在LINQ查询（EF 5）中包括编码字段值

Question

Not sure of the correct terminology, but is there any way to include a "coded field" that doesn't exist in the table, in an Entity Framework 5 LINQ query?

For example, in SQL you can do this:

SELECT 'Ledger' AS type, (...) FROM table

So that the query result includes a field called 'type' with the value 'Ledger'. Is there a way to do this in a LINQ query?

And before you ask why it has to be in the query, it's because the query is a union of multiple tables and I need a designation of which table the data came from. Without this I will need to query each one separately then merge them.

Answer 1

Sure, you can. It would look a bit like this:

var results = dbContext.Table.Select(r => new { type = "Ledger", ... });

Or if you need a named type, something like this should work:

public class UnionResult 
{
    string Type { get; set; }
    ...
}

var results = dbContext.Table.Select(r => new UnionResult { Type = "Ledger", ... });

Answer 2

Yes, you can totally do this.

pswg has used something that is called an anonymous type. Basically you define the outcome of the query on the fly. As you can see in his answer, in expression syntax this looks like Select(r => new { type = "Ledger", ... });

In query syntax it looks like this:

from xxx in y
select new { type = "Ledger" };

Behind new, there is no class / type or anything. Neither is type defined. But it will compile as a string natuarlly.

On the other hand you can define a custom ViewModel class for this

public class CustomResultVM
{
   //ignoring getter and setter
   public int Id;
   public string type;
   public string name;
}

your select would now be strongly typed and look like this:

Select(r => new CustomResultVM 
{ 
  Id = r.xxx,
  type = "Ledger", 
  Name = r.xxxx 
});

//query snytax
from xxx in y
select new CustomResultVM 
       {
          Id = r.xxx,
          type = "Ledger", 
          Name = r.xxxx 
       };

both ways are valid and it depends on what you need at any given point and time.