Xamarin Forms如何从指定路径调用外部默认应用打开文件？

Got it, let's figure out how to open a file with the default external app in Xamarin.Forms—since Process.Start doesn't translate directly to cross-platform mobile, we need to use Xamarin's Dependency Service to tap into native platform code (Android, iOS, UWP) to get this working.

Step 1: Define a Shared Interface #

First, in your shared Xamarin.Forms project, create an interface that declares the method we'll use across all platforms:

public interface IFileOpener
{
    Task OpenFileAsync(string filePath);
}

Step 2: Implement the Interface for Android #

In your Android project, add this native implementation. Don't forget the dependency attribute so Xamarin can find it:

using Android.Content;
using Android.Net;
using System.IO;
using Xamarin.Forms;

[assembly: Dependency(typeof(FileOpenerAndroid))]
namespace YourAppName.Droid
{
    public class FileOpenerAndroid : IFileOpener
    {
        public async Task OpenFileAsync(string filePath)
        {
            var targetFile = new Java.IO.File(filePath);
            if (!targetFile.Exists())
                throw new FileNotFoundException("Requested file doesn't exist", filePath);

            // Get the file's MIME type (fallback to "*/*" if we can't determine it)
            string mimeType = MimeTypeMap.Singleton.GetMimeTypeFromExtension(
                MimeTypeMap.GetFileExtensionFromUrl(targetFile.AbsolutePath).ToLower()
            );
            if (string.IsNullOrEmpty(mimeType))
                mimeType = "*/*";

            // Create and launch the intent to open the file
            var openIntent = new Intent(Intent.ActionView);
            openIntent.SetDataAndType(Uri.FromFile(targetFile), mimeType);
            openIntent.SetFlags(ActivityFlags.ClearWhenTaskReset | ActivityFlags.NewTask);

            try
            {
                Android.App.Application.Context.StartActivity(openIntent);
                await Task.CompletedTask;
            }
            catch (ActivityNotFoundException)
            {
                throw new InvalidOperationException("No app installed that can open this file type.");
            }
        }
    }
}

Heads up: For Android 7.0+, Uri.FromFile will trigger a FileUriExposedException. You'll need to set up a FileProvider in your Android manifest and adjust the URI creation to use FileProvider.GetUriForFile() instead.

Step 3: Implement the Interface for iOS #

In your iOS project, add this native implementation:

using Foundation;
using System.IO;
using UIKit;
using Xamarin.Forms;

[assembly: Dependency(typeof(FileOpeneriOS))]
namespace YourAppName.iOS
{
    public class FileOpeneriOS : IFileOpener
    {
        public async Task OpenFileAsync(string filePath)
        {
            if (!File.Exists(filePath))
                throw new FileNotFoundException("Requested file doesn't exist", filePath);

            var fileUrl = new NSUrl(filePath);
            var docController = UIDocumentInteractionController.FromUrl(fileUrl);

            // Find the topmost view controller to present the open-in menu
            var window = UIApplication.SharedApplication.KeyWindow;
            var topViewController = window.RootViewController;
            while (topViewController.PresentedViewController != null)
            {
                topViewController = topViewController.PresentedViewController;
            }

            docController.Delegate = new DocInteractionDelegate(topViewController);
            docController.PresentOpenInMenu(topViewController.View.Frame, topViewController.View, true);
            
            await Task.CompletedTask;
        }
    }

    // Helper delegate to handle preview presentation
    public class DocInteractionDelegate : UIDocumentInteractionControllerDelegate
    {
        private readonly UIViewController _parentViewController;

        public DocInteractionDelegate(UIViewController parent)
        {
            _parentViewController = parent;
        }

        public override UIViewController ViewControllerForPreview(UIDocumentInteractionController controller)
        {
            return _parentViewController;
        }
    }
}

Step 4: Use the Service in Shared Code #

Now, in your Xamarin.Forms shared code (like a ViewModel or Page code-behind), call the service like this:

try
{
    var fileOpener = DependencyService.Get<IFileOpener>();
    await fileOpener.OpenFileAsync(@"storage/emulated/0/DCIM/Camera/IMG_20180502_162205.jpg");
}
catch (Exception ex)
{
    // Handle errors (file missing, no app available, etc.)
    await DisplayAlert("Oops!", ex.Message, "Got it");
}

Bonus: UWP Implementation (If Needed) #

For UWP projects, here's the native implementation:

using Windows.Storage;
using Windows.System;
using Xamarin.Forms;

[assembly: Dependency(typeof(FileOpenerUWP))]
namespace YourAppName.UWP
{
    public class FileOpenerUWP : IFileOpener
    {
        public async Task OpenFileAsync(string filePath)
        {
            var targetFile = await StorageFile.GetFileFromPathAsync(filePath);
            await Launcher.LaunchFileAsync(targetFile);
        }
    }
}

This approach mirrors the behavior of Process.Start but works cross-platform for mobile, letting the system pick the default app to open your file.

内容的提问来源于stack exchange，提问作者ASalameh