How to retieve CRM Guid using LINQ and joins?

问题

We are using CRM 2011. We have Contracts with Entity Reference to Product and each Product has an Entity Reference to a Subject. Given a Contract Guid, I need to retrieve the Subject Guid.

I am a beginner at LINQ but I coded:

 var subject = from s in context.SubjectSet
                                       join product in context.ProductSet
                                           on s.Id equals product.SubjectId.Id
                                       join contract in context.ContractSet
                                           on product.Id equals contract.ce_ProductId.Id
                                       where contract.Id == gContractId
                                       select s;

                foreach (var s in subject)
                {
                    newReportableAction.ce_SupergroupRegarding =
                    new EntityReference(Xrm.Subject.EntityLogicalName, new Guid(s.Id.ToString()));
                }

This throws an error:

AttributeFrom and AttributeTo must be either both specified or both ommited. You can not pass only one or the other. AttributeFrom: , AttributeTo: ce_ProductId

What does this error mean?
How can I get the Guid?

Update:

I tried breaking the query into parts to see where the error was being generated from so I had:

var query = from product in context.ProductSet
                            join contract in context.ContractSet
                            on product.Id equals contract.ce_ProductId.Id

This gives:

"The type of one of expressions in the join clause is incorrect. Type inference failed in the call to 'Join'"

Thank you to all who help...

回答1:

I don't think you can use the .Id property right off the entity in Linq statements. Try this instead:

var query = from product in context.ProductSet
               join contract in context.ContractSet
               on product.ProductId equals contract.ce_ProductId.Id

Notice product.ProductId instead of product.Id.

来源：https://stackoverflow.com/questions/23373931/how-to-retieve-crm-guid-using-linq-and-joins