初步完成单个变量的历史记录查询

DMS.WPF/Converters/NullToVisibilityConverter.cs

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+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace DMS.WPF.Converters
+{
+    /// <summary>
+    /// Null值到可见性转换器。当绑定的值不为null时，返回Visible，否则返回Collapsed。
+    /// </summary>
+    public class NullToVisibilityConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            // 如果值不为null，则可见
+            return value != null ? Visibility.Visible : Visibility.Collapsed;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}