Dynamically build a int[] array from the database

I'm trying to build a int[] array from returned values from the database,

Normally for an int[] array that isn't database driven you would do something like this:

int[] arr1 = new int[] { 3, 4, 5 };

my public variable is as follows:

public int[] SelectedItems { get; set; } // Which is used for the multi select list box

I then when retrieving the values do something like this:

model.SelectedItems = (from x in collection select x.Id).ToArray();

but I get the error:

Cannot convert source type 'dynamic[]' to target type 'int[]'

I have googled this error but with no avail.

My collection looks like this:

public class UserActivities
{
    public int Id { get; set; }

    public string ActivityDesc { get; set; }
}

Update

I'm using dapper.net to retrieve the values so once I've made the call to the DB I then read it as follows:

var activities = sqlCon.Read().ToList();

I use this in two places, first place I display the string description which is shown on the page for the user, the Id's I try to reference and build an array which is used else where in the application.