simple: add IEquatable<Cell> to the list of interfaces implemented by Cell, and add a constraint on T to ensure that it implements IEquatable<T>:

public class State<T> where T : IEquatable<T> {
    ... // The rest of the code remains the same
}
...
public class Cell : IEquatable<Cell> {
    ... // The rest of the code remains the same
}

回答2:

Firstly, you should override object.Equals to defer to this Equals:

public override bool Equals(object obj) => Equals(obj as Cell);

And if you are overriding object.Equals then you also need to override object.GetHashCode():

public override int GetHashCode() => row * 31 + col;

Further to this, for better performance you could have Cell implement IEquatable<Cell>, and in your State<T>.Equals use EqualityComparer<T>.Default. This will be better because EqualityComparer<T>.Default uses IEquatable<T>.Equals(T) when implemented, falling back to object.Equals(object) when that isn't available.

回答3:

genericState.Equalsis the equality method derived from object. In your Cell class, you are not overriding object.Equals(object), so your equality method (Cell.Equals(Cell)) is not being called, but instead, Cell.Equals(object) is being called. So what you can do to fix this is change your Cell class to override the default equality method:

public class Cell
{
    public int row, col;
    public bool visited;
    public char value;
    public override bool Equals(object other)   //never gets here
    {            
       if(!(other is Cell)) return false;
       return other != null && other.row == row && other.col == col;    
    }
 }

来源：https://stackoverflow.com/questions/36235271/c-sharp-using-equals-method-in-a-generic-list-fails

标签

templates

generics

equals