Question

I have the following statement:

var itemsFromList = from item in ListItems 
                    where item.Countries != null 
                    select item;

So I can return all the values that are present in countries (some are empty).
in the same list I have other columns for instance cities and I have to change it to:

var itemsFromList = from item in ListItems 
                    where item.cities != null 
                    select item;

Is there a way to use the same statement to return either the cities or countries that are not empty by using a variable a bit like this:

var itemsFromList = from item in ListItems 
                    where item.variable != null 
                    select item;

Answer 1

It looks to me like Variable is meant to be just that, a variable, that is defined at a later date. If you really want to have your expression be modifiable like that, your best bet is to use Reflection.

First, you’ll need to get a reference to the PropertyInfo of the desired property. You can do this by calling Type.GetProperty(string name). Once you have a reference to the PropertyInfo, you can get the value of a specific instance by calling PropertyInfo.GetValue(Object obj, Object[] index).

Here is an example of creating a LINQ query that will get only items where the specified property is not null.

// Declare this as a Generic method of Type T so that we can pass in a
// List containing anything and easily get the appropriate Type object
public static IEnumerable<T> SelectNonNull<T>(
    IEnumerable<T> ListItems, string propertyName)
{
    IEnumerable<T> itemsFromList;
    // Get a reference to the PropertyInfo for the property
    // we're doing a null-check on.
    PropertyInfo variable = typeof(T).GetProperty(propertyName);
    if (variable == null)
    {
        // The property does not exist on this item type:
        // just return all items
        itemsFromList = from item in ListItems
                        select item;
    }
    else
    {
        itemsFromList = from item in ListItems
                        // GetValue will check the value of item's
                        // instance of the specified property.
                        where variable.GetValue(item, null) != null
                        select item;
    }
    return itemsFromList;
}

To get the results in your question, you could then use this function like so:

var NonNullCountries = SelectNonNull(ListItems, "Countries");
var NonNullCities = SelectNonNull(ListItems, "cities");

Alternately, we could declare this as an extension method (like the other Linq methods), like so:

public static IEnumerable<T> SelectNonNull<T>(
    this IEnumerable<T> source,
    string propertyName)
{
    PropertyInfo variable = typeof(T).GetProperty(propertyName);
    if (variable == null)
    {
        // Specified property does not exist on this item type:
        //just return all items
        return from item in source
                select item;
    }
    else
    {
        return from item in source
                where variable.GetValue(item, null) != null
                select item;
    }
}

We could then chain multiple calls together. For instance, if you wanted to filter out all entries where “cities” AND “Countries” are null, you could just use the following:

var NonNullCitiesOrCountries = ListItems.SelectNonNull("Countries")
                                        .SelectNonNull("cities");

Note: SelectNonNull just returns an IEnuerable. You will still need to enumerate over it to get at the results of the query.