将日期插入 SQL 数据库

所以我在做我的学校项目时遇到了另一个问题。仅供参考，我们不使用 sql 参数，并且还没有学习如何使用它们。

我正在尝试将生日插入sql数据库，但我尝试了所有方法，但总是存在数据类型不匹配。

你们能帮我吗（不改变代码的结构）？您可以通过搜索"生日"来查找它，因为其他所有内容都有德国名称。

我真的很感激你的帮助，因为我真的很绝望。编辑：有一个文本框，用户应该在其中输入生日。这就是我获得数据的地方。

编辑：我删除了所有其他不必要的字符串等。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data;
using System.Data.Common;
using System.Data.OleDb;
public class webUser
{
private DateTime _birthday;
public webUser()
{
    //
    // TODO: Add constructor logic here
    //
public DateTime birthday
{
    get { return _birhday; }
    set { _birthday= value; }
}
public bool checkUser(string eMail)
{
    string sql = "SELECT eMail, kennwort FROM Benutzerdatenbank WHERE eMail ='" + eMail + "'";
    string conStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + System.Web.HttpContext.Current.Server.MapPath("~/App_Data/Benutzerdatenbank.accdb");
    OleDbConnection con = new OleDbConnection(conStr);
    con.Open();
    OleDbDataAdapter da = new OleDbDataAdapter(sql, con);
    DataSet ds = new DataSet();
    da.Fill(ds);
    con.Close();
    if (ds.Tables[0].Rows.Count == 1)
        return true;
    else
        return false;
}
public bool addUser(string eMail, string kennwort, string vorname, string zuname, string telefonnummer, string strasse, string plz, string ort, string firma, string titel, DateTime birthday)
{
    if (this.checkUser(eMail) == true)
    {
        return false; 
    }
    else
    {
        string zeichen =   "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghjiklmnopqrstuvwxyz0123456789";
        string aktivierungscode = "";
        Random rnd = new Random();
        for (int i = 1; i < 62; i++)
        {
            aktivierungscode = aktivierungscode + zeichen.Substring(rnd.Next(0, zeichen.Length - 1), 1);
        }
        string sql = "INSERT INTO Benutzerdatenbank (eMail, kennwort, Titel, Vorname, Zuname, Firma, birthday, Telefonnummer, Strasse, PLZ, Ort, aktivierungscode) VALUES ('" +
        eMail + "','" + kennwort + "','" + titel + "','" + vorname + "','" + zuname + "','" + firma +  "','" + birthday+ "','" + telefonnummer + "','" + strasse + "','" + plz + "','" + ort + "','" + aktivierungscode + "');";

        string conStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + System.Web.HttpContext.Current.Server.MapPath("~/App_Data/Benutzerdatenbank.accdb");
        OleDbConnection con = new OleDbConnection(conStr);
        OleDbCommand cmd = new OleDbCommand(sql, con);
        con.Open();
        cmd.ExecuteNonQuery(); 
        con.Close();
        return true;
    }
}
public void ReadUser(string eMail, string kennwort)
{
    string sql = "SELECT * FROM Benutzerdatenbank WHERE eMail='" + eMail + "' AND kennwort ='" + kennwort + "'";
    string conStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + System.Web.HttpContext.Current.Server.MapPath("~/App_Data/Benutzerdatenbank.accdb");
    OleDbConnection con = new OleDbConnection(conStr);
    con.Open();
    OleDbDataAdapter da = new OleDbDataAdapter(sql, con);
    DataSet ds = new DataSet();
    da.Fill(ds);
    con.Close();
    if (ds.Tables[0].Rows.Count == 1)
    {
        this.eMail = (string)ds.Tables[0].Rows[0]["eMail"];
        this.vorname = (string)ds.Tables[0].Rows[0]["Vorname"];
        this.zuname = (string)ds.Tables[0].Rows[0]["Zuname"];
        this.telefonnummer = (string)ds.Tables[0].Rows[0]["Telefonnummer"];
        this.strasse = (string)ds.Tables[0].Rows[0]["strasse"];
        this.plz = (string)ds.Tables[0].Rows[0]["PLZ"];
        this.ort = (string)ds.Tables[0].Rows[0]["ORT"];
        this.titel = (string)ds.Tables[0].Rows[0]["Titel"];
        this.firma = (string)ds.Tables[0].Rows[0]["Firma"];
        this.birthday= Convert.ToDateTime(ds.Tables[0].Rows[0]["birthday"];
    }
    else
    {
        this.eMail = "";
        this.vorname = "";
        this.zuname = "";
    }
}

}