c# - 使用 Count 时出现空引用异常

Question

我有一种方法来计算项目，但遇到了一个愚蠢的例外：

 internal static bool IsCompanyInList(int companyId, int favoritecompanyId)
        {
            var result = _context.FavouriteCompanies.Count(c => c.CompanyId == companyId && c.FavouriteCompanyId == favoritecompanyId); // getting exception here
            if ( result >0)
                return true;
            return false;
        }

另外，还有一条线索，我的桌子是空的。根本没有行。如何处理这种情况。我也尝试过 Where 子句。见下文：

 var result = _context.FavouriteCompanies.Where(criteria_here);

Answer 1

FavouriteCompanies 为空。你可以这样做：

internal static bool IsCompanyInList(int companyId, int favoritecompanyId)
            {
                var result = _context.FavouriteCompanies;
                if (result!=null)
                {
                    return result.Any(c => c.CompanyId == companyId && c.FavouriteCompanyId == favoritecompanyId);
                } 

                return false;
            }

在那个解决方案if (result>0)中是多余的。

Answer 2

好吧，如果您可以包含堆栈，这会有所帮助，但我认为 FavouriteCompanies 集合中有一个 null （或者 _context 变量可能为 null - 如果您将许多方法与“。”链接在一起，您会遇到火车残骸很难说什么是空的）。您可以通过删除空值来防御性地解决这个问题：

var result = _context.FavouriteCompanies.Where( c => c != null ).Count(...)

although to be honest it would be much better to find out where a null has been inserted into FavourieCompanies and stop it there.

Answer 3

The error could be anywhere you attempt to access a member.

var result = _context.FavouriteCompanies.Count(c => c.CompanyId == companyId && c.FavouriteCompanyId == favoritecompanyId);

is _context null? is _context.FavoriteCompanies null? does FavoriteCompanies contain a record in which CompanyId or FavouriteCompanyId is null?

Since this is Linq2Sql, you can ask c.FavouriteCompanyId.Isnull() within the count clause before accessing the member itself.

Answer 4

For me, one of string (or nullable column) value was null.

        var result = _context.FavouriteCompanies.Count(c => c.CompanyId != null && c.CompanyId == companyId && c.FavouriteCompanyId != null && c.FavouriteCompanyId == favoritecompanyId);