how can i write a generic query with EF and C#

Question

i want to write a query in entity framework like this:

T entity = (from e in myContext.typeof(T) )

i tried to use string concat but this dose not work for get a member from myContext

more code:

public T GetEntity<T>(Guid entityId, string connectionString)
{
    using (RContext rpContext = new RContext(connectionString))
    {

        var entity = (from e in rpContext.Set<T>()
                                    where e.Id == entityId
                                    select e).FirstOrDefault();
                    return (T) Convert.ChangeType(entity, typeof(T));

    }
}

christophano · Accepted Answer · 2016-06-08T09:39:08.263

1

You can access the generic Set method like this:

var entities = (from e in myContext.Set<T>());

Update: You will need to add generic type constraints to your method, to ensure the generic type T will match the constraints already applied to the method DbContext.Set<T>.

Update 2: You don't need to cast your entity; it is already typeof(T).

public T GetEntity<T>(Guid entityId, string connectionString)
    where T : class
{
    using (RContext rpContext = new RContext(connectionString))
    {

        return (from e in rpContext.Set<T>()
                where e.Id == entityId
                select e).FirstOrDefault();
    }
}

Update 3: You can pass your predicate straight into the FirstOrDefault method, assuming you're not attached to the linq syntax.

public T GetEntity<T>(Guid entityId, string connectionString)
    where T : class
{
    using (RContext rpContext = new RContext(connectionString))
    {
        return rpContext.Set<T>().FirstOrDefault(e => e.Id == entityId);
    }
}

Update 4: As you're using the Id property inside your method, you will also need to constrain your method to types which have that property - either by constraining a common interface, or a common base class. If it's a base class, you can remove the class constraint.

edited Jun 08 '16 at 09:39

answered Jun 08 '16 at 08:47

christophano

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What type is `myContext`? – christophano Jun 08 '16 at 09:09
myContext is my DbContext – data Jun 08 '16 at 09:11
In what way does it not work? Compile error? Runtime exception? – christophano Jun 08 '16 at 09:13
the type T must be a refrence type in order to use it as a parametr 'TEntity' in the generic type or method 'System.Data.Entity.DbContext.Set()' – data Jun 08 '16 at 09:15
That's true. It must also be one of the entities that your `DbContext` serves. What are you trying to use as T? – christophano Jun 08 '16 at 09:17
T should be one of my entities. i have a function and many entities. i want to write a query for all of my entities in one function. i can write it with switch and if-else but i just trying generic, i think its better – data Jun 08 '16 at 09:19
Can you post some more of your code, including the part where you tried my suggestion? – christophano Jun 08 '16 at 09:22
sure, please check my post – data Jun 08 '16 at 09:27
can u give me a sample? – data Jun 08 '16 at 09:29
See my edit. As you're not defining `T` yet, you need to ensure that the constrainst that `DbContext.Set` applies to T are also applied by whoever calls your method. – christophano Jun 08 '16 at 09:35
oh, god i forgat the condition tnx bro – data Jun 08 '16 at 10:02

how can i write a generic query with EF and C#

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