public class Group
{
    public int GroupId { get; set; }
    public int Parent { get; set; }
    public string GroupName { get; set; }
}
public class MSSQLcontroller
{
    private string connString;
    public MSSQLcontroller(string cs)
    {
        connString = cs;
    }
    public IEnumerable<object> Dapper_Select(string sql)
    {
        IEnumerable<object> retobj;
        using (SqlConnection cn = new SqlConnection(connString))
        {
            cn.Open();
            var o = cn.Query(sql);
            retobj = o;
            cn.Close();
        }        
        return retobj;
    }
}

var gr = mssql.Dapper_Select("Select GroupId, Parent, GroupName From Groups");
foreach (var g in gr)
{

     MessageBox.Show(((Group)g).GroupName);
}

public object Dapper_Select(string sql, object typeobj)
{
    object o=null;
    using (SqlConnection cn = new SqlConnection(connString))
    {
        cn.Open();
        o = cn.Query<typeobj>(sql);
        cn.Close();
    }
    return o;
}
вызов
List<Group> gr;
gr = (List<Group>)mssql.Dapper_Select("Select GroupId, Parent, GroupName From Groups", Group);

public object Dapper_Select<T>(string sql) where T : class
{
    object retobj = null;
    using (SqlConnection cn = new SqlConnection(connString))
    {
        cn.Open();
        retobj = cn.Query<T>(sql);
        cn.Close();
    }
    return retobj;
}

использование 
List<Group> gr = (List<Group>)mssql.Dapper_Select<Group>("Select GroupId, Parent, GroupName From Groups");
List<User> us = (List<User>)mssql.Dapper_Select<User>("Select UserId, Login, FirstName, Middlename, Surname From Users");

public List<T> Dapper_Select<T>(string sql) where T : class
{
    List<T> retobj = null;
    using (SqlConnection cn = new SqlConnection(connString))
    {
        cn.Open();
        retobj = (List<T>)cn.Query<T>(sql);
        cn.Close();
    }
    return retobj;
}