Question

当我尝试使用System.Linq.Dynamic.Core库订购列表时，我得到了NullReferenceException。

var myFinalOrderedList = myList.AsQueryable()
    .OrderBy("Company.Name asc, Process.Name asc, Reference.Name asc")
    .ToList();

在Reference.Name OrderBy上发生空异常，因为Reference可为空。

如何按可为空的Reference对象排序？

StackTrace（敏感信息替换为“ ****”）：

    {
  "ClassName": "System.NullReferenceException",
  "Message": "Object reference not set to an instance of an object.",
  "Data": null,
  "InnerException": null,
  "HelpURL": null,
  "StackTraceString": "   at lambda_method(Closure , ControlActivity )\r\n   at System.Linq.EnumerableSorter`2.ComputeKeys(TElement[] elements, Int32 count)\r\n   at System.Linq.EnumerableSorter`2.ComputeKeys(TElement[] elements, Int32 count)\r\n   at System.Linq.EnumerableSorter`2.ComputeKeys(TElement[] elements, Int32 count)\r\n   at System.Linq.EnumerableSorter`1.ComputeMap(TElement[] elements, Int32 count)\r\n   at System.Linq.EnumerableSorter`1.Sort(TElement[] elements, Int32 count, Int32 minIdx, Int32 maxIdx)\r\n   at System.Linq.OrderedEnumerable`1.GetEnumerator(Int32 minIdx, Int32 maxIdx)+MoveNext()\r\n   at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)\r\n   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)\r\n   at ****.Dal.Repositories.ControlActivities.ControlActivityRepo.Search(ControlActivitySearchModel input) in C:\\Users\\****\\source\\Workspaces\\****\\Main\\****\\****.Dal\\Repositories\\ControlActivities\\ControlActivityRepo.cs:line 1189\r\n   at ****.Application.ControlActivities.ControlActivityService.Search(ControlActivitySearchModel input) in C:\\Users\\****\\source\\Workspaces\\****\\Main\\****\\****.Application\\ControlActivities\\ControlActivityService.cs:line 339\r\n   at ****.Api.Controllers.ControlActivityController.Search(String apikey, ControlActivitySearchModel input) in C:\\Users\\****\\source\\Workspaces\\****\\Main\\****\\****.Api\\Controllers\\ControlActivityController.cs:line 117",
  "RemoteStackTraceString": null,
  "RemoteStackIndex": 0,
  "ExceptionMethod": null,
  "HResult": -2147467261,
  "Source": "Anonymously Hosted DynamicMethods Assembly",
  "WatsonBuckets": null
}

公司，流程和引用是主列表（ControlActivity）的外键。这三个几乎都具有PK ID和NVARCHAR名称。所有控制活动都需要“公司”和“过程”，并且不需要参考。

Answer 1

我在@Alen发布的链接和https://github.com/StefH/System.Linq.Dynamic.Core/issues/98的链接的帮助下，找到了自己的问题的答案。下面，我展示了更多我的实际代码，这就是为什么它看起来与问题不同的原因。答案在于StaticMethod.ConvertToNullableNested。例如，我传入了{i.ColumnName} {i.SortOrder} = "Company.Name asc"，它返回了Company == null ? null : Company.Name asc。然后，我将每种排序附加在一起，并传递给OrderBy。

//TURN INTO QUERYABLE
var finalList = unionList.AsQueryable();

//DYNAMIC SORT
if (input.SortModel?.SortModelItems?.Count > 0)
{
    string sortQry = String.Empty;
    foreach (var i in input.SortModel.SortModelItems)
    {
        sortQry = sortQry + $"{StaticMethod.ConvertToNullableNested($"{i.ColumnName} {i.SortOrder}")}, ";
    }
    sortQry = sortQry.TrimEnd(", ");
    finalList = finalList.OrderBy(sortQry);
}

//RETURN AND REMEMBER TO .TAKE(Pagesize)
return Tuple.Create(finalList.Take(input.PageSize).ToList(), finalRowCount);

public static string ConvertToNullableNested(string expression, string result = "", int index = 0)
    {
        //Transforms => "a.b.c" to "(a != null ? (a.b != null ? a.b.c : null) : null)"
        if (string.IsNullOrEmpty(expression))
            return null;
        if (string.IsNullOrEmpty(result))
            result = expression;
        var properties = expression.Split(".");
        if (properties.Length == 0 || properties.Length - 1 == index)
            return result;
        var property = string.Join(".", properties.Take(index + 1));
        if (string.IsNullOrEmpty(property))
            return result;
        result = result.Replace(expression, $"{property} == null ? null : {expression}");
        return ConvertToNullableNested(expression, result, index + 1);
    }

动态Linq Core OrderBy可空子级属性

1 个答案: