实例化时“无法将对象从DBNull强制转换为其他类型”

Question

代码：

public List<ProductDTO> GetAllProducts()
{
    try
    {
        using (SqlConnection con = new SqlConnection(connectionString))
        {
            string query = "SELECT * FROM [dbo].[Product]";

            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                con.Open();
                SqlDataReader reader = cmd.ExecuteReader();
                List<ProductDTO> allProducts = new List<ProductDTO>();

                while (reader.Read())
                {
                    ProductDTO product = new ProductDTO
                    {
                        ProductID = Convert.ToInt32(reader["ProductID"]),
                        SellerID = Convert.ToInt32(reader["UserID"]),
                        Name = reader["Name"].ToString(),
                        Price = Convert.ToDecimal(reader["Price"]),
                        Photo = reader["Photo"].ToString(),
                        Likes = Convert.ToInt32(reader["Likes"]), //error occurs only when Likes is not commented out
                        Description = reader["Description"].ToString(),
                        Category = (Enums.ProductCategory.Category)reader["CategoryID"] - 1
                    };

                    allProducts.Add(product);
                }

                return allProducts;
            }
        }
    }
    catch (Exception)
    {
        return new List<ProductDTO>();
    }
}

我的问题是：在实例化ProductDTO时如何进行Object cannot be cast from DBNull to other types这样的检查？