class NumMultiConvert : IMultiValueConverter
    {

        #region Field 字段



        #endregion

        #region Constructor 构造函数



        #endregion

        #region Property 属性



        #endregion

        #region Event 事件



        #endregion

        #region Method 方法



        #endregion
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            return values.Where(i => i != null).Select(i => System.Convert.ToDouble(i)).Sum().ToString();
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

<StackPanel Orientation="Vertical">
            <TextBox Name="tb1"
                     Width="100"
                     FontSize="30"
                     Text="25" />
            <TextBox Name="tb2"
                     Width="100"
                     FontSize="30"
                     Text="75" />
            <TextBox Name="tb3"
                     Width="100"
                     FontSize="30"
                     Text="55" />
            <TextBox Name="tb4"
                     Width="100"
                     FontSize="30"
                     Text="10" />
            <TextBlock Width="100" FontSize="30">
                <TextBlock.Text>
                    <MultiBinding Converter="{StaticResource NumMultiConvert}">
                        <Binding ElementName="tb1" Path="Text" />
                        <Binding ElementName="tb2" Path="Text" />
                        <Binding ElementName="tb3" Path="Text" />
                        <Binding ElementName="tb4" Path="Text" />
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
        </StackPanel>