C#中双问号的意思

Question

从Codeplex下了一个代码，其中出异常的代码，出现这样的操作符。本人比较菜，希望高手指点下。代码如下， public static T Clone<T>(T source, Dictionary<Type, InstantiationHandler> typeInstantiationHandlers, List<PropertyInfo> ignoredProperties)
    {
      if (source == null)
        throw new ArgumentNullException("source");      ignoredProperties = ignoredProperties ?? new List<PropertyInfo>();
      typeInstantiationHandlers = typeInstantiationHandlers ?? new Dictionary<Type, InstantiationHandler>();
      Type sourceType = source.GetType();      T cloned;
      InstantiationHandler handler;
      if (typeInstantiationHandlers.TryGetValue(sourceType, out handler))
        cloned = (T)handler(source);
      else
        cloned = (T)Activator.CreateInstance(sourceType);      foreach (PropertyInfo curPropInfo in sourceType.GetProperties())
      {
        if (curPropInfo.GetGetMethod() != null
            && (curPropInfo.GetSetMethod() != null))
        {
          if (!ignoredProperties.Contains(curPropInfo))
          {
            // Handle Non-indexer properties
            if (curPropInfo.Name != "Item")
            {
              // get property from source
              object getValue = curPropInfo.GetGetMethod().Invoke(source, new object[] { });
              object clonedValue = curPropInfo.GetGetMethod().Invoke(cloned, new object[] { });              if (getValue != clonedValue)
              {
                // clone if needed
                if (getValue != null && getValue is DependencyObject)
                  getValue = Clone((DependencyObject)getValue, typeInstantiationHandlers);                // set property on cloned
                curPropInfo.GetSetMethod().Invoke(cloned, new object[] { getValue });
              }
            }
            // handle indexer
            else
            {
              // get count for indexer
              int numberofItemInColleciton =
                  (int)
                  curPropInfo.ReflectedType.GetProperty("Count").GetGetMethod().Invoke(source, new object[] { });              // run on indexer
              for (int i = 0; i < numberofItemInColleciton; i++)
              {
                // get item through Indexer
                object getValue = curPropInfo.GetGetMethod().Invoke(source, new object[] { i });                // clone if needed
                if (getValue != null && getValue is DependencyObject)
                  getValue = Clone((DependencyObject)getValue, typeInstantiationHandlers);
                // add item to collection
                curPropInfo.ReflectedType.GetMethod("Add").Invoke(cloned, new object[] { getValue });
              }
            }
          }
        }
      }      return cloned;
    }
先谢谢了

Answer 1

 ignoredProperties = ignoredProperties ?? new List<PropertyInfo>();在第五行。

Answer 2

int？a；//声明可空int a
a=null;//可空类型可以为null
int b=a??0;//判断a是否为null，如果为null则b=0，如果不为null则b等于a.value;

Answer 3

?? 
判断??左边表达式是否为null，如是null则取右边表达式的值，否则就取左边表达式的值

Answer 4

从楼上学习
            int? a;            a = null;
            a = 10;            int b = a ?? 0; //返回b=10