属性变为空C＃WPF

Question

I am facing the issue with the property becoming null when I set the data,

SelectedItem="{Binding PropertyName,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"

public className PropertyName
{
get{return this._PropertyName;}
set{this._PropertyName = value;}
}

The problem is when I assign the data in view model

PropertyName = new className ();

The "value" in the setter is null when I debug and check it

Answer 1

The problem is by the Mode=two-way which states that any changes which are done on UI will be reflected to the property being binded and vice-versa but since you have not added OnPropertyChanged event of INotifyPropertyChanged interface to the properties bound to UI , you face the problem. Just add PropertyChanged to all the properties which are binded to UI,

public className PropertyName
{
   get{return this._PropertyName;}
   set
   {
      this._PropertyName = value;
      OnPropertyChanged("PropertyName");
   }
}

#region INotifyPropertyChanged Members

public event PropertyChangedEventHandler PropertyChanged;

        /// <summary>
        /// Raises this object's PropertyChanged event.
        /// </summary>
        /// <param name="propertyName">The property that has a new value.</param>
        protected virtual void OnPropertyChanged(string propertyName)
        {    
            PropertyChangedEventHandler handler = this.PropertyChanged;
            if (handler != null)
            {
                var e = new PropertyChangedEventArgs(propertyName);
                handler(this, e);
            }
        }

        #endregion

Answer 2

Your class should implement INotifyPropertyChanged interface

public className PropertyName
{
    get { return this._PropertyName; }
    set 
    {
       this._PropertyName = value; 
       if(PropertyChanged != null)
       {
          PropertyChanged(this, new PropertyChangedEventArgs("PropertyName"));
       }
    }
}

When you changed value of your property in code UI don't know about it and reflect null value back to your property.