Using reflection to address a Linqed property

Question

I\'m trying to writing a generic method that will load a record of a specific type, with a specific ID. Here\'s one way that works:

    public abstract clas         


        

Answer 1

Having read these posts: Generic Data Access using LINQ to SQL and C#, LINQ-to-SQL: Generic Primary Key function and Calling a generic method with Type

My colleague and I came up with the following digest:

We added the following method to our datacontext (in a partial class).

public T GetInstanceByPrimaryKey<T>(object primaryKeyValue) where T : class
{
    var table = this.GetTable<T>();
    var mapping = this.Mapping.GetTable(typeof(T));
    var pkfield = mapping.RowType.DataMembers.SingleOrDefault(d => d.IsPrimaryKey);

    if (pkfield == null)
        throw new Exception(String.Format("Table {0} does not contain a Primary Key field", mapping.TableName));

    var param = Expression.Parameter(typeof(T), "e");

    var predicate =
        Expression.Lambda<Func<T, bool>>(Expression.Equal(Expression.Property(param, pkfield.Name), Expression.Constant(primaryKeyValue)), param);

    return table.SingleOrDefault(predicate);
}

Then, where we need to instanciate from the type name and primary key value:

string name = "LinqObjectName";
int primaryKey = 123;

var dc = new YourDataContext();

Type dcType = dc.GetType();
Type type = dcType.Assembly.GetType(String.Format("{0}.{1}", dcType.Namespace, name));

MethodInfo methodInfoOfMethodToExcute = dc.GetType().GetMethod("GetInstanceByPrimaryKey");
MethodInfo methodInfoOfTypeToGet = methodInfoOfMethodToExcute.MakeGenericMethod(name);
var instance = methodInfoOfTypeToGet.Invoke(dc, new object[] { primaryKey });

return instance;

Hope this helps!

Answer 2

Linq to SQL tries to translate your linq-query into SQL, but it does not know how to translate your property to a column name in the DB.

A good explanation can be found here on SO: simple linq to sql has no supported translation to SQL

But how to solve it, is another matter. I have with succes used the apporoach from this thread:

http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/df9dba6e-4615-478d-9d8a-9fd80c941ea2/

Or you can use dynamic query as mentioned here by scott guthrie:

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

Answer 3

Since LINQ statements referred to a LINQ-to-SQL IQueryable are translated to SQL queries, you will have to use the AsEnumerable extension (which will in turn cause a read of all the items in the database) and do reflection-related stuff on that IEnumerable.
EDIT
As required here's a clarification
As specified in a comment, what I meant was something like:

 (from obj in context.GetTable<T>() select obj).AsEnumerable().Where(x => x.IDValue == ID)

Unlike a query executed on an IQueryable, which can be perfectly translated to SQL such as

context.GetTable().Where(x => x.Text == "Hello")

which gets converted to something similar to

SELECT * FROM TABLE_MAPPED_TO_TYPE_T WHERE Text = 'Hello'

a query executed against an IEnumerable - in your case - will be executed by fetching all the entries of your table and then applying code-wise the specified filter.