Set null to property in mvvm WPF when nothing is f

问题:

I implemented a textbox, and this binds to a viewmodel. I fill in the textbox default by '100' for example, but if i change this to 10, the property always sets correctly. But when i delete all the numbers, i want that the property is set to null. But it just don't set the value when nothing is filled in. He just keeps the last value..

this is my code , viewmodel + xaml:

public double MaxTime
{
    get
    {
        return maxTime;
    }
    set
    {
        maxTime = value;
        OnPropertyChanged("MaxTime");
        if (SelectedQuestionDropList != null)
        {
            foreach (QuestionCluster cluster in this.Examination.QuestionClusters)
            {
                if (cluster == SelectedQuestionDropList)
                {
                    cluster.MaxTime = value;
                }
            }
        }
    }
}


<TextBox Height="23" 
         Visibility="{Binding Path=VisibleClusterDetails, Converter={StaticResource boolToVis},UpdateSourceTrigger=PropertyChanged}"  
         Text="{Binding Path=MaxTime,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}"
         HorizontalAlignment="Right" Margin="0,511,601,0" Name="textBox2" 
         VerticalAlignment="Top" Width="120" /> 

回答1:

Your property is a double so it won't support null or empty values.

Instead of using a double here I would expose the property as a string and convert between string and double in my property getter and setter. In the setter I could handle null or empty values as I wanted, and I could easily handle any special formatting that was needed.

回答2:

This is a binding issue you have to do three things

1) import the System namespace into your xaml file(notice the sys keyword):

xmlns:sys="clr-namespace:System;assembly=mscorlib"

2) Then in your binding you need to tell it to send null when the value in the textbox is string.Empty(which is the value when there is nothing in the textbox which is why it won't work because string.Empty can't convert to a double). By doing this:

<TextBox Text={Binding MaxTime,TargetNullValue={x:Static sys:String.Empty}}/>

3) Change your ViewModel MaxTime property to be of type double?

回答3:

The property can't be set to null, because double is non-nullable. Try using double? instead.