比较Linq中的可为空类型到Sql

首先要做的是进行日志记录，以查看生成了什么TSQL。例如：

ctx.Log = Console.Out;

LINQ-to-SQL似乎对null的处理有些不一致（取决于文字与值）：

using(var ctx = new DataClasses2DataContext()){    ctx.Log = Console.Out;    int? mgr = (int?)null; // redundant int? for comparison...    // 23 rows:    var bosses1 = ctx.Employees.Where(x => x.ReportsTo == (int?)null).ToList();    // 0 rows:    var bosses2 = ctx.Employees.Where(x => x.ReportsTo == mgr).ToList();}

因此，我只能建议使用带空值的顶级表格！

即

expression<Func<Category,bool>> predicate;if(categoryId == null) {    predicate = c=>c.ParentId == null;} else {    predicate = c=>c.ParentId == categoryId;}var subCategories = this.Repository.Categories.Where(predicate).ToList().Cast<ICategory>();

更新-我使用自定义功能使其“正常”工作

expression

    static void Main()    {        ShowEmps(29); // 4 rows        ShowEmps(null); // 23 rows    }    static void ShowEmps(int? manager)    {        using (var ctx = new DataClasses2DataContext())        { ctx.Log = Console.Out; var emps = ctx.Employees.Where(x => x.ReportsTo, manager).ToList(); Console.WriteLine(emps.Count);        }    }    static IQueryable<T> Where<T, TValue>(        this IQueryable<T> source,        expression<Func<T, TValue?>> selector,        TValue? value) where TValue : struct    {        var param = expression.Parameter(typeof (T), "x");        var member = expression.Invoke(selector, param);        var body = expression.Equal(     member, expression.Constant(value, typeof (TValue?)));        var lambda = expression.Lambda<Func<T,bool>>(body, param);        return source.Where(lambda);    }