public CalculatorViewModel()
        {
            this.calculation = new CalculatorModel();
            this.result = "0";
            this.FirstOperand = string.Empty;
            this.SecondOperand = string.Empty;
            this.Operation = string.Empty;

            calculateCommand = new CalculateCommand(this);
        }
        public CalculatorViewModel()
        {
            this.calculation   = new CalculatorModel();
            this.result        = "0";
            this.FirstOperand  = string.Empty;
            this.SecondOperand = string.Empty;
            this.Operation     = string.Empty;

            calculateCommand = new CalculateCommand(this);
        }