C#comboBox如何让下拉菜单中有数据库的某列多种选项

数据库中booktype表，有三个字段，自增长的编号id，int类型；名称name，nvarchar（20）类型。

我的combobox数据如下：
private void book__Load(object sender, EventArgs e)
{
//打开数据库
string con = "Data Source=(local);Initial Catalog=LBOS;Integrated Security=True";
SqlConnection sqlcon = new SqlConnection(con);
sqlcon.Open();

string sql = "select bookTypeName from bookType";
SqlCommand b = new SqlCommand(sql, sqlcon);
SqlDataReader A = b.ExecuteReader();
while (A.Read())
{
comboBox1.Items.Add(A[1].ToString());
A.Close();
}
}
我这样写了combobox下拉就只有一个选项 其他的都看不到

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private void book__Load(object sender, EventArgs e)
{
/族亮肢/打开数据库
string con = "键孝Data Source=(local);Initial Catalog=LBOS;Integrated Security=True";
SqlConnection sqlcon = new SqlConnection(con);
sqlcon.Open();

string sql = "select bookTypeName from bookType"兆世;
SqlCommand b = new SqlCommand(sql, sqlcon);
SqlDataReader A = b.ExecuteReader();
while (A.Read())
{
comboBox1.Items.Add(A[1].ToString());
}
A.Close();
}

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把A.Close();这一句删掉试试

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