Question

我正在绑定一个选中的列表框，其中包含后续列表列表，但如果列表为空，我该如何管理它？它会在ling语句中抛出异常，如果为null，我应该为checkedlist框设置什么

public List<dataObject> GetAllCustomItems(CategoryType currType, int mCategoryID)
        {
            List<dataObject> lst = null;
            try
            {
                switch (currType)
                {
                    case CategoryType.Dressing:
                        lst = (from xx in this.DressingItems 
                               where xx.DressingInfo.CatID == mCategoryID 
                               select new dataObject() { 
                                   ID = xx.DressingInfo.DressingID,
                                   Name = xx.DressingInfo.Description, 
                                   Selected = xx.IsDefault 
                               }).ToList();
                        break;
              }
        }

抛出异常

Value cannot be null.
Parameter name: source

Answer 1

您确定this.DressingItems不为空或空吗？

在LINQ查询之前检查：

if (this.DressingItems != null && this.DressingItems.Any()) {
    lst = (from xx in this.DressingItems where...
}

如果lst为空或空，请DressingItems返回null。

修改 :(在评论后）

另外，请检查DressingInfo属性：

where xx.DressingInfo != null &&...

Answer 2

如何在where子句

中首先检查DressingInfo是否为null

where xx.DressingInfo != null && xx.DressingInfo.CatID == mCategoryID

Answer 3

我不认为是xx.DressingInfo对象问题导致它会出现首先找不到Object引用，肯定是xx.DressingInfo中的值出了问题。你可以试试iif来检查它。

  var lst = (from xx in this.DressingItems 
                           where xx.DressingInfo.CatID == mCategoryID 
                           select new dataObject() { 
                               ID = xx.DressingInfo.DressingID==null?0:xx.DressingInfo.DressingID,
                               Name = xx.DressingInfo.Description==null? "":xx.DressingInfo.Description, 
                               Selected = xx.IsDefault 
                           });
return IsNullThenNew<dataObject>(lst);

如果您不想将null返回给调用者，则可以通过下面的泛型方法返回至少空对象。

 public static bool IsNullOrEmpty<T>(this IEnumerable<T> enumerable)
        {
            return enumerable == null || !enumerable.Any();
        }
 public static List<T> IsNullThenNew<T>(this IEnumerable<T> t)
        {
            if (!IsNullOrEmpty<T>(t))
            {
                return t.ToList();
            }
            else
            {
                Type genericListType = typeof(List<>);
                Type listType = genericListType.MakeGenericType(t.GetType());
                object listInstance = Activator.CreateInstance(listType);
                return (List<T>)listInstance;
            }//end if-else
        }

Answer 4

您可以更改List<dataObject> lst = null; 到List<dataObject> lst = new List<dataObject>; 或者做这样的事情：

public List<dataObject> GetAllCustomItems(CategoryType currType, int mCategoryID)
{
    try
    {
        switch (currType)
        {
            case CategoryType.Dressing:
                List<dataObject> lst = (from xx in this.DressingItems 
                       where xx.DressingInfo.CatID == mCategoryID 
                       select new dataObject() { 
                           ID = xx.DressingInfo.DressingID,
                           Name = xx.DressingInfo.Description, 
                           Selected = xx.IsDefault 
                       }).ToList();
                break;
      }
}

如果列表为空，如何检查linq？

4 个答案: