实体框架6：筛选表格

Question

I', approaching to EF6. I have understood that, if I want a single record, I can use the First() method (usable also like a "get", I think).

But what if I want a collection of records from my table? I mean, if the result of query can have a cardinality from 0 to N... what have I to do?

Silly Example:

Name table: SomeTable

ID    NAME    SOMEATTR
1      A       YELLOW
2      B        RED
3      C       YELLOW
4      D       YELLOW
5      E        BLUE

...

How can I get all the records where someAttr is "yellow"?

Answer 1

Once you have your Linq query...

var query = (from ... in ... where ... select ...);

Or, for your specific example:

var query = (from r in dbContext.MyTable where (r.someAttr == "YELLOW") select r);

You can either just iterate over it using foreach which will cause it to be executed once and then yield the results ...

foreach (var r in query)
{
}

... or call ToList() ( http://msdn.microsoft.com/en-us/library/vstudio/bb342261(v=vs.100).aspx ) if you want to acquire a list that can, later, be passed around, manipulated or examined many times. (This list is essentially disconnected from the server after it is returned.)

var list = query.ToList()

... or use it to construct a collection like a HashSet<T> ( http://msdn.microsoft.com/en-us/library/bb301504(v=vs.110).aspx )

var set = new HashSet<T>(query)

Any of these will cause it to be executed on the server, only once, and process the results.

After all, the query itself is IEnumerable . It is also IQueryable so you could manipulate it further and then enumerate the results, like...

var sortedList = query.OrderBy(...).ToList()

Answer 2

using linq & C#

private readonly myEntities db = new myEntities()
public list<SomeTable> MyRecords()
{
 return res = db.SomeTables.where(o => o.SOMEATTR == "YELLOW").ToList()
}

Answer 3

If you have a SOMEATTR class that maps to SOMEATTR table, you can query it:

var context = new YourDbContext();
var results = context.SomeTables.Where(x=>x.SOMEATTR == "Yellow").ToList();

//then you can iterate all results
foreach(var item in results)
{
    Console.WriteLine(item.ID + " " + item.NAME + " " + item.SOMEATTR);
}

The result will be:

1      A       YELLOW
3      C       YELLOW
4      D       YELLOW