Xamarin FindViewById 返回 null [英] Xamarin FindViewById returns null

问题描述

我正在尝试使用 Xamarin so C# 制作一个 android 应用程序.我做了两个布局，在每个布局中我都做了两个按钮来在它们之间导航.我试过这样:

I am trying to make an android app using Xamarin so C#. I made two layouts and in each one of them I made tow buttons to navigate between them.I tried like this:

using System;

using Android.App;
using Android.OS;
using Android.Views;
using Android.Widget;


namespace Example
{
    [Activity(Label = "Example", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            this.SetContentView(Resource.Layout.Main);

            this.FindViewById<Button>(Resource.Id.ForwardButton).Click += this.Forward;
            this.FindViewById<Button>(Resource.Id.BackButton).Click += this.Back;
        }

        public void Forward(object sender, EventArgs e)
        {
            this.SetContentView(Resource.Layout.Main2);
        }

        public void Back(object sender, EventArgs e)
        {
            this.SetContentView(Resource.Layout.Main);
        }
    }
}

但是每次当我启动应用程序时，我都会得到这个错误:System.NullReferenceException has been throw.Object reference not set to an instance of an object.有什么建议或更好的主意吗?

But every time when I start the app I get this errror: System.NullReferenceException has been thrown.Object reference not set to an instance of an object. Any advice or better idea?

推荐答案

您正在将 Main 设置为您的活动的布局，然后在以下代码行中您要求找到一个名为的按钮Back 在运行时不属于此布局的一部分.这意味着以下行将返回 null:

You are setting Main as the layout for your activity and then in the following lines of code you are asking to find a button named Back at runtime which is not part of this layout. This means the following line will return null:

FindViewById<Button>(Resource.Id.BackButton)

现在如果你执行FindViewById(Resource.Id.BackButton).Click，你肯定会得到一个System.NullReferenceException.

Now if you do FindViewById<Button>(Resource.Id.BackButton).Click, you will definitely get a System.NullReferenceException.

鉴于评论，以下是您应该做的事情以实现您的目标:

In view of the comments, here is what you should do to achieve what you are looking for:

创建两个不同的活动(Main1 和 Main2).在 Main1 中:

Create two different activities (Main1 and Main2). In Main1 you do:

    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        this.SetContentView(Resource.Layout.Main);

        this.FindViewById<Button>(Resource.Id.ForwardButton).Click += this.Forward;
    }

    public void Forward(object sender, EventArgs e)
    {
        this.StartActivity (typeof(Main2));
    }

然后在 Main2 中:

    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        this.SetContentView(Resource.Layout.Main2);

        this.FindViewById<Button>(Resource.Id.BackButton).Click += this.Back;
    }

    public void Back(object sender, EventArgs e)
    {
        this.StartActivity (typeof(Main));
    }

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