linq查询中的where子句

Question

在下面的代码transmittalno.TransID中总是有值，但doctranstocon.Transid有时有价值，有时却没有，所以当doctranstocon.Transid有价值时我没有问题，但是当没有任何价值时，在where子句中比较时：transmittalno.TransID == doctranstocon.Transid。 它返回错误“对象引用未设置为对象的实例。”

TranstoCons = from doctranstocon in _DocTranstoCons where 
              ( transmittalno.TransID == doctranstocon.Transid  ) 
              select doctranstocon.tblTranstoCon

我喜欢TranstoCons在“doctranstocon.Transid”没有任何值时返回null

如何在以下linq查询中处理此错误：

var query = from transmittalno in _Transmittals
            Select new TransmittaltoConPresentationModel 
            { 
              TransID = transmittalno.TransID, 
              Transmittal = transmittalno.TRANSMITTAL, 
              TranstoCons = from doctranstocon in _DocTranstoCons where 
                            ( transmittalno.TransID == doctranstocon.Transid  ) 
                            select doctranstocon.tblTranstoCon 
             };

Answer 1

您可以使用：

where doctranstocon != null && doctranstocon.Transid.Equals(transmittalno.TransID)

Answer 2

var query = from transmittalno in _Transmittals
            Select new TransmittaltoConPresentationModel 
            { 
              TransID = transmittalno.TransID, 
              Transmittal = transmittalno.TRANSMITTAL, 
              TranstoCons = from doctranstocon in _DocTranstoCons 
                            where doctranstocon == null || transmittalno.TransID == doctranstocon.Transid
                            select doctranstocon != null? doctranstocon.tblTranstoCon : null
             };