c# – 从资产中读取文本文件的问题(xamarin)

发布时间：2020-12-15 23:31:54 所属栏目：百科来源：网络整理

导读：我从资产目录中读取简单的txt有问题.我无法弄清楚它为什么不起作用以及出了什么问题. 这是我写的方法代码： private string ReadFile(){ var stream = Assets.Open ("sampleText.txt"); StreamReader sr = new StreamReader (stream); string text = sr.Read

我从资产目录中读取简单的txt有问题.我无法弄清楚它为什么不起作用以及出了什么问题.

这是我写的方法代码：

private string ReadFile(){
        var stream = Assets.Open ("sampleText.txt");
        StreamReader sr = new StreamReader (stream);
        string text = sr.ReadToEnd ();
        sr.Close ();
        return text;
    }

这是错误：

Java.Lang.NullPointerException: Exception of type 'Java.Lang.NullPointerException' was thrown.
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/data/lanes/2970/46c3f7e0/source/mono/external/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143 
at Android.Runtime.JNIEnv.CallNonvirtualObjectMethod (IntPtr jobject,IntPtr jclass,IntPtr jmethod) [0x00084] in /Users/builder/data/lanes/2970/46c3f7e0/source/monodroid/src/Mono.Android/src/Runtime/JNIEnv.g.cs:596 
at Android.Content.ContextWrapper.get_Assets () [0x0005f] in /Users/builder/data/lanes/2970/46c3f7e0/source/monodroid/src/Mono.Android/platforms/android-19/src/generated/Android.Content.ContextWrapper.cs:154

我在Windows上运行我的整个应用程序没有问题,只是我在android上运行它有问题.尝试过各种各样的东西,比如AssetManager的实例,但是效果不好.

文本文件标记为AndroidAsset.

这是我的TextChangeBench：

public class TextChangeBench : Activity
{
    public TextChangeBench (){}

    private void SaveFile(string[] tab){

        string saveLocation = "sampleTextReworked.txt";
        StreamWriter sw = new StreamWriter (saveLocation);
        foreach (string s in tab) {
            sw.Write (s);
        }
        sw.Close ();
    }

    private string ReadFile(){
        var stream = Assets.Open ("sampleText.txt");
        StreamReader sr = new StreamReader (stream);
        string text = sr.ReadToEnd ();
        sr.Close ();
        return text;
    }
    public void ChangeText(){
        try{
            File.Delete("sampleTextReworked.txt");
        }catch(FileNotFoundException e){Console.WriteLine (e);}

        try{
            string text = ReadFile ();
            char c;
            string[] newTab = new string[text.Length];
            for (int i = 0; i < text.Length; i++)
            {
                c = (char)text [i];
                if (Char.IsUpper(c))
                {
                    newTab[i] = text[i].ToString().ToLower();
                }
                else if (Char.IsLower(c))
                {
                    newTab[i] = text[i].ToString().ToUpper();
                }
                else
                {
                    newTab[i] = text[i].ToString();
                }
            }
            SaveFile(newTab);
        }
        catch(Exception e){Console.WriteLine ("{0} ",e);}
    }
}

这是我的MainActivity：

[Activity (Label = "csBench",MainLauncher = true,Icon = "@mipmap/icon")]
public class MainActivity : Activity
{
    protected override void OnCreate (Bundle savedInstanceState)
    {
        base.OnCreate (savedInstanceState);

        SetContentView (Resource.Layout.Main);
        Button startButton = FindViewById<Button> (Resource.Id.start);
        startButton.Click += delegate {

            MathBench mb = new MathBench ();
            TextChangeBench tcb = new TextChangeBench ();
            PassedTime pt = new PassedTime ();
            EditText et = (EditText)FindViewById (Resource.Id.textInfo);

            for (int i = 0; i < 5; i++) {
                pt.StartMeasuring ();
                //mb.Silnia (25);
                //mb.Fibonacci (32);
                //mb.BubbleSort ();
                tcb.ChangeText ();
                if (i == 4) {
                    pt.StopMeasuring ();

                    et.SetText(pt.ReturnResult(),TextView.BufferType.Normal);
                    //et.SetText(pt.ReturnResult().ToString());
                    //Console.WriteLine(pt.ReturnResult ());
                    pt.ResetTimers ();
                }
            }  
        };

    }
}

欢迎任何帮助,提前谢谢.

解决方法

发生此错误的原因是手动创建活动实例(如TextChangeBench)会绕过Android框架设置过程,从而使活动处于无效状态.

活动用作用户交互点,将视图/屏幕绑定到应用程序中的逻辑.由于TextChangeBench仅实现应用程序逻辑,因此不需要从Activity派生,并且更适合作为普通的C#类.

删除继承为activity以简化TextChangeBench的代码.这意味着我们无法再访问Assets属性,因此无法检索资产.我们可以通过使用全局应用程序上下文来解决此问题：

var stream = Android.App.Application.Context.Assets.Open("sampleText.txt");

删除活动继承和使用全局上下文访问资产管理器的组合将修复Java.Lang.NullPointerException.

最终的代码如下所示：

public class TextChangeBench 
{
    private void SaveFile(string[] tab)
    {

        string saveLocation = "sampleTextReworked.txt";
        StreamWriter sw = new StreamWriter(saveLocation);
        foreach (string s in tab)
        {
            sw.Write(s);
        }
        sw.Close();
    }

    private string ReadFile()
    {
        var stream = Android.App.Application.Context.Assets.Open("sampleText.txt");

        StreamReader sr = new StreamReader(stream);
        string text = sr.ReadToEnd();
        sr.Close();
        return text;
    }
    public void ChangeText()
    {
        try
        {
            File.Delete("sampleTextReworked.txt");
        }
        catch (FileNotFoundException e) { Console.WriteLine(e); }

        try
        {
            string text = ReadFile();
            char c;
            string[] newTab = new string[text.Length];
            for (int i = 0; i < text.Length; i++)
            {
                c = (char)text[i];
                if (Char.IsUpper(c))
                {
                    newTab[i] = text[i].ToString().ToLower();
                }
                else if (Char.IsLower(c))
                {
                    newTab[i] = text[i].ToString().ToUpper();
                }
                else
                {
                    newTab[i] = text[i].ToString();
                }
            }
            SaveFile(newTab);
        }
        catch (Exception e) { Console.WriteLine("{0} ",e); }
    }
}

看到：

> Xamarin Toast Message error (C#)
> How to use the method of an Activity in a DialogFragment?

（编辑：李大同）

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