Update Sharepoint List Item

I got following error...

System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.SharePoint.SPListItem.get_UniqueId() at ConsoleApplication1.Program.Main(String[] args) in Program.cs:line 21

running following code

using (SPSite site = new SPSite("http://site/"))
{    
    using (SPWeb 开发者_如何学运维web = site.OpenWeb())
    {
        try
        {
            SPList list = web.Lists["ListName"]; // 2        
            SPListItem item = list.Items.Add();
            Guid itemId = item.UniqueId;
            SPListItem itemUpdate = web.Lists["ListName"].Items[itemId];
            itemUpdate["PercentComplete"] = .45; // 45%        
            itemUpdate.Update();
        }
        catch (Exception e)
        { 
            Console.WriteLine(e);
            Console.ReadLine();

        }
    }
}

What's the problem?

If you're trying to alter values for a just inserted list item, you should go with:

SPList list = web.Lists["ListName"];
//SPListItem item = list.Items.Add();
//item["PercentComplete"] = .45; // 45%
//item.Update();

SPListItemCollection items = list.GetItems(new SPQuery()
{
    Query = @"<Where>
                <Eq>
                   <FieldRef Name='Title' />
                   <Value Type='Text'>Desigining</Value>
                </Eq>
              </Where>"
});

foreach (SPListItem item in items)
{
    item["PercentComplete"] = .45; // 45%
    item.Update();
}

You just need to use list.Items[uniqueId] or faster list.GetItemByUniqueId(uniqueId) if you needs to find a particular item to update; what can be accomplished by using SPQuery class.

Ruben's answer was correct but was getting few errors (may be its was only for me) therefore i tweaked little bit and then it was working fine. Below is the code which i used if anyone needs it

 SPList list = web.Lists["ListName"];
                    //SPListItem item = list.Items.Add();
                    //item["PercentComplete"] = .45; 
                    // 45%//item.Update();
                   SPQuery oQuery = new SPQuery();

                        oQuery.Query = @"<Where>               
                                    <Eq>                   
                                        <FieldRef Name='Title' />                   
                                        <Value Type='Text'>Design</Value>              
                                    </Eq>            
                                  </Where>";
                        SPListItemCollection collListItems = list.GetItems(oQuery);
                        foreach (SPListItem item in collListItems)
                    {    item["PercentComplete"] = .55;   
                        item.Update();}

Try calling Update () on the list before getting the UniqueID

 SPList list = web.Lists["ListName"]; // 2        
 SPListItem item = list.Items.Add();
 item["Title"] = "Test";
 item.Update ();
 list.Update ();
 Guid itemId = item.UniqueId;

My best quess is that your item is not yet created in the list when you do:

Guid itemId = item.UniqueId;
SPListItem itemUpdate = web.Lists["ListName"].Items[itemId];

First do a item.Update() before requesting the uniqueId and/or getting the item back from a list.

PS : I see no reason why you should get a second SPItem object for updating the 'PercentComplete' information.