System.InvalidCastException: “Не удалось привести тип объекта ”System.Windows.Controls.TextBox“ к типу ”System.IConvertible“.”

Здравствуйте! Помогите, пожалуйста, в чем ошибка? System.InvalidCastException: "Не удалось привести тип объекта "System.Windows.Controls.TextBox" к типу "System.IConvertible"."

Данное ошибка возникает при отрисовки прямоугольника

Код xaml.cs

private void Draw_rectangle(object sender, RoutedEventArgs e)
    {
        /*RectangleGeometry rectangle = new RectangleGeometry();
        rectangle.Rect = new Rect(1, 20, 120, 100);
        //координаты(x, y), width, height
        Path myPath = new Path();
        myPath.Fill = Brushes.LemonChiffon;
        myPath.Stroke = Brushes.Red;
        myPath.StrokeThickness = 2;
        myPath.Data = rectangle;
        t.Children.Add(myPath);*/
        RectangleGeometry rectangle = new RectangleGeometry();
        Window_Draw_rectangle dlg = new Window_Draw_rectangle();
        if(dlg.ShowDialog() == true)
        {
            rectangle.Rect = new Rect(Convert.ToInt32(dlg.x_begin),
                Convert.ToInt32(dlg.y_begin), Convert.ToInt32
                (dlg.width_textbox), Convert.ToInt32(dlg.height_textbox));
            Path myPath = new Path();
            myPath.Fill = Brushes.LemonChiffon;
            myPath.Stroke = Brushes.Red;
            myPath.StrokeThickness = 2;
            myPath.Data = rectangle;
            t.Children.Add(myPath);
        }
    }

Код xaml диалогового окна

<Grid>
    <Label 
        Content="Координаты: " 
        HorizontalAlignment="Left" 
        Height="25" 
        Margin="10,10,0,0" 
        VerticalAlignment="Top" 
        Width="90">
    </Label>
    <Label 
        Content="X: " 
        HorizontalAlignment="Left" 
        Height="25" 
        Margin="10,35,0,0" 
        VerticalAlignment="Top" 
        Width="25">
    </Label>
    <TextBox 
        HorizontalAlignment="Left"
        PreviewTextInput="TextBox_PreviewTextInput_x_begin" 
        PreviewKeyDown="TextBox_PreviewKeyDown_x_begin" 
        Height="20" 
        Name="x_begin"
        Margin="35,40,0,0" 
        TextWrapping="Wrap" 
        VerticalAlignment="Top" 
        Width="80">
    </TextBox>
    <Label 
        Content="Y: " 
        HorizontalAlignment="Left" 
        Height="25" 
        Margin="10,60,0,0" 
        VerticalAlignment="Top" 
        Width="25">
    </Label>
    <TextBox 
        HorizontalAlignment="Left" 
        PreviewTextInput="TextBox_PreviewTextInput_y_begin" 
        PreviewKeyDown="TextBox_PreviewKeyDown_y_begin" 
        Height="20" 
        x:Name="y_begin"
        Margin="35,65,0,0" 
        TextWrapping="Wrap" 
        VerticalAlignment="Top" 
        Width="80">
    </TextBox>
    <Label 
        Content="Размеры:" 
        HorizontalAlignment="Left" 
        Height="25" 
        Margin="133,10,0,0" 
        VerticalAlignment="Top" 
        Width="90">
    </Label>
    <Label 
        Content="Ширина: " 
        HorizontalAlignment="Left" 
        Height="25" 
        Margin="133,35,0,0" 
        VerticalAlignment="Top" 
        Width="65">
    </Label>
    <TextBox 
        HorizontalAlignment="Left" 
        PreviewTextInput="TextBox_PreviewTextInput_width_textbox" 
        PreviewKeyDown="TextBox_PreviewKeyDown_width_textbox" 
        Height="20" 
        x:Name="width_textbox"
        Margin="202,40,0,0" 
        TextWrapping="Wrap" 
        VerticalAlignment="Top" 
        Width="80">
    </TextBox>
    <Label 
        Content="Высота: " 
        HorizontalAlignment="Left" 
        Height="25" 
        Margin="133,60,0,0" 
        VerticalAlignment="Top" 
        Width="55">
    </Label>
    <TextBox 
        HorizontalAlignment="Left" 
        PreviewTextInput="TextBox_PreviewTextInput_height_textbox" 
        PreviewKeyDown="TextBox_PreviewKeyDown_height_textbox" 
        Height="20" 
        x:Name="height_textbox"
        Margin="202,65,0,0" 
        TextWrapping="Wrap" 
        VerticalAlignment="Top" 
        Width="80">
    </TextBox>
    <Button Name="okButton" 
            Click="okButton_Click" 
            IsDefault="True" 
            Margin="47,99,185,10"
            Content="OK">
    </Button>
    <Button x:Name="cancelButton" 
            IsCancel="True" 
            Content="Cancel" 
            Margin="150,99,65,10"
            Click="Cancel_click">
    </Button>
</Grid>