Access single file or folder from IStorageFolder by name

src/Android/Avalonia.Android/Platform/Storage/AndroidStorageItem.cs

@@ -294,6 +294,79 @@ public async IAsyncEnumerable<IStorageItem> GetItemsAsync()
             return destination;
         }
     }
+
+    private async Task<IStorageItem?> GetItemAsync(string name, bool isDirectory)
+    {
+        if (!await EnsureExternalFilesPermission(false))
+        {
+            return null;
+        }
+
+        var contentResolver = Activity.ContentResolver;
+        if (contentResolver == null)
+        {
+            return null;
+        }
+
+        var root = PermissionRoot ?? Uri;
+        var folderId = root != Uri ? DocumentsContract.GetDocumentId(Uri) : DocumentsContract.GetTreeDocumentId(Uri);
+        var childrenUri = DocumentsContract.BuildChildDocumentsUriUsingTree(root, folderId);
+
+        var projection = new[]
+        {
+            DocumentsContract.Document.ColumnDocumentId,
+            DocumentsContract.Document.ColumnMimeType,
+            DocumentsContract.Document.ColumnDisplayName
+        };
+
+        if (childrenUri != null)
+        {
+            using var cursor = contentResolver.Query(childrenUri, projection, null, null, null);
+            if (cursor != null)
+            {
+                while (cursor.MoveToNext())
+                {
+                    var id = cursor.GetString(0);
+                    var mime = cursor.GetString(1);
+
+                    var fileName = cursor.GetString(2);
+                    if (fileName != name)
+                    {
+                        continue;
+                    }                   
+
+                    bool mineDirectory = mime == DocumentsContract.Document.MimeTypeDir;
+                    if (isDirectory != mineDirectory)
+                    {
+                        return null;
+                    }
+
+                    var uri = DocumentsContract.BuildDocumentUriUsingTree(root, id);
+                    if (uri == null)
+                    {
+                        return null;
+                    }
+
+                    return isDirectory ? new AndroidStorageFolder(Activity, uri, false, this, root) :
+                        new AndroidStorageFile(Activity, uri, this, root);
+                }
+            }
+        }
+
+        return null;
+    }
+
+    public async Task<IStorageFolder?> GetFolderAsync(string name)
+    {
+        var folder = await GetItemAsync(name, true);
+        return (IStorageFolder?)folder;
+    }
+
+    public async Task<IStorageFile?> GetFileAsync(string name)
+    {
+        var file = await GetItemAsync(name, false);
+        return (IStorageFile?)file;
+    }
 }
 
 internal sealed class WellKnownAndroidStorageFolder : AndroidStorageFolder

src/Avalonia.Base/Platform/Storage/FileIO/BclStorageFolder.cs

@@ -19,4 +19,10 @@ public IAsyncEnumerable<IStorageItem> GetItemsAsync() => GetItemsCore(directoryI
 
     public Task<IStorageFolder?> CreateFolderAsync(string name) => Task.FromResult(
         (IStorageFolder?)WrapFileSystemInfo(CreateFolderCore(directoryInfo, name)));
+
+    public Task<IStorageFolder?> GetFolderAsync(string name) => Task.FromResult(
+        (IStorageFolder?)WrapFileSystemInfo(GetFolderCore(directoryInfo, name)));
+
+    public Task<IStorageFile?> GetFileAsync(string name) => Task.FromResult(
+       (IStorageFile?)WrapFileSystemInfo(GetFileCore(directoryInfo, name)));
 }

src/Avalonia.Base/Platform/Storage/FileIO/BclStorageItem.cs

@@ -127,6 +127,28 @@ internal static IEnumerable<FileSystemInfo> GetItemsCore(DirectoryInfo directory
         .OfType<FileSystemInfo>()
         .Concat(directoryInfo.EnumerateFiles());
 
+    internal static FileSystemInfo? GetFolderCore(DirectoryInfo directoryInfo, string name)
+    {
+        var path = System.IO.Path.Combine(directoryInfo.FullName, name);
+        if (Directory.Exists(path))
+        {
+            return new DirectoryInfo(path);
+        }
+
+        return null;
+    }
+
+    internal static FileSystemInfo? GetFileCore(DirectoryInfo directoryInfo, string name)
+    {
+        var path = System.IO.Path.Combine(directoryInfo.FullName, name);
+        if (File.Exists(path))
+        {
+            return new FileInfo(path);
+        }
+
+        return null;
+    }
+
     internal static FileInfo CreateFileCore(DirectoryInfo directoryInfo, string name)
     {
         var fileName = System.IO.Path.Combine(directoryInfo.FullName, name);

src/Avalonia.Base/Platform/Storage/IStorageFolder.cs

@@ -18,6 +18,24 @@ public interface IStorageFolder : IStorageItem
     /// </returns>
     IAsyncEnumerable<IStorageItem> GetItemsAsync();
 
+    /// <summary>
+    /// Gets the folder with the specified name from the current folder.
+    /// </summary>
+    /// <param name="name">The name of the folder to get</param>
+    /// <returns>
+    /// When this method completes successfully, it returns the folder with the specified name from the current folder.
+    /// </returns>
+    Task<IStorageFolder?> GetFolderAsync(string name);
+
+    /// <summary>
+    /// Gets the file with the specified name from the current folder.
+    /// </summary>
+    /// <param name="name">The name of the file to get</param>
+    /// <returns>
+    /// When this method completes successfully, it returns the file with the specified name from the current folder.
+    /// </returns>
+    Task<IStorageFile?> GetFileAsync(string name);
+
     /// <summary>
     /// Creates a file with specified name as a child of the current storage folder
     /// </summary>

src/Avalonia.Native/StorageItem.cs

@@ -149,4 +149,18 @@ IEnumerable<IStorageItem> GetItems()
         var folder = BclStorageItem.CreateFolderCore(directoryInfo, name);
         return Task.FromResult((IStorageFolder?)WrapFileSystemInfo(folder, ScopeOwnerUri));
     }
+
+    public Task<IStorageFolder?> GetFolderAsync(string name)
+    {
+        using var scope = OpenScope();
+        var item = BclStorageItem.GetFolderCore(directoryInfo, name);
+        return Task.FromResult((IStorageFolder?)WrapFileSystemInfo(item, ScopeOwnerUri));
+    }
+
+    public Task<IStorageFile?> GetFileAsync(string name)
+    {
+        using var scope = OpenScope();
+        var item = BclStorageItem.GetFileCore(directoryInfo, name);
+        return Task.FromResult((IStorageFile?)WrapFileSystemInfo(item, ScopeOwnerUri));
+    }
 }

src/Browser/Avalonia.Browser/Interop/StorageHelper.cs

@@ -67,4 +67,10 @@ internal static partial class StorageHelper
 
     [JSImport("StorageProvider.createFolder", AvaloniaModule.StorageModuleName)]
     public static partial Task<JSObject?> CreateFolder(JSObject folderHandle, string name);
+
+    [JSImport("StorageItem.getFile", AvaloniaModule.StorageModuleName)]
+    public static partial Task<JSObject?> GetFile(JSObject folderHandle, string name);
+
+    [JSImport("StorageItem.getFolder", AvaloniaModule.StorageModuleName)]
+    public static partial Task<JSObject?> GetFolder(JSObject folderHandle, string name);
 }

src/Browser/Avalonia.Browser/Storage/BrowserStorageProvider.cs

@@ -385,4 +385,40 @@ public async IAsyncEnumerable<IStorageItem> GetItemsAsync()
             throw new UnauthorizedAccessException("User denied permissions to open the file", ex);
         }
     }
+
+    public async Task<IStorageFolder?> GetFolderAsync(string name)
+    {
+        try
+        {
+            var storageFile = await StorageHelper.GetFolder(FileHandle, name);
+            if (storageFile is null)
+            {
+                return null;
+            }
+
+            return new JSStorageFolder(storageFile);
+        }
+        catch (JSException ex) when (ex.Message == BrowserStorageProvider.NoPermissionsMessage)
+        {
+            throw new UnauthorizedAccessException("User denied permissions to open the folder", ex);
+        }
+    }
+
+    public async Task<IStorageFile?> GetFileAsync(string name)
+    {
+        try
+        {
+            var storageFile = await StorageHelper.GetFile(FileHandle, name);
+            if (storageFile is null)
+            {
+                return null;
+            }
+
+            return new JSStorageFile(storageFile);
+        }
+        catch (JSException ex) when (ex.Message == BrowserStorageProvider.NoPermissionsMessage)
+        {
+            throw new UnauthorizedAccessException("User denied permissions to open the file", ex);
+        }
+    }
 }

src/Browser/Avalonia.Browser/webapp/modules/storage/storageItem.ts

@@ -107,6 +107,16 @@ export class StorageItem {
         return await ((item.handle as any).getFileHandle(name, { create: true }) as Promise<any>);
     }
 
+    public static async getFile(item: StorageItem, name: string): Promise<any | null> {
+        if (item.kind !== "directory" || !item.handle) {
+            throw new TypeError("Unable to create item in the requested directory");
+        }
+
+        await item.verityPermissions("read");
+
+        return await ((item.handle as any).getFileHandle(name) as Promise<any>);
+    }
+
     public static async createFolder(item: StorageItem, name: string): Promise<any | null> {
         if (item.kind !== "directory" || !item.handle) {
             throw new TypeError("Unable to create item in the requested directory");
@@ -117,6 +127,16 @@ export class StorageItem {
         return await ((item.handle as any).getDirectoryHandle(name, { create: true }) as Promise<any>);
     }
 
+    public static async getFolder(item: StorageItem, name: string): Promise<any | null> {
+        if (item.kind !== "directory" || !item.handle) {
+            throw new TypeError("Unable to create item in the requested directory");
+        }
+
+        await item.verityPermissions("read");
+
+        return await ((item.handle as any).getDirectoryHandle(name) as Promise<any>);
+    }
+
     public static async deleteAsync(item: StorageItem): Promise<any | null> {
         if (!item.handle) {
             return null;

src/iOS/Avalonia.iOS/Storage/IOSStorageItem.cs

@@ -306,4 +306,36 @@ public async IAsyncEnumerable<IStorageItem> GetItemsAsync()
             SecurityScopedAncestorUrl.StopAccessingSecurityScopedResource();
         }
     }
+
+    private NSUrl? GetItem(string name, bool isDirectory)
+    {
+        try
+        {
+            SecurityScopedAncestorUrl.StartAccessingSecurityScopedResource();
+
+            var path = System.IO.Path.Combine(FilePath, name);
+            if (NSFileManager.DefaultManager.FileExists(path, ref isDirectory))
+            {
+                return new NSUrl(path, isDirectory);
+            }
+
+            return null;
+        }
+        finally
+        {
+            SecurityScopedAncestorUrl.StopAccessingSecurityScopedResource();
+        }
+    }
+
+    public Task<IStorageFolder?> GetFolderAsync(string name)
+    {
+        var url = GetItem(name, true);
+        return Task.FromResult<IStorageFolder?>(url is null ? null : new IOSStorageFolder(url));
+    }
+
+    public Task<IStorageFile?> GetFileAsync(string name)
+    {
+        var url = GetItem(name, false);
+        return Task.FromResult<IStorageFile?>(url is null ? null : new IOSStorageFile(url));
+    }
 }