C# &SQL Server:如果字符串值为空，如何在命令参数中插入 DBNull.Value? [英] C# & SQL Server : how to Insert DBNull.Value in command parameter if string value is empty?

问题描述

我已经搜索了几个小时，但找不到解决方案.

I've been searching for a couple of hours now and cannot find a solution.

我将一些字符串插入到 SQL 中，但有时，我用来执行此操作的方法可能包含空字符串，即 ""

I am inserting some strings into SQL, however on on occasions, the method I use to do this may contain strings that are empty, i.e ""

因此，我想在 SQL Server 中插入一个空值.

I therefore want to insert a null value into SQL Server instead.

首先，我测试我的方法以确保我能够使用以下内容手动插入 DBNull.Value:

Firstly I test my method to make sure I am able to insert a DBNull.Value manually by using the following:

cmd.Parameters.AddWithValue("@surname", DBNull.Value);  // THIS WORKS OK

然而，当我尝试以下操作时，该方法运行时没有任何错误异常，但 SQL Server 列不显示空值，它只是空的，没有任何内容:

HOWEVER when I try the following below, the method runs without any error exception BUT the SQL Server column does not show a null value, it's just empty with nothing in it:

cmd.Parameters.AddWithValue("@surname", (object)surname ?? DBNull.Value);

我只想要一个有效的解决方案，而不需要处理任何更常见的类扩展和编写大量额外的代码.我在论坛上看到的每一个答案都太复杂了，这本该是一个简单的任务.

I just want an efficient solution without messing about with any more common class extensions and writing lots of extra code. Every answer I see on the forums is just over complicated for what should be such a simple task.

有什么想法吗?提前致谢...

Any ideas? Thanks in advance...

这里是完整的方法:

public static void AddUserToUserTable(string username, 
        string forename, 
        string surname, 
        string emailAddress, 
        string password, 
        string accountActive, 
        string userGroup)
{
    string cmdText1 = "INSERT INTO Users (Username, Forename, Surname, EmailAddress, Password, AccountActive, UserGroup) VALUES (@username, @forename, @surname, @emailAddress, @password, @userGroup, @accountActive)";

    try
    {
        // The using statement will take care of the disposing of the reader and the command object.
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            SqlCommand cmd = new SqlCommand(cmdText1, connection);

            cmd.Parameters.AddWithValue("@username", username);
            cmd.Parameters.AddWithValue("@forename", forename);
            cmd.Parameters.AddWithValue("@surname", surname, null);
            cmd.Parameters.AddWithValue("@emailAddress", emailAddress);
            cmd.Parameters.AddWithValue("@password", password);
            cmd.Parameters.AddWithValue("@userGroup", accountActive);
            cmd.Parameters.AddWithValue("@accountActive", userGroup);

            connection.Open();
            cmd.ExecuteNonQuery();

            log.Info("NEW User [" + username + "] Created");
        }
    }
    catch (Exception ex)
    {
        log.Error(ex);
    }
}

推荐答案

1) 最简单的方案大家都已经提过:

1) The simplest solution everyone has already mentioned:

cmd.Parameters.AddWithValue("@surname", 
    String.IsNullOrEmpty(surname) ? DBNull.Value : surname);

<小时>

2) 如果你可以修改数据库本身，你可以添加一个触发器来在 INSERT 和 上用 NULL 替换空字符串>UPDATE 操作.如果有其他开发人员和/或应用程序更改数据库，这具有确保一致性的优势.

---

2) If you can modify the database itself, you could add a trigger to replace empty strings with NULL on INSERT and UPDATE operations. This has an advantage of ensuring consistency if there are other developers and/or applications altering the database..

CREATE TRIGGER User_ReplaceEmptyWithNull
    ON 
        Users 
    AFTER 
        INSERT, 
        UPDATE 
    AS
        UPDATE 
            Users
        SET 
            Users.Forename = IIF(inserted.Forename != '', inserted.Forename, NULL),
            Users.Surname = IIF(inserted.Surname != '', inserted.Surname, NULL)
        FROM 
            inserted INNER JOIN Users
                ON inserted.Username = Users.Username

免责声明:我不是数据库触发器方面的专家.我根据另一个SO问题的答案

3) 您可以为 String 对象创建一个扩展方法.

3) You could make an extension method for String objects.

namespace YOUR_NAMESPACE
{
    public static class MyExtensions
    {
        public static object OrDBNull( this String value )
        {
            return String.IsNullOrEmpty(value) ? DBNull.Value : value;
        }
    }   
}

...

cmd.Parameters.AddWithValue("@surname", surname.OrDBNull());

<小时>

4) 您可以为 SqlParameterCollection 对象创建一个扩展方法.

---

4) You could make an extension method for SqlParameterCollection objects.

namespace YOUR_NAMESPACE
{
    public static class MyExtensions
    {
        public static void AddString( this SqlParameterCollection collection, string parameterName, string value )
        {
            collection.AddWithValue(parameterName, String.IsNullOrEmpty(value) ? DBNull.Value : value);
        }
    }   
}

...

cmd.Parameters.AddString("@surname", surname);

免责声明:未经测试.我可能在某个地方搞砸了.

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