how to return selected columns based on user choice

Question

I'm trying to limit which fields are returned by an API based on a parameter called fields which I accept multiple strings doing this

private readonly string[] fields;

public string[] SelectiveSerializer(string fields)
{
    string[] _fields;
    var fieldColl = fields.Split(',');
    _fields = fieldColl
        .Select(f => f.ToLower().Trim())
        .ToArray();
    return _fields;

}

I want to be able to choose what I return based on whatever the user gives me in _fields. Normal way to do it:

 var linq = (from entity in db.users
             where entity.ID== id
              && entity.ON== false
             select( new {
             ID = entity.ID,
             FirstName = entity.FirstName,
             LastName =entity.LastName,
             FotherName = entity.FotherName
             }).ToList();

but here I have to specify the fields in Select (ID, FirstName ..etc), which I want it to be dynamic based on what fields[] has. Is there a way to do this?

sort of this (which is wrong):

var linq = (from entity in db.users
             where entity.ID== id
              && entity.ON== false
             select( new {
             foreach (string s in _fields)
             {
                    entity.s;
             }
             }).ToList();

Perhaps something like [this](https://stackoverflow.com/questions/16516971/linq-dynamic-select)? — Steve Greene, Nov 28 '17 at 18:54

Olivier Jacot-Descombes · Answer 1 · 2017-11-29T14:10:24.863

Use a ternary expression for each assignment

var user = entityContext.Users.Where(u => u.ID == id)
    .Select(u => new {
        ID =  _fields.Contains['id'] ? u.ID : 0,
        FirstName = _fields.Contains['firstname'] ? u.FirstName : null,
        LastName = _fields.Contains['lastname'] ? u.LastName : null,
        otherName = _fields.Contains['othername'] ? u.otherName : null
    })
    .ToList();

I also would put the field names in a HashSet<string> for a better performance.

var _fields = new HashSet<string>(fields.Split(',').Select(f => f.ToLower().Trim()));

This solution keeps all the properties but sets the unwanted ones to null. If you want to dynamically add properties, see this other SO question: How to dynamically create a class in C#?. But note that this only useful in scenarios where objects of this type are processed dynamically as well.

is't this going to return all fields but with null if not chosen? — 0x2bad, Nov 28 '17 at 18:47
Yes. You cannot have a type with variable fileds unless using some heavy reflection stuff. Even if it is an anonymous type. See SO question [How to dynamically create a class in C#?](https://stackoverflow.com/q/3862226/880990). — Olivier Jacot-Descombes, Nov 28 '17 at 18:51

score 0 · Accepted Answer · answered Feb 02 '18 at 20:00

I was finally able to do this with minimal work. assuming the filter is a string list. string array.

so to avoid reflection and all that jazz, I iterate over each record and see if the variable is in the filter list, then create a dic entry with (var,val) assuming that no duplicate var in the same record, which can be catch if you want but I don't have this issue. Then at the end add that dic to a list. the method accept anonymous type list and a filter list.

public static List<Dictionary<string, object>> filteredList(IEnumerable source, string[] filter)
    {
        var filteredList = new List<Dictionary<string, object>>();
        foreach (var single in source)
        {
            var type = single.GetType();
            var props = type.GetProperties();
            var singleRecord = new Dictionary<string, object>();
            foreach (var v in props)
            {
                if (filter.Contains(v.Name))
                {
                    var tempValue = type.GetProperty(v.Name).GetValue(single, null);
                    singleRecord.Add(v.Name, tempValue);
                }

            }
            filteredList.Add(singleRecord);
        }
        return filteredList;
    }

how to return selected columns based on user choice

2 Answers2