I get Null Object in MVVM Model with silverlight two way binding

I'm new to silverlight and trying to save a form to the database via RIA Services using MVVM Pattern.

I get a textbox value in ViewModel when I bind a textbox to a string in twoway binding mode.

B开发者_JS百科ut When I bind a Object.Property to the textbox (Twoway binding) I get a null object in the ViewModel after I click on the save button.

Here is my code, please help me figure out where I am going wrong.

private tblSchool _school;
public tblSchool thisschool
    {
        get 
        {
            return _school;
        }

        set 
        {
            if (_school != value)
            {
                _school = value;
                OnPropertyChanged("thisschool");
            }
        }
    }

    private void SaveSchool()
    {

        DomainServiceForDatabaseData service = new DomainServiceForDatabaseData();
        service.tblSchools.Add(thisschool); //HERE I GET NULL VALUE
        service.SubmitChanges();
    }

Here is my XAML:

<Grid x:Name="LayoutRoot"
      DataContext="{Binding Source={StaticResource SignUpViewModel}}">
    <TextBox Height="23"
             HorizontalAlignment="Right"
             Margin="0,55,160,0"
             Name="textBox1"
             VerticalAlignment="Top"
             Width="213"
             Text="{Binding Path= thisschool.School_Name, Mode=TwoWay}" />

The backing field _school doesn't get initialized in your code sample.

Somewhere you will need to do _school = new tblSchool() or it will stay null forever.