Question

我正在尝试根据组合框中选择的项目填充文本框。问题是它在加载时抛出以下错误，我在visual studio中按next .. next。它做了我真正想做的事情。

enter image description here 如何解决负载问题。

表格加载的代码是

private void UpdateProduct_Load(object sender, EventArgs e)
        {
            DataSet ds = GetAllItems();
            comboBox2.DataSource = ds.Tables[0];
            comboBox2.DisplayMember = "Product Name";
            comboBox2.SelectedIndex = 0;


        }

组合框的选定索引的代码是

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
           // string selectedText = this.comboBox2.GetItemText(this.comboBox2.SelectedItem);
            DataSet d = GetProductInfo(comboBox2.Text);
            if (d.Tables.Count > 0)
            {
                textBox2.Text = d.Tables[0].Rows[0]["Quantity"].ToString();
                textBox3.Text = d.Tables[0].Rows[0]["Color"].ToString();
                textBox4.Text = d.Tables[0].Rows[0]["Size"].ToString();
                textBox5.Text = d.Tables[0].Rows[0]["Price"].ToString();
            }

        }

第一次加载表单时我只有问题。

GetProductInfo代码

  public DataSet GetProductInfo(string product)
        {
            DataSet dataSet = new DataSet();
            OleDbConnection oleConn = new OleDbConnection(connString);

            try
            {
                oleConn.Open();
                string sql = "SELECT [Quantity], [Color], [Size], [Price] FROM [Product] WHERE [Product Name]= '" + product + "'";
                OleDbDataAdapter dataAdapter = new OleDbDataAdapter(sql, oleConn);
                dataAdapter.Fill(dataSet, "Product");
            }
            catch (Exception ex)
            {
                MessageBox.Show("An exception has been occured\n" + ex.ToString());
                Console.WriteLine(ex.ToString());
            }
            finally
            {
                oleConn.Close();
            }
            if (dataSet.Tables["Product"].Rows.Count <= 0)
                return null;

            return dataSet;
        }

堆栈跟踪

System.NullReferenceException occurred
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Purchase Management
  StackTrace:
       at Purchase_Management.UpdateProduct.comboBox2_SelectedIndexChanged(Object sender, EventArgs e) in c:\Users\Amrit\Desktop\Purchase Management\Purchase Management\UpdateProduct.cs:line 99
  InnerException:

Answer 1

使用 SelectionChangeCommitted 事件，而不是使用SelectedIndexChanged。这样可以避免所有这些问题，因为它只会在用户更改所选项目时触发，而不是在初始化组合框时触发。

Answer 2

您的方法返回null。这就是抛出错误的原因

DataSet dstProduct = new DataSet();
dstProduct = GetProductInfo(comboBox2.Text);

然后尝试

if(dstProduct.Tables.Count>0 && dstProduct.Tables[0].Rows.Count >0)
{
 // then do stuff. 
}

Answer 3

因为.Count在d = null的情况下触发了异常。

声明DataSet d = new DataSet();

Answer 4

只需添加if (!string.IsNullOrEmpty(productName))语句，您就可以了：

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
    var productName = comboBox2.Text;
    if (!string.IsNullOrEmpty(productName)) {

        DataSet d = GetProductInfo(productName);
        if (d.Tables.Count > 0)
        {
            textBox2.Text = d.Tables[0].Rows[0]["Quantity"].ToString();
            textBox3.Text = d.Tables[0].Rows[0]["Color"].ToString();
            textBox4.Text = d.Tables[0].Rows[0]["Size"].ToString();
            textBox5.Text = d.Tables[0].Rows[0]["Price"].ToString();
        }
    }
}

由于组合框选择了索引，表单加载出错

4 个答案: