如何捕获空参数并允许在ADO.NET中使用DBNull进行解析

Question

So I am passing these 18 parameters to my DAL and then inserting or updating my Access database.

I have 2 fields that are DateTime and one that is and int . Both DateTime and the int parameters are 'not required', which means that can be empty strings.

I've have a problem when I pass my txtBirthDate.Text, txtHireDate.Text, and txtReportsTo.Text that if they have been left null, my DAL screams at me with a FormatException that can't get past parsing a null element.

So my problem right now is, I'm unsure where to Parse the DateTime and int variable to allow my Parameters to accept them. I am also unsure how to set these variables to DBNull.Value that are have been past through to my DAL as empty strings.

Here is a sample of my DAL method call:

rowsAdded = ((DataAccessLayer)Application["dbAccess"]).insertEmployees(txtLname.Text, txtFname.Text, txtTitle.Text, 
         txtCourt.Text,txtBday.Text,txtHireDate.Text, txtAddress.Text, txtCity.Text,txtRegion.Text,txtPostalCode.Text,
         txtCountry.Text, txtHomePhone.Text, txtExtension.Text, upPhoto.FileName.ToString(),txtNotes.Text,txtReports.Text,txtPassword.Text);

Here is my DAL method:

 public int insertEmployees(string lname, string fname, string title, string toc, string birth, string hire, string address, string city,
    string region,string postal,string country, string phone, string ext,string photo, string notes, string report, string pass)
{
    string last = lname;
    string first = fname;
    string tlt = title;
    string tOfc = toc;
    string addy = address;
    string town = city;
    string reg = country;
    string phum = phone;
    string exten = ext;
    string rep = report;// THIS IS Int
    string pas = pass;
    string pc = postal;
    string note = notes;
    string regions = region;
    string hD = hire;// THIS IS DATETIME
    string bD = birth;// THIS IS DATETIME
    string pho = photo;
    int rows = 0;
        dbString = "INSERT INTO [Employees] ([LastName],[FirstName],[Title],[TitleOfCourtesy],[BirthDate],[HireDate],[Address],[City],[Region],[PostalCode]," +
        "[Country],[HomePhone],[Extension],[Photo],[Notes],[ReportsTo],[Password]) VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)";
        string queryLast = "Select @@Identity";

        conn.Open();
        oleCommand = new OleDbCommand(dbString, conn);

        oleCommand.Parameters.Add(new OleDbParameter("@LastName", OleDbType.VarChar, 20, ParameterDirection.Input, false, 10, 0, "LastName", DataRowVersion.Original, null)).Value = last;
        oleCommand.Parameters.Add(new OleDbParameter("@FirstName", OleDbType.VarChar, 10, ParameterDirection.Input, false, 10, 0, "FirstName", DataRowVersion.Original, null)).Value = first;
        oleCommand.Parameters.Add(new OleDbParameter("@Title", OleDbType.VarChar, 30, ParameterDirection.Input, true, 10, 0, "Title", DataRowVersion.Original, null)).Value = tlt ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@TitleOfCourtesy", OleDbType.VarChar, 25, ParameterDirection.Input, true, 10, 0, "TitleOfCourtesy", DataRowVersion.Original, null)).Value = tOfc ?? (object)DBNull.Value;
//DATE  oleCommand.Parameters.Add(new OleDbParameter("@BirthDate", OleDbType.DBDate, 20, ParameterDirection.Input, true, 10, 0, "BirthDate", DataRowVersion.Original, null)).Value = (object)bD ?? DBNull.Value;
//DATE  oleCommand.Parameters.Add(new OleDbParameter("@HireDate", OleDbType.DBDate, 20, ParameterDirection.Input, true, 10, 0, "HireDate", DataRowVersion.Original, null)).Value = (object)hD ?? DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@Address", OleDbType.VarChar, 60, ParameterDirection.Input, true, 10, 0, "Address", DataRowVersion.Original, null)).Value = addy ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@City", OleDbType.VarChar, 15, ParameterDirection.Input, true, 10, 10, "City", DataRowVersion.Original, null)).Value = phum ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@Region", OleDbType.VarChar, 15, ParameterDirection.Input, true, 10, 0, "Region", DataRowVersion.Original, null)).Value = regions ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@PostalCode", OleDbType.VarChar, 10, ParameterDirection.Input, true, 10, 0, "PostalCode", DataRowVersion.Original, null)).Value = pc ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@Country", OleDbType.VarChar, 15, ParameterDirection.Input, true, 10, 0, "Country", DataRowVersion.Original, null)).Value = reg ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@HomePhone", OleDbType.VarChar, 24, ParameterDirection.Input, true, 10, 0, "HomePhone", DataRowVersion.Original, null)).Value = phum ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@Extension", OleDbType.VarChar, 4, ParameterDirection.Input, true, 10, 0, "Extension", DataRowVersion.Original, null)).Value = exten ?? (object)DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@Photo", OleDbType.VarChar, 255, ParameterDirection.Input, true, 10, 0, "Photo", DataRowVersion.Original, null)).Value = pho ?? (object)DBNull.Value; ;
        oleCommand.Parameters.Add(new OleDbParameter("@Notes", OleDbType.VarWChar, 255, ParameterDirection.Input, true, 10, 0, "Notes", DataRowVersion.Original, null)).Value = note ?? (object)DBNull.Value;
  //INT oleCommand.Parameters.Add(new OleDbParameter("@ReportsTo", OleDbType.Integer, 2, ParameterDirection.Input, true, 10, 0, "ReportsTo", DataRowVersion.Original, null)).Value = (object)rep ?? DBNull.Value;
        oleCommand.Parameters.Add(new OleDbParameter("@Password", OleDbType.VarChar, 255, ParameterDirection.Input, false, 10, 0, "Password", DataRowVersion.Original, null)).Value = pas ?? (object)DBNull.Value;
        rows = (int)oleCommand.ExecuteNonQuery();
        oleCommand.CommandText = queryLast;

Answer 1

for string values you can use this expression:

oleCommand.Parameters.Add(new OleDbParameter("@LastName", OleDbType.VarChar, 20, 
                                             ParameterDirection.Input, false, 10, 
                                             0, "LastName", 
                                             DataRowVersion.Original, null)
                         ).Value = String.IsNullOrEmpty(last) ? DBNull.Value : last;

Repeat it for the rest of string values