maui-media-picker（毛伊岛媒体选择器）

.NET MAUI Media Picker — Gotchas & Best Practices

Common Mistakes

1. Forgetting to check for cancellation

Single-select returns null, multi-select returns an empty list. Not checking causes NullReferenceException.

// ❌ Crashes when user cancels
var photo = await MediaPicker.Default.PickPhotoAsync();
using var stream = await photo.OpenReadAsync();

// ✅ Always check for null / empty
var photo = await MediaPicker.Default.PickPhotoAsync();
if (photo is null) return;
using var stream = await photo.OpenReadAsync();

2. Calling from a background thread

All MediaPicker methods must run on the UI thread or they throw.

// ❌ Will throw on some platforms
await Task.Run(async () => await MediaPicker.Default.PickPhotoAsync());

// ✅ Call directly from a command or event handler on the UI thread
var photo = await MediaPicker.Default.PickPhotoAsync();

3. Not checking IsCaptureSupported

Devices without cameras (emulators, some tablets) will throw if you call capture methods.

// ❌ Crashes on devices without a camera
var photo = await MediaPicker.Default.CapturePhotoAsync();

// ✅ Guard with capability check
if (MediaPicker.Default.IsCaptureSupported)
{
    var photo = await MediaPicker.Default.CapturePhotoAsync();
}

4. Not disposing streams

OpenReadAsync() returns a stream that must be disposed.

// ❌ Leaks the stream
var stream = await photo.OpenReadAsync();
var bytes = ReadAllBytes(stream);

// ✅ Dispose with using
using var stream = await photo.OpenReadAsync();

Platform Pitfalls

⚠️ SelectionLimit is not enforced on Android/Windows

MediaPickerOptions.SelectionLimit is advisory. Always validate the count yourself:

var options = new MediaPickerOptions { SelectionLimit = 5 };
var results = await MediaPicker.Default.PickPhotosAsync(options);
if (results.Count() > 5)
{
    // Warn user or take only the first 5
    results = results.Take(5);
}

⚠️ Android storage permissions split at API 33

• API ≤ 32: needs READ_EXTERNAL_STORAGE / WRITE_EXTERNAL_STORAGE
• API ≥ 33: needs READ_MEDIA_IMAGES / READ_MEDIA_VIDEO (broad storage permissions are ignored)
• Use android:maxSdkVersion="32" on the old permissions to avoid Play Store warnings

⚠️ iOS requires all four plist keys for full functionality

Missing any one of NSCameraUsageDescription, NSMicrophoneUsageDescription, NSPhotoLibraryUsageDescription, or NSPhotoLibraryAddUsageDescription causes a runtime crash when that feature is accessed.

⚠️ Android requires <queries> for camera intents

Without the IMAGE_CAPTURE query in AndroidManifest.xml, CapturePhotoAsync may fail silently on Android 11+ due to package visibility restrictions.

Decision Framework

Checklist

• All picker calls run on the UI thread
• Null/empty checks after every pick/capture call
• IsCaptureSupported guard before capture methods
• Streams disposed with using
• Android manifest has both old and new storage permissions with maxSdkVersion
• Android manifest has <queries> block for camera intent
• iOS Info.plist has all four usage description keys
• SelectionLimit validated in code, not trusted from platform