使用AutocompleteStringCollection时C＃无法将对象类型数组转换为字符串类型数组

我想使用使用数据库的texbox自动完成功能。数据库查询使用来自三个表的join语句返回名称列表。现在我在使用.ToArray（）时遇到了erorr：

错误：“无法将源类型Purchase []转换为目标类型字符串[]”

这是我的代码：

PurchaseManager _aPurchaseManager = new PurchaseManager();

string[] productNameFromJoinedDatabase = _aPurchaseManager.GetProductNameWithCategoryAndBrandName();

productNameTextBox.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
productNameTextBox.AutoCompleteSource = AutoCompleteSource.CustomSource;


var autoComplete = new AutoCompleteStringCollection();
autoComplete.AddRange(productNameFromJoinedDatabase);
productNameTextBox.AutoCompleteCustomSource = autoComplete;

更新：

使用后：

       string[] productNameFromJoinedDatabase = _aPurchaseManager.GetProductNameWithCategoryAndBrandName().Select(x => x.ProductName).ToArray();

先前的错误已消失。但这不适用于自动完成功能！

这是我的购买课程：

class Purchase
{
    public Int64 PurchaseId { get; set; }
    public string ProductName { get; set; }
    public double Price { get; set; }
    public int Quantity { get; set; }
    public double TotalPurchasePrice { get; set; }
    public DateTime DateTime { get; set; }

    public CategoryEntry CategoryEntry { get; set; }
    public BrandEntry BrandEntry { get; set; }
    public Product Product { get; set; }
    public Purchase()
    {
        CategoryEntry = new CategoryEntry();
        BrandEntry = new BrandEntry();
        Product = new Product();
    }
}

这是我的数据库类：

public List<Purchase> GetProductNameWithCategoryAndBrandName()
    {

        List<Purchase> _aPurchaseList = new List<Purchase>();

        _connection.Open();
        string query = string.Format("SELECT CategoryEntryTable.Name, BrandEntryTable.Name, ProductNameExtentionEntryTable.ProductNameExtention " +
                                     "FROM ProductNameExtentionEntryTable " +
                                     "JOIN CategoryEntryTable " +
                                     "ON ProductNameExtentionEntryTable.CategoryId = CategoryEntryTable.CategoryId " +
                                     "JOIN BrandEntryTable " +
                                     "ON ProductNameExtentionEntryTable.BrandId = BrandEntryTable.BrandId");
        _command = new SqlCommand(query, _connection);
        SqlDataReader aReader = _command.ExecuteReader();

        if (aReader.HasRows)
        {
            while (aReader.Read())
            {
                _aPurchase = new Purchase();
                _aPurchase.CategoryEntry.Name = (aReader[0]).ToString();
                _aPurchase.BrandEntry.Name = (string)aReader[1];
                _aPurchase.Product.ProductNameExtention = (string)aReader[2];

                _aPurchaseList.Add(_aPurchase);
            }
        }
        _connection.Close();
        return _aPurchaseList;
    }