如果返回值为null，则程序崩溃

时间：2010-01-13 00:21:08

标签： c# mysql mono

我试图从mysql（c＃，mono）中检索一些数据，问题是如果返回的值为null，程序崩溃，我知道它必须对返回的null值做一些事情，因为如果我想要检索数据库中有效的东西，任何人都可以帮我解决这个问题吗？

代码：

MySqlConnection dbcon;
dbcon = new MySqlConnection(conString);
try
{
    dbcon.Open();
}
catch (Exception)
{
    Console.WriteLine("MySQL Database Connection Problem !!!");

}

//reading data from mysql   
MySqlCommand dbcmd = new MySqlCommand(sql, dbcon);

MySqlDataReader reader = dbcmd.ExecuteReader();

while(reader.Read()){

    txtFirstname.Text = reader["first_name"].ToString();
    txtLastname.Text  = reader["last_name"].ToString();
    imgUser.File      = path+reader["photo"].ToString();
    expDate           = reader["expiration_datetime"].ToString();
    usrName           = reader["username"].ToString();

}

dbcon.Close();

4 个答案:

答案 0 :(得分：2)

您正在对MySQL返回的对象调用ToString方法。

如果MySQL返回null，您将在ToString对象上调用null方法，并提供NullReferenceException。

假设SQL实际上是返回字符串（而不是数字或日期），您可以简单地转换为字符串，如下所示：(string)reader["username"]。
如果SQL返回非字符串数据类型，则可以调用Convert.ToString(reader["username"])。

如果MySQL返回null，这两个都会导致null。

如果您希望MySQL null导致null以外的其他内容，请使用null合并运算符，如下所示：Convert.ToString(reader["username"]) ?? "It's Null!"。

答案 1 :(得分：1)

例如，让我们假设您的“first_name”在数据库中为空; reader["first_name"]将返回null，但它没有.ToString()方法，因此您的代码失败。

下面，我改变了你获取文件的方式;我尝试将其强制转换为字符串，如果我得到null值，则使用??（null-coalescing运算符）返回空字符串。

txtFirstname.Text = reader["first_name"  ] as string ?? "";
txtLastname.Text  = reader["last_name"   ] as string ?? "";
imgUser.File      = path + reader["photo"] as string ?? "";
expDate           = reader["expiration_datetime"] as string ?? "";
usrName           = reader["username"] as string ?? "";

HTH

答案 2 :(得分：0)

根据评论中的例外情况，您似乎在其他地方遇到了完全不同的问题。

我猜您正在读取.resources文件并从SQL查询生成文件名。如果是这样，null可能会导致您生成错误的文件名。

您需要调试代码。

答案 3 :(得分：0)

所以我想这将有助于发布整个代码，就在这里。我只是想通过扫描打印在卡片上的条形码来激活ldap中的帐户。我还从mysql数据库中检索有关该帐户用户的信息。

<小时/> 

using System;
using System.Collections;
using Gtk;
using MySql.Data.MySqlClient;
using Novell.Directory.Ldap;



public partial class MainWindow: Gtk.Window
{

    public MainWindow (): base (Gtk.WindowType.Toplevel)
    {
        Build ();
    }


protected void OnDeleteEvent (object sender, DeleteEventArgs a)
{
    Application.Quit ();
    a.RetVal = true;
}

protected virtual void OnEntry1FocusOutEvent (object o, Gtk.FocusOutEventArgs args)
{
    string conString = "server=localhost;database=ldap;User ID=someid;password=somepassword;";
    string sql = String.Format("Select * From user where barcode='{0}'",txtBarcode.Text);
    string path = "/home/path/to/project/Photo/";
    string expDate = "";
    string usrName = "";
    //creating connection with database
    MySqlConnection dbcon;
    dbcon = new MySqlConnection(conString);
    try

    {

        dbcon.Open();

    }

    catch (Exception ex)

    {
        Console.WriteLine(ex.ToString());

        //Console.WriteLine("MySQL Database Connection Problem !!!");

    }
    //reading data from mysql
    MySqlCommand dbcmd = new MySqlCommand(sql, dbcon);

    MySqlDataReader reader = dbcmd.ExecuteReader();

    while(reader.Read()){


        txtFirstname.Text = Convert.ToString(reader["first_name"]);
        txtLastname.Text  = Convert.ToString(reader["last_name"]);
        imgUser.File      = path+Convert.ToString(reader["photo"]);
        expDate           = Convert.ToString(reader["expiration_datetime"]);
        usrName           = Convert.ToString(reader["username"]);

        }

    dbcon.Close();

    //changeing time from sting to datetime
    DateTime dt = Convert.ToDateTime(expDate);
    DateTime now = DateTime.Now;
    txtStatus.Text = dt.ToString();
    //checking if the account has expired
    if(dt > now){
        //connecting with ldap server
        string server = "ip of the server";
        string dn = "cn=Manager,dc=itc";
        string up = "password";
        string dn1 = "uid="+usrName+",ou=people,dc=itc";

        // Create a new LdapConnection instance
        LdapConnection connl = new LdapConnection();

        // Connect to the server
        connl.Connect(server, 389);

        //Bind
        connl.Bind(dn,up);

        //Modify user in ldap entry
        ArrayList modList = new ArrayList();
        LdapAttribute attribute;
        //Replace the value of loginshell attribute
        attribute = new LdapAttribute( "loginShell", "/bin/sh");
        modList.Add( new LdapModification(LdapModification.REPLACE, attribute));

        LdapModification[] mods = new LdapModification[modList.Count];  
        //Type mtype=Type.GetType("Novell.Directory.LdapModification");
        mods = (LdapModification[])modList.ToArray(typeof(LdapModification));

        //Modify the entry in the directory
        connl.Modify ( dn1, mods );
        txtStatus.Text="Account Activated!";
        // Disconnect from server
        connl.Disconnect();
    }else{
        txtStatus.Text="Account Expired!";

    }
}
protected void OnDeleteEvent (object sender, DeleteEventArgs a) { Application.Quit (); a.RetVal = true; } protected virtual void OnEntry1FocusOutEvent (object o, Gtk.FocusOutEventArgs args) { string conString = "server=localhost;database=ldap;User ID=someid;password=somepassword;"; string sql = String.Format("Select * From user where barcode='{0}'",txtBarcode.Text); string path = "/home/path/to/project/Photo/"; string expDate = ""; string usrName = ""; //creating connection with database MySqlConnection dbcon; dbcon = new MySqlConnection(conString); try { dbcon.Open(); } catch (Exception ex) { Console.WriteLine(ex.ToString()); //Console.WriteLine("MySQL Database Connection Problem !!!"); } //reading data from mysql MySqlCommand dbcmd = new MySqlCommand(sql, dbcon); MySqlDataReader reader = dbcmd.ExecuteReader(); while(reader.Read()){ txtFirstname.Text = Convert.ToString(reader["first_name"]); txtLastname.Text = Convert.ToString(reader["last_name"]); imgUser.File = path+Convert.ToString(reader["photo"]); expDate = Convert.ToString(reader["expiration_datetime"]); usrName = Convert.ToString(reader["username"]); } dbcon.Close(); //changeing time from sting to datetime DateTime dt = Convert.ToDateTime(expDate); DateTime now = DateTime.Now; txtStatus.Text = dt.ToString(); //checking if the account has expired if(dt > now){ //connecting with ldap server string server = "ip of the server"; string dn = "cn=Manager,dc=itc"; string up = "password"; string dn1 = "uid="+usrName+",ou=people,dc=itc"; // Create a new LdapConnection instance LdapConnection connl = new LdapConnection(); // Connect to the server connl.Connect(server, 389); //Bind connl.Bind(dn,up); //Modify user in ldap entry ArrayList modList = new ArrayList(); LdapAttribute attribute; //Replace the value of loginshell attribute attribute = new LdapAttribute( "loginShell", "/bin/sh"); modList.Add( new LdapModification(LdapModification.REPLACE, attribute)); LdapModification[] mods = new LdapModification[modList.Count]; //Type mtype=Type.GetType("Novell.Directory.LdapModification"); mods = (LdapModification[])modList.ToArray(typeof(LdapModification)); //Modify the entry in the directory connl.Modify ( dn1, mods ); txtStatus.Text="Account Activated!"; // Disconnect from server connl.Disconnect(); }else{ txtStatus.Text="Account Expired!"; } }

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