对象被实例化但仍然收到警告:“字段'x'永远不会分配给....”

时间:2017-05-03 07:08:57

标签: c# class visual-studio-2017

描述

我想我声明并实例化了这个类:_teamsVM,但我不断得到警告,引用:

  

字段'Sta​​rtpageVM2._teamsVM'永远不会分配给,并且将永远分配   将其默认值设为null。

同样:在运行应用程序时,它确实给了我一个错误,这个类没有实例化(证明VS2017在某种程度上是正确的)。

问题 我错过了什么?

环境:

  • Microsoft Visual Studio 2017
  • 项目类型:通用Windows

代码

以下是涉及的4个类的代码。

基类viewmodel:

using System.ComponentModel;

namespace MyApp.ViewModels
{
    public class BaseViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

    }
}

主Viewmodel(包含警告):

using System.Collections.ObjectModel;
using MyApp.Models;

namespace MyApp.ViewModels
{

    public class StartPageVM2 : BaseViewModel
    {
        #region Declarations
        private TeamsVM2 _teamsVM;
        public ObservableCollection<Team2> Teams {
            get { return _teamsVM.Teams; }
            set { _teamsVM.Teams = value; }
        }
        #endregion Declarations

        #region Constructor
        public StartPageVM2()
        {
            TeamsVM2 _teamsVM = new TeamsVM2();
        }
        #endregion
    }
}

子视图模型:

using MyApp.Models;
using System.Collections.ObjectModel;

namespace MyApp.ViewModels
{
    public class TeamsVM2 : BaseViewModel
    {
        private ObservableCollection<Team2> _teams;
        public ObservableCollection<Team2> Teams {
            get { return _teams; }
            set { _teams = value; }
        }

        public TeamsVM2()
        {
            ObservableCollection<Team2> _teams = new ObservableCollection<Team2>();
        }
    }
}

使用的Model类:

namespace MyApp.Models
{
    public class Team2
    {
        private string _sTeamName;
        public string TeamName {
            get { return _sTeamName; }
            set { _sTeamName = value; }
        }

        public Team2() { }
        public Team2(string sTeamName)
        {
            _sTeamName = sTeamName;
        }
    }
}

1 个答案:

答案 0 :(得分:3)

在构造函数中,您永远不会分配成员变量,而是在构造函数的范围内创建一个新变量:

#region Constructor
public StartPageVM2()
{
    TeamsVM2 _teamsVM = new TeamsVM2();
}
#endregion

注意范围。那时_teamsVM != this._teamsVM

请改为关注:

#region Constructor
public StartPageVM2()
{
    _teamsVM = new TeamsVM2(); //or write  this._teamsVM = new TeamsVM2(); to highlight the scope
}
#endregion