Question

我有以下代码通过存储过程查询数据库。当ReturnObject.familymembers = multi.Read<InternalObjects.Person>().AsQueryable();不为空时，返回正常;但是我找不到通过Dapper / linq处理带有集合类型的null返回的方法。

我的存储过程：

SELECT    states.name as stateName,   fam.*
FROM            Family fam
WHERE        fam.idUserName=@username 

SELECT      person.*
FROM            person
INNER JOIN Family fam on fam.Id = person.idFamily
WHERE        fam.idUserName=@username

当第二个SQL语句有记录时，设置ReturnObject.familymembers没有任何问题，但是我无法确定如何指定默认值或从Dapper处理null返回。调试时，如果没有返回任何行，ReturnObject.familymembers = multi.Read<InternalObjects.Person>().AsQueryable()将抛出一个空异常。

这是我认为可行的，但不是：

ReturnObject.familymembers = multi.Read<InternalObjects.Person>().AsQueryable().DefaultIfEmpty<InternalObjects.Person>();

        public InternalObjects.FamilyDashboard GetDashboardInfo(string username)
    {
        InternalObjects.FamilyDashboard ReturnObject = new InternalObjects.FamilyDashboard();

        using (var dbConnection = _dbConnectionFactory.CreateConnection())
        {
            using (var multi = dbConnection.QueryMultiple("DashboardSP", new { username = username }, commandType: CommandType.StoredProcedure))
            {
                ReturnObject  = multi.Read<InternalObjects.FamilyDashboard>().SingleOrDefault();
                ReturnObject.familymembers = multi.Read<InternalObjects.Person>().AsQueryable();

            }
        }
        return ReturnObject;
    }

我的DTO：

public class InternalObjects
{
    public class FamilyDashboard
    {
        public string physicalAddress1 { get; set; }
        public string MailingAddress1 { get; set; }
        public string physicalAddressCity { get; set; }
        public string physicalAddressZip { get; set; }
        public string MailingAddressCity { get; set; }
        public string NumInHousehold { get; set; }
        public string capidCounty { get; set; }
        public string physicalUnit { get; set; }
        public string mailingUnit { get; set; }
        public string familyPhone { get; set; }
        public string capCharacteristics { get; set; }
        public IEnumerable<Person> familymembers {get; set;}

    }
    public class Person
    {
        public int Id { get; set; }
        public string firstName { get; set; }
        public string lastName { get; set; }
        public string middleName { get; set; }
        public int gender { get; set; }
        public DateTime birthdate { get; set; }
        public string emailPersonal { get; set; }
        public string SSN { get; set; }
        public string relationshipName { get; set; }
        public string primaryLanguageName { get; set; }

        public int age { get; set; }

    }
}

Answer 1

我找到了答案：

基本上，如果没有返回任何行，那么dapper不会返回任何值，而这些行不会实例化我的对象。我的父对象ReturnObject没有被实例化，但仍然正常运行。

下一行ReturnObject.familymembers = multi.Read<InternalObjects.Person>().AsQueryable()尝试访问该对象，但DTO不存在。

一个简单的条件节省了一天：

if (ReturnObject != null)
{
    ReturnObject.familymembers = multi.Read<InternalObjects.Person>().ToList<InternalObjects.Person>();
}

参与其他任何人的参考！：

How to return null from a Dapper query rather than default(T)?

Getting Dapper to return an empty string instead of a null string

当结果为null时，dapper多映射IEnumerable列表的默认值

1 个答案: