查询实体框架4，查询问题的方法

Question

i have a little misunderstanding and i hope you can clear that for me. i have a table called : Requests.

I want to do query, lets say by Request Id but it doesn't work.

public Requests SearchById(int id)
{
    Model.ModelContainer cont = new Model.ModelContainer();
    return (cont.Requests.Where(req => req.ReqId == id));
    //when i try to cast to Request i get an error too
}

i found an example which this line should work :

cont.Requests.First(req => req.ReqId == id)

but i get error during compilation that it cannot contain lambda expression.

i have few question:

1. How cant i handle the upper example ?(querying from entity)
2. How can i work with multi-line results besides converting to list
3. i need to build and advanced search (i have 6-7 columns ), is there a common way to do it?

thanks !!

Answer 1

This should work:

public Request SearchById(int id)
        {
           using(Model.ModelContainer cont = new Model.ModelContainer())
           {
                return cont.Requests.Where(req => req.ReqId == id).FirstOrDefault();
           }
        }

What you had was an IEnumerable<Request> , not a single request. Also make sure you have a

using System.Linq;

in your code file. Also always dispose the context when you can using a using block.

cont.Requests.First(req => req.ReqId == id)

This should also work - make sure you have a using System.Linq in your code file.

How can i work with multi-line results besides converting to list

Since your context is local and not available outside of your method (it gets disposed afterwards) a list is your best option. If you keep it an IQueryable then you have not really retrieved the results from the DB yet, you have just defined your query, this leads to a problem when the results are retrieved by the consumer since only then the DB query is executed - but the corresponding DB context might have been disposed already. Append .ToList() at the end of your query to materialize an IQuerable<T>

I need to build and advanced search (i have 6-7 columns ), is there a common way to do it?

You can combine multiple clauses in the .Where() extension method, ie Where( x=> xA=="foo" && xB == 42) . You can also chain multiple Where() methods if necessary.

Answer 2

Your problem is that Where returns a collection of objects (zero to many). You only want a single object. You can't simply cast a collection of objects to a single object.

Instead you should get the First (or if you know there's definitely going to be only one you can call Single ).

Regarding 2 - that depends what you want to do. You can do foreach over the resultset, or select from the resultset before putting it into a list - but sometimes the most efficient option is to put in a list first.

And point 3, there are plenty of patterns available. Again it depends on what you're doing and how the 'search terms' work. For example,

public List<Requests> SearchById(int? id, DateTime? date, string name)
{
    var cont = new Model.ModelContainer();
    var query = cont.Requests;

    if (id != null)
        query = query.Where(req => req.ReqId == id.Value);

    if (date != null)
        query = query.Where(req => req.Date == date.Value);

    if (!String.IsNullOrEmpty(name))
        query = query.Where(req => req.Name == name);

    return query.ToList();
}