即制作一个wpf,其中一个组合框根据另一个组合框填充.但是,只有一个组合框填充.

这是我的代码如下.

 public partial class Form4 : Form
    {
        public Form4()
        {
            InitializeComponent();
            this.Load += Form4_Load;
        }

        string connstring = ("Server=localhost;Port=5432;User Id=postgres;Password=021393;Database=postgres;");

        private void Form4_Load(object sender, EventArgs e)
        {
            string query = "SELECT * FROM data_organsystem";            
            fillCombo(comboBox3, query, "name", "id");
            comboBox3_SelectedIndexChanged(null, null);
        }

        private void fillCombo(ComboBox combo, string query, string displayMember, string valueMember)
        {
            NpgsqlConnection conn = new NpgsqlConnection(connstring);
            NpgsqlCommand cmd = new NpgsqlCommand(query, conn);
            NpgsqlDataAdapter da = new NpgsqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);
            combo.DataSource = dt;             
            combo.DisplayMember = displayMember;            
            combo.ValueMember = valueMember;
        }

        private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
        {
            int val;
            Int32.TryParse(comboBox3.SelectedValue.ToString(), out val);
            string query = "SELECT * FROM data_symptom WHERE organ_system_id = " + val;
            fillCombo(comboBox4, query, "name", "id");
        }
    }
}

如果您对如何编辑此代码有任何想法,那将是一个很大的帮助.谢谢！

解决方法:

执行该行的方法comboBox3_SelectedIndexChanged时,您将收到错误

Int32.TryParse(comboBox3.SelectedValue.ToString(), out val);

因为如果在ComboBox中没有选择任何项目,comboBox3.SelectedValue为null,我在您的代码中没有看到您在第一次调用comboBox3_SelectedIndexChanged之前选择了一些项目.

因为没有显示在Form.Load事件处理程序异常中执行的方法comboBox3_SelectedIndexChanged.检查一下：https://*.com/a/3209813/1565525.
这就是为什么你没有得到任何错误

在使用之前,您需要检查SelectedValue是否为null

If(this.comboBox3.SelectedValue is null)
{
    this.comboBox4.DataSource = null; //Remove all items if nothing selected
}
else
{
    Int32 val= (Int32)this.ComboBox3.SelectedValue;
    string query = "SELECT id, name FROM data_symptom WHERE organ_system_id = " + val;
    fillCombo(this.comboBox4, query, "name", "id");
 }

因为在使用项填充ComboBox时使用DataBinding,所以逻辑上使用SelectedValueChanged事件处理程序

private void comboBox3_SelectedValueChanged(object sender, EventsArgs e)
{
    //same code
}