I have this entities:

public class Parent: AllDependant
{
    /*Properties goes here*/
}

public class Children: AllDependant
{
    /*Properties goes here*/
}

Then I have allDependants variable with typeof List<AllDependant> which will keep some of parents and childrens entitiy in mix.

Later on, I want to select from them and do something like this:

var selectedDependantInfos = allDependants
        .Select(dependant =>
        {
            if (dependant is Parent)
            {
                var parent = dependant as Parent;
                return new { Name = parent.Name, SomeSpecialInfo = parent.ParentInfo };
            }
            else
            {
                var child = dependant as Children;
                return new { Name = child.Name, SomeSpecialInfo = child.ChildInfo }
            }
        });

Note the specific property for child and parent require me to cast and get the property to a new model for UI display which is not entity concern. I cannot put the special property in AllDependant base class because I need to refactor the property name on so many files including *.ascx which is troublesome. However it done by using the Linq Select extension method above but I just thinking of this:

Question: How can I do the same in Linq Query?

This will give error on the select keyword and the curly braces:

var selectedDependantInfos = from dependant in allDependants
                            select
                            {
                                /* the same if statement goes here */
                            }

• c#
• linq
• linq-to-objects

You would use the conditional operator and get something like

  from dependant in allDependants             
  select dependant is Parent 
         ? new { Name = (dependant as Parent).Name,  /* Parent fields */ }
         : new { Name = (dependant as Children).Name, /* Child fields */ }

But as you see that is not a great improvement. There's no convenient place to do the type-cast.

The better option would seem to move the Name and SpecialInfo properties to a base class (AllDependant or a special intermediate class).

An alternative would be:

var parents = allDependants.OfType<Parent>.Select(p => new { Name =  p.Name, .... };
var children = allDependants.OfType<Children>.Select(c => new { Name =  c.Name, .... };

var combined = parents.Concat(children);

The disadvantage of this approach would be that addDependants would be iterated over twice.

Another way of using Reflection

var selectedDependantInfos = from p in allDependants
                         let key = p is Parent ? "ParentInfo" : "ChildInfo"
                         select new { 
                             Name = p.GetType().GetProperty("Name").GetValue(p, null).ToString(), 
                             SomeSpecialInfo = p.GetType().GetProperty(key).GetValue(p, null).ToString() 
                         };