WPF中用MVVM绑定ContentControl界面无变化

在WPF中使用MVVM绑定ContentControl，调试的时候Query和Refund方法都是进的去的，问题就在
ViewModel = _vMQueryPayListCtrl这些东西都无效，界面没有变化，仔细对比了别人的demo，也没找到问题。
VMMainWindow中继承了INotifyPropertyChanged，由于代码较多，只贴了主要部分代码。
xaml部分

 <Window.Resources>

        <v:VMMainWindow x:Key="MainViewModel" />

        <DataTemplate DataType="{x:Type v:VMNumberArea}">

            <c:NumberArea />

        </DataTemplate>

        <DataTemplate DataType="{x:Type v:VMQueryPayListCtrl}">

            <c:QueryPayListCtrl/>

        </DataTemplate>

        <DataTemplate DataType="{x:Type v:VMRefundCtrl}">

            <c:RefundCtrl/>

        </DataTemplate>

        

    </Window.Resources>

    <Grid DataContext="{StaticResource MainViewModel}">

 <ContentControl x:Name="workArea" Background="White" Content="{Binding ViewModel}"  HorizontalAlignment="Left" Height="{Binding Wheight}"  Width="850" Grid.Row="2" VerticalAlignment="Top"/>

        <UserCtrl:ShadeCtrl  Tag="查询" HorizontalAlignment="Left" Grid.Row="0" VerticalAlignment="Top" Height="85" Margin="131,0,0,0" Width="58">

            <i:Interaction.Triggers>

                <i:EventTrigger EventName="MouseLeftButtonUp">

                    <i:InvokeCommandAction Command="{Binding TextBlockCommand}"  CommandParameter="查询"  />

                </i:EventTrigger>

            </i:Interaction.Triggers>

        </UserCtrl:ShadeCtrl>

 <UserCtrl:ShadeCtrl  Tag="退款" HorizontalAlignment="Left" Grid.Row="0" VerticalAlignment="Top" Height="85" Margin="63,0,0,0" Width="68" >

            <i:Interaction.Triggers>

                <i:EventTrigger EventName="MouseLeftButtonUp">

                    <i:InvokeCommandAction Command="{Binding TextBlockCommand}"  CommandParameter="退款"  />

                </i:EventTrigger>

            </i:Interaction.Triggers>

        </UserCtrl:ShadeCtrl>

VM部分

 

   private readonly VMNumberArea _vMNumberArea = new VMNumberArea();

        private readonly VMQueryPayListCtrl _vMQueryPayListCtrl = new VMQueryPayListCtrl();

        private readonly VMRefundCtrl _vMRefundCtrl = new VMRefundCtrl();

     private object _viewModel;



        public object ViewModel

        {

            get { return _viewModel; }

            set

            {

                if (_viewModel == value)

                {

                    return;

                }

                _viewModel = value;

                OnPropertyChanged("ViewModel");

            }

        }

        public DelegateCommand TextBlockCommand { get; set; }



 public VMMainWindow()

        {

            this.TextBlockCommand = new DelegateCommand();

            this.TextBlockCommand.ExecuteAction = new Action<object>(ShowTextBlock);

        }



        private void ShowTextBlock(object obj)

        {

            switch (obj.ToString())

            {

                case "收款":

                    Receivables();

                    break;

                case "退款":

                    Refund();

                    break;

                case "查询":

                    Query();

                    break;

                case "统计":

                    Statistics();

                    break;

                case "解绑":

                    Untying();

                    break;

                case "设置":

                    Setup();

                    break;

                case "账户":

                    Account();

                    break;

                case "退出":

                default:

                    Closed?.Invoke();

                    break;

            }

        }

   private void Query()

        {

            Height = new GridLength(0);

            Wheight = 475;

            ViewModel = _vMQueryPayListCtrl;

        }



        private void Refund()

        {

            ViewModel = new VMRefundCtrl();

        }