linq-to-entity动态查询

Question

I'am currently migrating our old system to .Net and I encountered this problem. I want to return the result but I still need to refine it after the function, but using this code, I don't have a choice but to call the result from the database and filter an in-memory result instead which is poor performance.

public IQueryable<User> GetUser(string[] accessCodes)
{
    string condition = "";
    if (accessCodes == null)
    {
         condition = " AccessCode IS NOT NULL "
    }
    else
    {
        for (int i = 0; i <= accessCodes.Length - 1; i++)
        {
            condition += " AccessCode LIKE '%" + accessCodes[i].ToString() + "%' ";
            if (i + 1 <= code.Length - 1)
            {
                condition += " OR ";
            }
        }
    }

    return context.ExecuteQuery<User>("SELECT * FROM User WHERE " + condition, null).ToList();
}

I've tried this approach this but i'm stuck:

public IQueryable<User> GetUser(string[] accessCodes)
{
    IQueryable<User> basequery = from u in context.User 
                                 select u;

    if (accessCodes == null)
    {
         basequery = basequery.Where(n => n.AccessCode != null);
    }
    else
    {
        for (int i = 0; i <= accessCodes.Length - 1; i++)
        {
            // what am I supposed to do here?
        }
    }

    return basequery;
}

I'm hoping that there are solutions which do not require third party libraries.

Answer 1

You can try with Any :

else
{
    output = output.Where(u => accessCodes.Any(a => u.AccessCode.Contains(a)));
}

or you can use PredicateBuilder :

if (accessCodes == null)
{
    output = output.Where(u => u.AccessCode == null);
}
else
{
    var predicate = PredicateBuilder.False<User>();

    for (int i = 0; i <= accessCodes.Length - 1; i++)
    {
        predicate = predicate.Or(u => u.AccessCode.Contains(accessCodes[i]))
    }

    output = output.Where(predicate);
}

I also changed your if part: Where method does not modify source, it returns new query definition, so you have to assign it back to output to make it work.

Answer 2

This should work for you:

IQueryable<User> basequery = from u in context.User 
                             select u;
if (accessCodes == null)
{
    basequery = basequery.Where(u => u.AccessCode != null);
}
else
{
    basequery = basequery.Where(u => accessCodes.Contains(u=>u.AccessCode));
}

also make sure you return basequery , since output in your method is not defined and not used.