使用c＃从字符串转换日期和/或时间从字符串转换时失败，以便检查SQL Server数据库中的表

Question

I am beginner in C# and I am developing an import system. In the code below I am trying to check if the date in the imported Excel file corresponds to the first or the second half year. Before the actual import, all registrations need to be deleted. The code works fine but when I am executing the delete command I get an error:

Conversion failed when converting date and/or time from character string

Please help!

PS: I have read the other posts about this error but I couldn't identify similarities with my code.

Thank you!

string input = (string)table.Rows[2][27];
string neededDate = input.Substring(29);
string getmonth = input.Substring(32, 2);
string getyear = input.Substring(35, 4);

if (int.Parse(getmonth) <= 6 && int.Parse(getyear) == DateTime.Now.Year)
{
    var date1 = DateTime.Today;
    var year = date1.Year;

    DateTime start1 = new DateTime(year, 1, 1);
    DateTime end1 = new DateTime(year, 6, 30);

    db.ExecuteCommand("Delete from Table where Date between '" + start1 + "' AND '" + end1 +"'");                  
}
else if (int.Parse(getmonth) > 6 && int.Parse(getyear) == DateTime.Now.Year)
{
    var date2 = DateTime.Today;
    var year = date2.Year;

    DateTime  start2 = new DateTime(year, 7, 1);
    DateTime  end2 = new DateTime(year, 12, 31);

    db.ExecuteCommand("Delete from Table where Date  between '" + start2 + "' AND '" + end2 + "'");                     
}
else
{
    Console.Write("");
}

Answer 1

A couple of issues and observations for you.

Why are you casting the value (row 2, column 27) to a string? Is it not a DateTime value already? If not, it probably should. This would remove the need for you to manually parse the data. By manually parsing the data, you are not taking into account different DateTime formats. eg

DateTime input = DateTime.Parse(table.Rows[2][27].ToString());

You can check for null values using:

table.Rows[2].IsNull(27);

Is column 27 the [Date] column you reference in your delete statement? If it's a string, then you'll have to cast the column and the values to DateTime types to use the BETWEEN syntax.

When dealing with dates, try specify the correct format SQL Server and .Net have different date formats by default.

You should also use parameterized SQL, not string concatenation as it leaves you potentially open to sql injection.

DELETE FROM [Table] WHERE [Date] BETWEEN @StartDate AND @EndDate

Answer 2

Its probably due to a wrong date format used. I believe that using .ToString("yyyy-MM-dd") on the dates will work for you.

db.ExecuteCommand("Delete from Table where Date between '" + start1.ToString("yyyy-MM-dd")
                   + "' AND '" +  end1.ToString("yyyy-MM-dd") +"'");

But, using these kind of queries is considered bad-practice and can lead to sql-injection (although not in your specific case because the data is not coming from the users).

You should start using parameterized queries instead:

SqlConnection con = new SqlConnection(connectionString);

var command = con.CreateCommand();
command.CommandText = "Delete from Table where Date between @start AND @end";
command.Parameters.Add("@start", SqlDbType.DateTime).Value = start1;
command.Parameters.Add("@end", SqlDbType.DateTime).Value = end1;
command.ExecuteNonQuery();