根据其他下拉列表中的值下拉显示值c＃

Question

我有两个下拉列表，第一个显示不同种类的动物，我希望第二个显示品种，具体取决于所选择的物种。

在我的数据库中，所有品种的品种都有匹配的ID。

首次下拉代码：

protected void Page_Load(object sender, EventArgs e)
{
    DropDown_Species();
}

public void DropDown_Species()
{
    if (!Page.IsPostBack)
    {

        MySqlCommand sql_country = new MySqlCommand("SELECT Species FROM breed", cs);
        //connection string opend
        cs.Open();

        MySqlDataReader ddlvalue;
        ddlvalue = sql_country.ExecuteReader();

        petSpecies.DataSource = ddlvalue;
        petSpecies.DataValueField = "Species";
        petSpecies.DataTextField = "Species";
        petSpecies.DataBind();
        petSpecies.Items.Insert(0, "Select Species");
        // connection string closed
        cs.Close();
        cs.Dispose();
    }
}

第二次下拉代码：

protected void petsBreed_SelectedIndexChanged(object sender, EventArgs e)
 {
    if (petSpecies.Text != string.Empty)
    {
        MySqlCommand cd = new MySqlCommand(string.Format(
             "SELECT * FROM (Breed.breed)", petSpecies.Text), cs);
        cs.Open();
        MySqlDataReader petsSpecies = cd.ExecuteReader();
        petsBreed.DataSource = petsSpecies;
        petsBreed.DataValueField = "Breed";
        petsBreed.DataTextField = "Breed";
        petsBreed.DataBind();
        petsBreed.Items.Insert(0, "Select Breed");
        cs.Close();
        cs.Dispose();
    }

我很确定问题出在第二次下拉。

对C＃来说很新。

有人能告诉我问题出在哪里和/或如何使这个想法发挥作用？

Answer 1

你正在使用string.Format但没有使用那里的参数......？

MySqlCommand cd = new MySqlCommand(string.Format(
         "SELECT * FROM (Breed.breed)", petSpecies.Text), cs);

你不应该使用字符串格式，如：

 var use_it_properly = string.Format ("Hi {0}, now you're using it properly", name);

否则，你不会转发输入，在你的情况下应该是所选择的物种？

---

在MSDN上阅读更多内容：String.Format Method。

该网站的示例：

DateTime dat = new DateTime(2012, 1, 17, 9, 30, 0); 
string city = "Chicago";
int temp = -16;
string output = String.Format("At {0} in {1}, the temperature was {2} degrees.",
                              dat, city, temp);
Console.WriteLine(output);
// The example displays the following output: 
//    At 1/17/2012 9:30:00 AM in Chicago, the temperature was -16 degrees. 

Answer 2

我不熟悉MySQL。以下是您可以使用SqlDataSource执行的操作。

数据库（表名为品种）

BreedId    Species    Breed
1          dog        Alsatian
2          dog        pitbull
3          dog        Shetland sheepdog
4          dog        Boxer
5          cat        Dragon Li
6          cat        Australian Mist
7          cat        Korat

截图

ASPX

<asp:DropDownList ID="DropDownListSpecies" runat="server" DataSourceID="Species"
    DataTextField="Species" DataValueField="Species" AutoPostBack="True">
</asp:DropDownList>
<asp:SqlDataSource ID="Species" runat="server"
    ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
    SelectCommand="SELECT DISTINCT [Species] FROM [Breed]"></asp:SqlDataSource>

<asp:DropDownList ID="DropDownListBreed" runat="server"
   DataSourceID="breed" DataTextField="Breed" DataValueField="Breed">
</asp:DropDownList>
<asp:SqlDataSource ID="breed" runat="server"
    ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
    SelectCommand="SELECT DISTINCT [Breed] FROM [Breed] WHERE Species=@Species">
    <SelectParameters>
        <asp:ControlParameter ControlID="DropDownListSpecies" 
            PropertyName="SelectedValue" Name="Species" Type="String" 
           DefaultValue="cat" />
    </SelectParameters>
</asp:SqlDataSource>

确保AutoPostBack="True"

DropDownListSpecies