Xamarin custom UITableViewCell throws System NullReferenceException

问题

I'm creating a Xamarin app for iOS and I've added a UITableViewCell to a storyboard to give it my own style. I did add a class to this custom UITableViewCell, namely MainMenuCell. I added two labels to the cell and connected them with the MainMenuCell.h file, resulting in the following code:

MainMenuCell.cs

using System;
using Foundation;
using UIKit;

namespace MyProjectNamespace
{
    public partial class MainMenuCell : UITableViewCell
    {
        public MainMenuCell (IntPtr handle) : base (handle)
        {
        }

        public MainMenuCell () : base ()
        {
        }

        public void SetCellData()
        {
            projectNameLabel.Text = "Project name";
            projectDateLabel.Text = "Project date";
        }
    }
}

MainMenuCell.h (auto-generated):

using Foundation;
using System.CodeDom.Compiler;

namespace MyProjectNamespace
{
[Register ("MainMenuCell")]
partial class MainMenuCell
{
    [Outlet]
    UIKit.UILabel projectDateLabel { get; set; }

    [Outlet]
    UIKit.UILabel projectNameLabel { get; set; }

    void ReleaseDesignerOutlets ()
    {
        if (projectNameLabel != null) {
            projectNameLabel.Dispose ();
            projectNameLabel = null;
        }

        if (projectDateLabel != null) {
            projectDateLabel.Dispose ();
            projectDateLabel = null;
        }
    }
}
}

Now I have my UITableViewSource here, and I'm trying to initialize the MainMenuCell from the GetCell method:

using System;
using UIKit;
using Foundation;

namespace MyProjectNamespace
{
public class MainMenuSource : UITableViewSource
{
    public MainMenuSource ()
    {

    }

    public override nint NumberOfSections (UITableView tableView)
    {
        return 1;
    }

    public override string TitleForHeader (UITableView tableView, nint section)
    {
        return "Projects";
    }

    public override nint RowsInSection (UITableView tableview, nint section)
    {
        return 1;
    }

    public override UITableViewCell GetCell (UITableView tableView, NSIndexPath indexPath)
    {
        MainMenuCell cell = new MainMenuCell();
        cell.SetCellData ();
        return cell;
    }
}
}

However, it keeps throwing me a System.NullReferenceException at the line:

projectNameLabel.Text = "Project name";

It says: object reference not set to an instance of an object.

What am I missing here? Any help would be highly appreciated.

回答1:

You're almost there – instead of creating a new cell by yourself, let iOS do the work and dequeue the result.

public override UITableViewCell GetCell(UITableView tableView, NSIndexPath indexPath)
{
    var cell = (MainMenuCell)tableView.DequeueReusableCell("MainMenuCell");
    cell.SetCellData();

    return cell;
}

Note, that "MainMenuCell" is a identifier for the dynamic prototype cell from the Storyboard, you can name it whatever you want, but it has to be the same withing Storyboard and your Datasource.

来源：https://stackoverflow.com/questions/29430879/xamarin-custom-uitableviewcell-throws-system-nullreferenceexception

标签

uitableview

xamarin.ios

Xamarin

nullreferenceexception