如何使用日期时间处理 asp.net 中的 null 值？

Question

this is my code to save the student record,

int x = clsFunctions.InsertStudent(txtStudent.Text,Convert.ToDateTime(txtContractDate.Text))

and the function definition is,

public static int InsertStudent(string Name, DateTime ContractDate)
{
int isaved = 0;
SqlParameter[] sqlprm = new SqlParameter[2];

sqlprm[0] = new SqlParameter("@Name", SqlDbType.VarChar, 100);
sqlprm[0].Value = Name;

sqlprm[1] = new SqlParameter("@ContractDate", SqlDbType.Date);
sqlprm[1].Value = ContractDate;

isaved = db.ExecuteNonQueryProcedure("ProcedureName", sqlprm);
return isaved;
}

db.ExecuteNonQueryProcedure is the function to call ExecuteNonQuery()...

My question is that when I enter the value in ContractDate field, it works fine, the data is saved in database.. But when I left it empty, I got an error "String was not recognized as a valid DateTime". The ContractDate field in a table in database is 'Allow null'. How to handle this null value through this code as ContractDate field is not mandatory on the form?

Answer 1

System.DateTime is a struct, which is non-nullable. I suggest you use String.IsNullOrEmpty(txtContractDate.Text) before attempting to use Convert.ToDateTime so you can know if a blank string is passed in or not and change the parameter DateTime ContractDate to DateTime? ContractDate DateTime? ContractDate which allows you to pass in null. See MSDN Nullable .

DateTime? contractDate = null; 
if (!String.IsNullOrEmpty(txtContractDate.Text))
    contractDate = Convert.ToDateTime(txtContractDate.Text);
int x = InsertStudent(txtStudent.Text, contractDate);

Edit: I made a small mistake with ??, the operator expects the same types on both sides. You'll need to use more verbose code. I've ran a test successfully on SQL Server Express 2014 LocalDB using code similar as below.

Finally, you need to handle the null case of contractDate when you create your SqlParameter using DBNull.Value as such:

if (ContractDate != null)
    sqlprm[1].Value = ContractDate.Value;
else
    sqlprm[1].Value = DBNull.Value;

Answer 2

<asp:TextBox ID="TextBox1" TextMode="Date" Text='<%# Eval("ColumnName").ToString() == "" ? Eval("ColumnName") : Convert.ToDateTime(Eval("ColumnName").ToString()).ToString("yyyy-MM-dd") %>' runat="server" CssClass="form-control"></asp:TextBox>