已解决

/// <summary>
/// 将部分需要的属性值转json
/// </summary>
/// <typeparam name="T">要转化的类</typeparam>
/// <param name="t">要转化的实例</param>
/// <param name="jsonString">要转化的属性值</param>
/// <returns></returns>
public static string JsonSerializerListIgnoredNull<T>(List<T> t,string jsonString)
where T:new()
{
return JsonConvert.SerializeObject(TemporaryClass.GetObjectListUsefulProperties(t, jsonString), Formatting.Indented,
new JsonSerializerSettings {DefaultValueHandling = DefaultValueHandling.Ignore});
}

public static string JsonSerializerListIgnoredNull<T>(T t, string jsonString)
where T : new()
{
return JsonConvert.SerializeObject(TemporaryClass.GetObjectUsefulProperties(t, jsonString), Formatting.Indented,
new JsonSerializerSettings { DefaultValueHandling = DefaultValueHandling.Ignore });
}

/// <summary>
/// 获取一个类的需要的属性值
/// </summary>
/// <typeparam name="T">要处理的类型</typeparam>
/// <param name="obj"></param>
/// <param name="propertiesStr">使用时用'_'分开需要的属性值的各个属性名</param>
/// <returns></returns>
public static T GetObjectUsefulProperties<T>(T obj, string propertiesStr) where T : new()
{
string[] properties = propertiesStr.Split('_');
T returnObj = new T();

foreach (var propertyName in properties)
{
//将obj中的propertyName这个属性赋值到returnObj上;
returnObj.GetType().GetProperty(propertyName).SetValue(returnObj, obj.GetType().GetProperty(propertyName).GetValue(obj));
}
return returnObj;
}

/// <summary>
/// 获取一个List<类>的需要的各个类属性值
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="objs"></param>
/// <param name="propertiesStr">使用时用'_'分开需要的属性值的各个属性名</param>
/// <returns></returns>
public static List<T> GetObjectListUsefulProperties<T>(List<T> objs, string propertiesStr) where T : new()
{
var returnObjs = new List<T>();
foreach (var obj in objs)
{
returnObjs.Add(GetObjectUsefulProperties(obj, propertiesStr));
}
return returnObjs;
}