NodeEx.cs

public interface INode
{
    double Eval(Dictionary<char, double> variablesValue);
}

public class AddNode : INode
{
    private readonly INode _left;
    private readonly INode _right;

    public AddNode(INode left, INode right)
    {
        _left = left;
        _right = right;
    }

    public double Eval(Dictionary<char, double> variablesValue) => _left.Eval(variablesValue) + _right.Eval(variablesValue);

    public override string ToString()
    {
        return $"Add({_left},{_right})";
    }
}