我应该使用isDbNull吗？如何在这种特殊情况下使用它？
我应该使用isDbNull吗？如何在这种特殊情况下使用它？
提问于 2018-09-05 07:59:25
我知道Null的值不能转换成字符串值，所以我在网上到处寻找，确实找到了一些答案，比如使用IsDBNull()，但我似乎不能让它工作。我是新手，所以请耐心听我说。我只需要将可为空的字段作为""填充到适当的TextBox中，当它为Null时。如果你能帮我找出合适的语法，那就太好了。可能是Null (或不是)的字段是projects_project_number，我返回到我的原始代码，以显示我开始的内容。这就是它：
ConnectionStringSettings conSettings = ConfigurationManager.ConnectionStrings["shopmanagerConnectionString1"];
MySqlConnection con = new MySqlConnection(conSettings.ToString());
string Query = "select * from shopmanager.quotes where idquotes = @search_quote_number;";
MySqlCommand cmdDataBase = new MySqlCommand(Query, con);
MySqlDataReader myReader;
try
{
con.Open();
cmdDataBase.Parameters.AddWithValue("@search_quote_number", search_quote_number.Text);
myReader = cmdDataBase.ExecuteReader();
while (myReader.Read())
{
string sClient_number = myReader.GetString("client_info_client_number");
string sId = myReader.GetInt16("idquotes").ToString();
string sClientName = myReader.GetString("client_name");
string spredicted_start_date = myReader.GetString("predicted_start_date");
string sdate_required = myReader.GetString("requested_date");
string sdate_predicted = myReader.GetString("delivery_expected_date");
string sdate_received = myReader.GetString("date_received");
string squote_amount = myReader.GetString("quote_amount");
string sproject_number = myReader.GetString("projects_project_number");
quote_id.Text = sId;
client_name.Text = sClientName;
predicted_start_date.Text = spredicted_start_date;
date_required.Text = sdate_required;
date_predicted.Text = sdate_predicted;
date_received.Text = sdate_received;
quote_amount.Text = squote_amount;
project_number.Text = sproject_number;
temp_client_id.client_id = sClient_number;
search_quote_number.Text = "";
}
cmdDataBase.Parameters.Clear();
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
con.Close();回答 2
Stack Overflow用户
回答已采纳
发布于 2018-09-05 08:15:44
要使用DBNull，请遵循MSDN上关于Is.DBNull()的指导
使用DBNull有多种方法，但使用您在问题中给出的方法，最好的用法是：
string sproject_number = string.Empty;
if (!Convert.IsDBNull(myReader.GetString("projects_project_number"))) //check if value is not null
sproject_number = myReader.GetString("projects_project_number");
project_number.Text = sproject_number;您还可以使用SqlDataReader .IsDBNull的内置函数
int columnIndex = myReader.GetOrdinal("projects_project_number"); //get column index
string sproject_number = string.Empty;
if (!myReader.IsDBNull(columnIndex)) //check if value is not null
sproject_number = myReader[columnIndex].ToString();
project_number.Text = sproject_number;为了完整性：
int columnIndex = myReader.GetOrdinal();
string sproject_number = string.Empty;
if (DBNull.Value.Equals(myReader.GetString("projects_project_number"))) //check if value is not null
sproject_number = myReader.GetString("projects_project_number");
project_number.Text = sproject_number;评估原始帖子中关于直接赋值到文本框并整理所有内容的评论(并将其添加到using块中是很好的做法)：
ConnectionStringSettings conSettings = ConfigurationManager.ConnectionStrings["shopmanagerConnectionString1"]
string Query = "select * from shopmanager.quotes where idquotes = @search_quote_number;";
using(MySqlConnection con = new MySqlConnection(conSettings.ToString()))
{
using(MySqlCommand cmdDataBase = new MySqlCommand(Query, con))
{
cmdDataBase.Parameters.AddWithValue("@search_quote_number", search_quote_number.Text.Trim());
con.Open();
var myReader = cmdDataBase.ExecuteReader();
if(myReader.Read())
{
temp_client_id.client_id = myReader.GetString("client_info_client_number");
quote_id.Text = myReader.GetInt16("idquotes").ToString();
client_name.Text = myReader.GetString("client_name");
predicted_start_date.Text = myReader.GetString("predicted_start_date");
date_required.Text = sdate_required = myReader.GetString("requested_date");
date_predicted.Tex = myReader.GetString("delivery_expected_date");
date_received.Text = myReader.GetString("date_received");
quote_amount.Text = myReader.GetString("quote_amount");
project_number.Text = DBNull.Value.Equals(myReader.GetString("projects_project_number")) ? "" : myReader.GetString("projects_project_number");
search_quote_number.Text = "";
}
}
}Stack Overflow用户
发布于 2018-09-05 08:16:47
您可以这样做：
string sproject_number = DBNull.Value.Equals(myReader.GetString("projects_project_number")) ? "" : myReader.GetString("projects_project_number");或者，您可以使用如下SO Post中的扩展方法
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原文链接：
https://stackoverflow.com/questions/52175357
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