如何在非静态类中使用静态方法

我想在非静态类中使用此方法：

public class LogicalExpressionCalculator : MeasurementInterpreter
{
    private static Boolean Operator(this string logic, string x, string y)
    {
        switch (logic)
        {
            case "==": return x == y;
            case "!=": return x != y;
            case ">": return Int32.Parse(x) > Int32.Parse(y);
            case ">=": return Int32.Parse(x) >= Int32.Parse(y);
            case "<": return Int32.Parse(x) < Int32.Parse(y);
            case "<=": return Int32.Parse(x) <= Int32.Parse(y);
            case "&&": return bool.Parse(x) && bool.Parse(y);
            case "||": return bool.Parse(x) || bool.Parse(y);
            default: throw new Exception("invalid logic");
        }
    }
 }

如何将此方法更改为在非静态类中工作？

我想在此方法中的类内部使用的方法：

protected void CalculateExpression()
{
    Stack variableStack = new Stack();
    Stack operatorStack = new Stack();
    foreach (string field in contextString)
    {
        switch (field)
        {
            case "(":
            case "<":
            case ">":
            case "==":
                operatorStack.Push(field);
                break;
            case ")":
                string operatorPop = null;
                while (operatorPop == "(")
                {
                    operatorPop = operatorStack.Pop().ToString();
                    resultStack = Operator(operatorPop, variableStack.Pop().ToString(), variableStack.Pop().ToString());
                    variableStack.Push(resultStack);
                }
                break;
            case "UserInput":
            case "EngineRpm":
            case "VehicleSpeed":
                variableStack.Push((string)hashtable[field]);
                break;
            default:
                int integer;
                if(Int32.TryParse(field, out integer)) variableStack.Push(integer);
                if (bool.Parse(field)) variableStack.Push(field);
                break;
        }
    }
    Result = resultStack;
    if (Result)
    {
    }
}

我收到错误

扩展方法必须是静态的

private static Boolean Operator(string logic, string x, string y)
{
    switch (logic)
    {
        case "==": return x == y;
        case "!=": return x != y;
        case ">": return Int32.Parse(x) > Int32.Parse(y);
        case ">=": return Int32.Parse(x) >= Int32.Parse(y);
        case "<": return Int32.Parse(x) < Int32.Parse(y);
        case "<=": return Int32.Parse(x) <= Int32.Parse(y);
        case "&&": return bool.Parse(x) && bool.Parse(y);
        case "||": return bool.Parse(x) || bool.Parse(y);
        default: throw new Exception("invalid logic");
    }
}