将数据发送给微信

var dbFile = Path.Combine(DBSetting.GetSetting().DBDirectory, $"{BLL.SelectProject.DBName}.db");
                        var sharefile = new Xamarin.Essentials.ShareFile(dbFile)
                        {
                            ContentType = "application/db"
                        };
                        await Share.RequestAsync(new ShareFileRequest
                        {
                            Title = $"分享项目文件{BLL.SelectProject.DBName}.db",
                            File = sharefile
                        });

接收微信分享的文件

在MainActivity.cs中

  //参考：https://blog.csdn.net/j5856004/article/details/102651886
    //参考：https://www.jianshu.com/p/2f2ffb6ec4bb
    //必须要单独写这个IntentFilter，否则桌面就没有图标了!!!!!重点中的重点
    [IntentFilter(new[] { Intent.ActionMain }, Categories = new[] { Intent.CategoryLauncher })]
    [IntentFilter(
        new[] { Intent.ActionView, Intent.ActionSend },// Intent.ActionView ，确保微信点击【使用第三方应用打开】时可以看到图标
        Categories = new[] { Intent.CategoryDefault },
        /*DataScheme = "file",*/ //如果QQ也要打开，这里要设置file
                                 //DataMimeType确保特定的文件，可以使用此APP打开
        /*DataMimeType = "application/octet-stream"*/ //设置了application/octet-stream，就可以接.db.octet-stream文件
        DataMimeType = "text/comma-separated-values" //*/*表示接收所有文件
        )]
    //这个是项目默认的，不用改
    [Activity(Label = "FmosMobile", Icon = "@mipmap/fmos", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {

     }

在MainActivity的OnResume方法重写中，加入：

   protected override void OnResume()
   {
            base.OnResume();
            this.SaveFile(Intent);
   }

 public static PYLMath.CommonExecuteResult<string> SaveFile(this Activity content, Intent Intent)
        {
            var res = new PYLMath.CommonExecuteResult<string>();
            //使用微信第三方应用打开文件时
            var extras = Intent.Extras;
            if (extras == null)
            {
                res.IsSuccess = false;
                return res;
            }
            string tempPath;
            try
            {
                tempPath = System.IO.Path.Combine(System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal), "temp");
                if (!System.IO.Directory.Exists(tempPath))
                {
                    System.IO.Directory.CreateDirectory(tempPath);
                }
            }
            catch (Exception ex)
            {
                res.InnerException = ex;
                return res;
            }
            //
            string action = Intent.Action; //如果从微信中点击【使用第三方应用打开】，就是ActionView
                                           //如果微信分享的是*.db，那么 "application/db" ；如果是*.db..octet-stream，那么就application/octet-stream
            string type = Intent.Type;
            var uri = Intent.Data;
            int readCount = 0;
            char[] buffer = null;
            //创建一个请求
            try
            {
                //参考：https://segmentfault.com/a/1190000021357383，关键是这几行！！！！！！
                ParcelFileDescriptor parcelFileDescriptor = content.ContentResolver.OpenFileDescriptor(Intent.Data, "r");
                var reader = new Java.IO.FileReader(parcelFileDescriptor.FileDescriptor);
                var size = parcelFileDescriptor.StatSize;
                if (reader.Ready() == false)
                {
                    return res;
                }
                buffer = new char[size];
                readCount = reader.Read(buffer, 0, (int)size);
                //
                parcelFileDescriptor.Close();
                reader.Close();
            }
            catch (Java.IO.FileNotFoundException e)
            {
                res.InnerException = e;
                res.IsSuccess = false;
                return res;
            }
            catch (Exception ex)
            {
                res.InnerException = ex;
                res.IsSuccess = false;
                return res;
            }
            //
            if (readCount <= 0)
            {
                return res;
            } 
            byte[] bytes = new byte[readCount];
            for (int i = 0; i < readCount; i++)
            {
                bytes[i] = (byte)buffer[i];
            }
            try
            {
                //
                var fileName = System.IO.Path.GetFileName(Nancy.Helpers.HttpUtility.UrlDecode(Intent.DataString, System.Text.Encoding.UTF8));
                var saveFile = System.IO.Path.Combine(tempPath, fileName);
                System.IO.File.WriteAllBytes(saveFile, bytes);
                //
                res.Content = saveFile;
                res.IsSuccess = true;
                return res;
            }
            catch (Exception ex)
            {
                res.InnerException = ex;
                return res;
            }
        }