C#NullReferenceException未被用户代码处理

Question

任何有关这方面的帮助将不胜感激.每当我运行我的程序时,我得到一个NullReferenceException未被用户代码处理.

因此,当我启动程序时,我会看到一个表单,我应该从名单列表中进行选择,并能够更新详细信息或删除它们,但下拉列表中没有任何内容.

我得到空引用:

comm.Parameters["@EmployeeID"].Value =
employeesList.SelectedItem.Value;

这是CS文件中的选择按钮:

protected void selectButton_Click(object sender, EventArgs e)
{
    // Declare objects
    SqlConnection conn;
    SqlCommand comm;
    SqlDataReader reader;
    // Read the connection string from Web.config
    string connectionString =
        ConfigurationManager.ConnectionStrings[
        "HelpDesk"].ConnectionString;
    // Initialize connection
    conn = new SqlConnection(connectionString);
    // Create command
    comm = new SqlCommand(
        "SELECT FirstName, LastName, Username, Address, City, County, Post Code, " +
        "HomePhone, Extension, MobilePhone FROM Employees " +
        "WHERE EmployeeID = @EmployeeID", conn);
    // Add command parameters
    comm.Parameters.Add("@EmployeeID", System.Data.SqlDbType.Int);
    comm.Parameters["@EmployeeID"].Value =
        employeesList.SelectedItem.Value;
    // Enclose database code in Try-Catch-Finally
    try
    {
        // Open the connection
        conn.Open();
        // Execute the command
        reader = comm.ExecuteReader();
        // Display the data on the form
        if (reader.Read())
        {
            firstnameTextBox.Text = reader["FirstName"].ToString();
            lastnameTextBox.Text = reader["lastName"].ToString();
            userNameTextBox.Text = reader["Username"].ToString();
            addressTextBox.Text = reader["Address"].ToString();
            cityTextBox.Text = reader["City"].ToString();
            countyTextBox.Text = reader["county"].ToString();
            postcodeTextBox.Text = reader["Post Code"].ToString();
            homePhoneTextBox.Text = reader["HomePhone"].ToString();
            extensionTextBox.Text = reader["Extension"].ToString();
            mobilePhoneTextBox.Text = reader["MobilePhone"].ToString();
        }
        // Close the reader 
        reader.Close();
        // Enable the Update button
        updateButton.Enabled = true;
        // Enable the Delete button
        deleteButton.Enabled = true;
    }
    catch
    {
        // Display error message
        dbErrorLabel.Text =
            "Error loading the employee details!<br />";
    }
    finally
    {
        // Close the connection
        conn.Close();
    }
}

这是aspx文件中的代码:

<p>
<asp:Label ID="dbErrorLabel" ForeColor="Red" runat="server" />
Select an employee to update:<br />
<asp:DropDownList ID="employeesList" runat="server" />
<asp:Button ID="selectButton" Text="Select" runat="server"
    onclick="selectButton_Click" />

Answer 1

把断点放在:

comm.Parameters["@EmployeeID"].Value =
        employeesList.SelectedItem.Value;

当你运行程序时检查以下值:

employeesList.SelectedItem

如果没有选择任何项目,则很可能为空.在使用之前尝试检查空值:

 if (employeesList.SelectedItem != null)
 {
comm.Parameters["@EmployeeID"].Value =
    employeesList.SelectedItem.Value;
 }
 else
 {
// Handle this case
 }

希望这可以帮助!