[英]How can I check if a class property is set in C#?
我目前正在為我的 C# 課程開發 BMI 計算器。 我的應用程序會詢問用戶以英尺和英寸為單位的身高和以磅為單位的體重，或者以厘米為單位的身高和以公斤為單位的體重。 它們是一個選項卡控件，具有用於英制模式或公制模式的按鈕。
如果單擊了英制選項卡中的“計算”按鈕，我需要確保已填寫 heightFt、heightIn 和 weightLbs 文本框。 然后我將它們轉換為公制單位。 如果單擊公制選項卡中的計算按鈕，我需要確保已填寫 heightCm 和 weightKg 文本框。
一旦提供了這些，我就會將身高和體重轉換為 BMI。 如果我使用if(heightFt == null)我得到以下錯誤：
The result of the expression is always 'false' since a value of type 'double' is never equal to 'null' of type 'double?'
我該如何解決？
這是我的 MainWindow.xaml.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace BMICalculator
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private BMICalc _BMICalc;
public MainWindow()
{
_BMICalc = new BMICalc();
InitializeComponent();
}
private void calculateImperial_Click(object sender, RoutedEventArgs e)
{
_BMICalc.heightFt = Convert.ToDouble(heightFt.Text);
_BMICalc.heightIn = Convert.ToDouble(heightIn.Text);
_BMICalc.weightLbs = Convert.ToDouble(weightLbs.Text);
_BMICalc.doCalculation("Imperial");
if (_BMICalc.error == null)
{
MessageBox.Show("Your BMI is " + _BMICalc.bmi + " and you are " + _BMICalc.category, "Your BMI");
}
}
}
}
這是 BMICalc.cs
using System;
using System.Windows;
using System.ComponentModel;
namespace BMICalculator
{
class BMICalc :INotifyPropertyChanged
{
public double _heightFt { get; set; }
public double _heightIn { get; set; }
public double _heightCm { get; set; }
public double _weightLbs { get; set; }
public double _weightKg { get; set; }
public double _bmi { get; set; }
public string _category { get; set; }
public string _error { get; set; }
public double heightFt
{
get { return heightFt; }
set
{
_heightFt = value;
OnPropertyChanged("heightFt");
}
}
public double heightIn
{
get { return _heightIn; }
set
{
_heightIn = value;
OnPropertyChanged("heightIn");
}
}
public double heightCm
{
get { return _heightCm; }
set
{
_heightCm = value;
OnPropertyChanged("heightCm");
}
}
public double weightLbs
{
get { return _weightLbs; }
set
{
_weightLbs = value;
OnPropertyChanged("weightLbs");
}
}
public double weightKg
{
get { return _weightKg; }
set
{
_weightKg = value;
OnPropertyChanged("weightKg");
}
}
public double bmi
{
get { return _bmi; }
set
{
_bmi = value;
OnPropertyChanged("bmi");
}
}
public string error
{
get { return _error; }
set
{
_error = value;
OnPropertyChanged("error");
}
}
public string category
{
get { return _category; }
set
{
_category = value;
OnPropertyChanged("category");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string propertyName){
if(PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public void doCalculation(string calculationMode){
if(calculationMode == "Imperial"){
if (heightFt == null)
{
this.error = "You must provide a value for your height in feet!";
}
else if (heightIn == null)
{
this.error = "You must provide a value for your height in inches!";
}
else if (weightLbs == null)
{
this.error = "You must provide a value for your weight in pounds!";
}
else
{
heightFt = Convert.ToDouble(heightFt);
heightIn = Convert.ToDouble(heightIn);
weightLbs = Convert.ToDouble(weightLbs);
_weightKg = Convert.ToDouble(_weightLbs * (1 / 2.2));
_heightCm = Convert.ToDouble((_heightFt + (_heightIn / 12) / 0.032808));
}
} else if(calculationMode == "Metric"){
this.bmi = weightKg / Math.Pow((heightCm / 100), 2);
if (this.bmi < 18.5)
{
this.category = "underweight";
}
else if (this.bmi >= 18.5 && this.bmi < 24.9)
{
this.category = "normalweight";
}
else if (this.bmi >= 25 && this.bmi <= 29.9)
{
this.category = "overweight";
}
else if (this.bmi > 30)
{
this.category = "obese";
}
}
}
}
}
}
您可以將double屬性設為可為空，以便與 null（默認值）進行比較。
例子：
public double? heightIn
{
get { return _heightIn; }
set {
_heightIn = value;
OnPropertyChanged("heightIn");
}
}
在這種情況下，請確保更改相應私有變量的類型以匹配。
您永遠不會為您的屬性設置默認值。 由於double不能為null您可能希望改為檢查 0。 以下划線開頭的屬性應該只是具有默認值的私有變量。 還可以考慮僅在內部使用指標。
class BMICalc :INotifyPropertyChanged
{
private double _heightFt = 0;
private double _heightIn = 0;
private double _heightCm = 0;
private double _weightLbs = 0;
private double _weightKg = 0;
private double _bmi = 0;
private string _category = null;
private string _error = null;
public double heightFt
{
get { return _heightFt; }
set
{
_heightFt = value;
OnPropertyChanged("heightFt");
}
}
public double heightIn
{
get { return _heightIn; }
set
{
_heightIn = value;
OnPropertyChanged("heightIn");
}
}
public double heightCm
{
get { return _heightCm; }
set
{
_heightCm = value;
OnPropertyChanged("heightCm");
}
}
public double weightLbs
{
get { return _weightLbs; }
set
{
_weightLbs = value;
OnPropertyChanged("weightLbs");
}
}
public double weightKg
{
get { return _weightKg; }
set
{
_weightKg = value;
OnPropertyChanged("weightKg");
}
}
public double bmi
{
get { return _bmi; }
set
{
_bmi = value;
OnPropertyChanged("bmi");
}
}
public string error
{
get { return _error; }
set
{
_error = value;
OnPropertyChanged("error");
}
}
public string category
{
get { return _category; }
set
{
_category = value;
OnPropertyChanged("category");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string propertyName){
if(PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public void doCalculation(string calculationMode){
if(calculationMode == "Imperial"){
if (heightFt == null)
{
this.error = "You must provide a value for your height in feet!";
}
else if (heightIn == null)
{
this.error = "You must provide a value for your height in inches!";
}
else if (weightLbs == null)
{
this.error = "You must provide a value for your weight in pounds!";
}
else
{
heightFt = Convert.ToDouble(heightFt);
heightIn = Convert.ToDouble(heightIn);
weightLbs = Convert.ToDouble(weightLbs);
_weightKg = Convert.ToDouble(_weightLbs * (1 / 2.2));
_heightCm = Convert.ToDouble((_heightFt + (_heightIn / 12) / 0.032808));
}
} else if(calculationMode == "Metric"){
this.bmi = weightKg / Math.Pow((heightCm / 100), 2);
if (this.bmi < 18.5)
{
this.category = "underweight";
}
else if (this.bmi >= 18.5 && this.bmi < 24.9)
{
this.category = "normalweight";
}
else if (this.bmi >= 25 && this.bmi <= 29.9)
{
this.category = "overweight";
}
else if (this.bmi > 30)
{
this.category = "obese";
}
}
}
}
}
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