using System;

using System.Data;

using System.Collections;

using System.Linq;

using System.Data.DataSetExtensions;

​

public class Program

{

    public static void Main()

    {

​

                DataTable dt = new DataTable();

                dt.Columns.Add("Col0", typeof(string));

                dt.Columns.Add("Col1", typeof(bool)); //nie ustawisz tutaj bool?

                dt.Rows.Add(new Object[] { "1", true }); 

                dt.Rows.Add(new Object[] { "2", false });

                dt.Rows.Add(new Object[] { "3", false });

                dt.Rows.Add(new Object[] { "5", null });

                var onlyTrues = dt.AsEnumerable().Where(w => w.Field<bool?>("Col1") == true); //a tutaj bez bool? nie przepuszcza

        foreach(DataRow row in onlyTrues)

        {

            Console.WriteLine(row[0].ToString());

        }

    }

}