Question

我有一个方法，并试图弄清楚如何处理使用null参数调用该方法。

public static List<Incident> GetIncidentByTechnician(int techID)
    {
        List<Incident> incidents = new List<Incident>();

            Incident inc = null; // found incident
                                 // define connection
            SqlConnection connection = TechSupportDB.GetConnection();

            // define the select query command
            string selectQuery = "select IncidentID, CustomerID, ProductCode, TechID, DateOpened, DateClosed, Title, Description " +
                                 "from Incidents " +
                                 "where TechID = @TechID";

        SqlCommand selectCommand = new SqlCommand(selectQuery, connection);
            selectCommand.Parameters.AddWithValue("@TechID", techID);
            try
            {   
                // open the connection
                connection.Open();

                // execute the query
                SqlDataReader reader = selectCommand.ExecuteReader();

                // process the result while there are incidents
                while (reader.Read())
                {
                    inc = new Incident();
                    inc.IncidentID = (int)reader["IncidentID"];
                    inc.CustomerID = (int)reader["CustomerId"];
                    inc.ProductCode = reader["ProductCode"].ToString();
                    inc.TechID = (int)reader["TechID"];
                    inc.DateOpened = (DateTime)reader["DateOpened"];
                    inc.DateClosed = (DateTime?)reader["DateClosed"];
                    inc.Title = reader["Title"].ToString();
                    inc.Description = reader["Description"].ToString();
                    incidents.Add(inc); //add to the list
                }
            }
            catch (Exception ex)
            {
                throw ex; // let the form handle it
            }
            finally
            {
                connection.Close(); // close connecto no matter what
            }
        return incidents;
    }

需要＆＃34; techID＆＃34; integer作为参数，并使用该技术人员ID填充事件列表。但是，某些事件的techID值为null。我不希望将它们添加到我的列表中。

我尝试了一个if（techID！= null），但是它给出了一个错误，说它永远都是真的。我认为我的sql语句不好，因为＆＃34;其中TechID = null＆＃34;不会返回结果，它应该是＆＃34;其中TechID为null＆＃34;在sql server management studio中测试此语句时。有任何处理这些空值的建议吗？我的事故课也如下所示。我将techID设置为接受int值的空值。

    public class Incident
{
    public Incident() { }
    public int IncidentID { get; set; }
    public int CustomerID { get; set; }
    public string ProductCode { get; set; }
    public int? TechID { get; set; }
    public DateTime DateOpened { get; set; }
    public DateTime? DateClosed { get; set; }
    public string Title { get; set; }
    public string Description { get; set; }
}

Answer 1

您应该添加另一个条件TechID IS NOT NULL来检查SELECT查询中的不存在，例如

SELECT IncidentID, CustomerID, ProductCode, TechID, DateOpened, DateClosed, 
Title, Description from Incidents 
where TechID = @TechID AND TechID IS NOT NULL;

Answer 2

怎么样

while (reader.Read())
{
    if (reader["Tech"] != null)
    {
        inc = new Incident();
        inc.IncidentID = (int)reader["IncidentID"];
        inc.CustomerID = (int)reader["CustomerId"];
        inc.ProductCode = reader["ProductCode"].ToString();
        inc.TechID = (int)reader["TechID"];
        inc.DateOpened = (DateTime)reader["DateOpened"];
        inc.DateClosed = (DateTime?)reader["DateClosed"];
        inc.Title = reader["Title"].ToString();
        inc.Description = reader["Description"].ToString();
        incidents.Add(inc); //add to the list
    }
}

如果您的值为空，则不要创建或添加Incident对象...

Answer 3

根据你的函数原型，techID是一个整数，不可为空：

public static List<Incident> GetIncidentByTechnician(int techID)

这意味着SQL调用永远不会包含空@techID，因为它是从非可空techID分配的：

selectCommand.Parameters.AddWithValue("@TechID", techID);

因此，您的select语句应仅返回具有非null techID的行。毕竟它必须匹配。

select IncidentID, CustomerID, ProductCode, TechID, DateOpened, DateClosed, Title, Description 
from Incidents
where TechID = @TechID

因此，读者不应该返回空techID：

inc.TechID = (int)reader["TechID"];

鉴于您提供的代码示例，null TechID值不应成为问题。您不必添加if或修改where子句来处理空techID值，因为c＃代码不允许这些值。

您已尝试按原样运行代码？

使用（有时）null参数调用方法来执行SQL语句。 C＃/ ASP.NET

3 个答案: