我有一個(gè)問題，如果我通過 a 搜索數(shù)據(jù)TextBox并且數(shù)據(jù)庫中不存在該數(shù)據(jù)，我會(huì)收到錯(cuò)誤消息“對(duì)象不能從 DBNull 轉(zhuǎn)換為其他類型?！蔽以噲D讓自己MessageBox看起來像是在說數(shù)據(jù)不存在并且無法弄清楚如何做到這一點(diǎn)。我嘗試使用一個(gè)if語句，如果TextBox等于DBNull則MessageBox出現(xiàn) a。這不是t work and im not too sure why. The error occurs with me trying to **equal** toDBNull . How do i produce aTextBox` 說數(shù)據(jù)不存在嗎？{ SqlConnection con = new SqlConnection("***COnString**"); con.Open(); SqlCommand comm = new SqlCommand("SELECT SUM (Total_Hours_Day) FROM Sign_In_Out_Table, User_Table WHERE User_Table.FirstName = '" + Search_Username_Alerts_Admin_txtbox.Text + "' AND Sign_In_Out_Table.eb_number = User_Table.eb_number AND Date between GETDATE()-14 and GETDATE()", con); decimal TotalHoursFortnight = Convert.ToDecimal(comm.ExecuteScalar()); con.Close(); decimal sum = 0; sum += Convert.ToDecimal(TotalHoursFortnight); if (Search_Username_Alerts_Admin_txtbox.Text == DBNull) { MessageBox.Show("No Data Exists"); } else { MessageBox.Show(Search_Username_Alerts_Admin_txtbox.Text + ":" + Environment.NewLine + " Hours Worked = " + TotalHoursFortnight, ("Working Info Admin"), MessageBoxButtons.OK, MessageBoxIcon.Information); }}我的預(yù)期結(jié)果是出現(xiàn)一個(gè)消息框，提示搜索時(shí)數(shù)據(jù)不存在。但如果確實(shí)如此，數(shù)據(jù)將顯示在消息框中。
3 回答
慕的地10843
TA貢獻(xiàn)1785條經(jīng)驗(yàn) 獲得超8個(gè)贊
您有3 個(gè)案例需要實(shí)施：
根本沒有數(shù)據(jù)：檢查
null無效數(shù)據(jù)，例如
1 + 2 + NULL + 3 == NULL：檢查DBNull.Value有效數(shù)據(jù)，例如
1 + 2 + 3 == 6：借助Convert.ToDecimal()
代碼：
// wrap IDisposable into using
using (SqlConnection con = new SqlConnection("***COnString**")) {
con.Open();
//DONE: Make sql readable
//DONE: Make sql parametrized
//TODO: you may want to change eb_number = eb_number into INNER JOIN
string sql =
@"SELECT SUM (Total_Hours_Day)
FROM Sign_In_Out_Table,
User_Table
WHERE User_Table.FirstName = @prm_FirstName
AND Sign_In_Out_Table.eb_number = User_Table.eb_number
AND Date BETWEEN GETDATE() - 14 AND GETDATE()";
using (SqlCommand comm = new SqlCommand(sql, con)) {
//TODO: Better specify RDBMS type explictly with "comm.Parameters.Add(...)"
comm.Parameters.AddWithValue(
"@prm_FirstName", Search_Username_Alerts_Admin_txtbox.Text);
var result = comm.ExecuteScalar();
if (null == result) { // No Data
MessageBox.Show("No Data Exist");
}
else if (DBNull.Value == result) { // We have the Data and it's RDBMS Null
MessageBox.Show("Data Exist, but not valid.");
}
else { // We have a valid Decimal
Decimal sum = Convert.ToDecimal(result);
//TODO: put the relevant code here
}
}
}
縹緲止盈
TA貢獻(xiàn)2041條經(jīng)驗(yàn) 獲得超4個(gè)贊
我會(huì)使用 decimal.tryparse 來查看是否返回了數(shù)據(jù)
SqlConnection con = new SqlConnection("***COnString**");
con.Open();
SqlCommand comm = new SqlCommand("SELECT SUM (Total_Hours_Day) FROM Sign_In_Out_Table, User_Table WHERE User_Table.FirstName = '" + Search_Username_Alerts_Admin_txtbox.Text + "' AND Sign_In_Out_Table.eb_number = User_Table.eb_number AND Date between GETDATE()-14 and GETDATE()", con);
string TotalHoursFortnight = (comm.ExecuteScalar()).ToString();
con.Close();
decimal sum = 0;
decimal temp;
if(!decimal.TryParse(TotalHoursFortnight, out temp))
{
MessageBox.Show("No Data Exists");
}
else
{
sum += temp;
MessageBox.Show(Search_Username_Alerts_Admin_txtbox.Text + ":" + Environment.NewLine + " Hours Worked = " + TotalHoursFortnight, ("Working Info Admin"), MessageBoxButtons.OK, MessageBoxIcon.Information);
}
一只斗牛犬
TA貢獻(xiàn)1784條經(jīng)驗(yàn) 獲得超2個(gè)贊
您需要為查詢結(jié)果檢查 null
var result = comm.ExecuteScalar();
if(result != null){
decimal TotalHoursFortnight = Convert.ToDecimal(comm.ExecuteScalar());
// move remain code to if block
}
并且if (Search_Username_Alerts_Admin_txtbox.Text == DBNull)應(yīng)該改為
if (Convert.IsDBNull(Search_Username_Alerts_Admin_txtbox.Text){
}
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