NodeVariable.cs

// Expression context
public interface IContext
{
    double ResolveVariable(string name);
}

// Represents a variable (or a constant) in an expression.  eg: "2 * pi"
public class NodeVariable : Node
{
    public NodeVariable(string variableName)
    {
        _variableName = variableName;
    }

    string _variableName;

    public override double Eval(IContext ctx)
    {
        return ctx.ResolveVariable(_variableName);
    }
}