LINQ左联接错误
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我已经在LINQ中编写了以下查询以执行左联接，但其抛出错误：
var qry = from c in dc.category_feature_Name_trans_SelectAll_Active()
join p in dc.product_category_feature_trans_SelectAll()
on c.cft_id equals p.cft_id into cp
from p in cp.DefaultIfEmpty()
select new
{
c.cft_id,
c.feature_id,
c.feature_name,
p.product_id ,
p.value
};
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information about
the error and where it originated in the code.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Source Error:
Line 57: on c.cft_id equals p.cft_id into cp
Line 58: from p in cp.DefaultIfEmpty()
error Line 59: select new
Line 60: {
Line 61: c.cft_id,
没有找到相关结果
已邀请:
4 个回复
土投
将返回一个具有单个null值的序列。 这意味着您必须考虑以下事实：可以为null。现在，您还没有真正说出在这种情况下要发生的事情。您可能想要这样的东西：var qry = from c in dc.category_feature_Name_trans_SelectAll_Active() join p in dc.product_category_feature_trans_SelectAll() on c.cft_id equals p.cft_id into cp from p in cp.DefaultIfEmpty() select new { c.cft_id, c.feature_id, c.feature_name, product_id = p == null ? null : p.product_id, value = p == null ? null : p.value };或的类型，这没有帮助。 （例如，如果是值类型，则您需要对上述代码进行更多处理。）哭木算
可以是。您需要考虑到这一点。 这是应该工作的查询，尽管我不确定是哪种值。如果value是引用类型，则查询将起作用。如果值是值类型，则使用类似于“ 9”的类型转换。var qry = from c in dc.category_feature_Name_trans_SelectAll_Active() join p in dc.product_category_feature_trans_SelectAll() on c.cft_id equals p.cft_id into cp from p in cp.DefaultIfEmpty() select new { c.cft_id, c.feature_id, c.feature_name, product_id = p == null ? (int?)null : p.product_id, value = p == null ? null : p.value, };森含械
var deviceResultDetails = from pa in programAliasrecords join pgm in Newprogramrecords on pa.program_id equals pgm.id into pgm_join from pgm2 in pgm_join.DefaultIfEmpty() join latest_fw in firmwareWithIdrecords on pgm2?.latest_firmware_id equals latest_fw?.id into latest_fw_join from latest_fw2 in latest_fw_join.DefaultIfEmpty() join dev_fw in firmwareWithIdrecords on pgm2?.firmware_group_id equals dev_fw?.firmware_group_id into dev_fw_join from dev_fw2 in dev_fw_join.DefaultIfEmpty() join vv in vulnerabilityrecords on pgm2?.id equals vv?.program_id into vv_join from vv2 in vv_join.DefaultIfEmpty() where dev_fw2?.version == row.firmware && pa?.keyword == row.model select new _deviceResults { model = row.model, serial = row.serial, firmware = row.firmware, firmware_date = dev_fw2.br_date == null ? null: dev_fw2.br_date, latest_firmware = latest_fw2.version == null ? null : latest_fw2.version, latest_firmware_date = latest_fw2.br_date == null ? null : latest_fw2.br_date, status = Convert.ToInt32(vulnerability_count) > 0 && pgm2.out_of_support == \"TRUE\" ? \"Vulnerable (End of Suport)\" : Convert.ToInt32(vulnerability_count) > 0 && pgm2.out_of_support == \" \" ? \"Vulnerable (Upgradeable)\" : pgm2.out_of_support == \"TRUE\" ? \"Out-of-support\" : Convert.ToInt32(dev_fw2.revs_out_of_date) > 1 ? \"Out-of-date\" : pgm2.id == \"NonHP\" ? \"NonHP\" : pgm2.id == \"NoFirmware\" ? \"No Firmware\" : pgm2.id == \"HPink\" || pgm2?.id == \"HPOther\" ? \"Not evaluated (model not included yet)\" : pgm2.id == \" \" ? \"Not evaluated (model not recognized)\" : dev_fw2.version == \" \" ? \"Not evaluated (firmware version missing)\" : dev_fw2.id == \" \" ? \"Not evaluated (firmware version mismatch)\" : // && dev_fw.id in (select version from firmware) dev_fw2.id == \" \" ? \"Not evaluated (firmware version unrecognized)\" : dev_fw2.br_date == \" \" || pgm2.id == \" \" || dev_fw2.revs_out_of_date == \" \" ? \"Not evaluated\" : \"OK\", out_of_support = pgm2.out_of_support == \"\" ? \"false\" : pgm2.out_of_support, revs_out_of_date = dev_fw2.revs_out_of_date == null ? null : dev_fw2.revs_out_of_date, program_id = pgm2.id == null ? null : pgm2.id, firmware_id = dev_fw2.id == null ? null : latest_fw2.br_date };眠缝