fix: Map FileAccessMode to appropriate FileMode in StorageFile.OpenStreamAsync

src/Uno.UI.RuntimeTests/Tests/Windows_Storage/Given_StorageFile_Native.base.cs

@@ -290,6 +290,45 @@ public async Task When_InputStreamFlush()
 			}
 		}
 
+		[TestMethod]
+		public async Task When_OpenStreamForWriteAsync_On_New_File()
+		{
+			var rootFolder = await GetRootFolderAsync();
+			var fileName = GetRandomTextFileName();
+			var filePath = Path.Combine(rootFolder.Path, fileName);
+
+			try
+			{
+				// Ensure the file does not exist
+				if (File.Exists(filePath))
+				{
+					File.Delete(filePath);
+				}
+
+				// Get a reference to a file that doesn't exist yet
+				var storageFile = await StorageFile.GetFileFromPathAsync(filePath);
+
+				// This should not throw FileNotFoundException
+				using var stream = await storageFile.OpenStreamForWriteAsync();
+				using var writer = new StreamWriter(stream);
+				await writer.WriteAsync("Test content");
+				await writer.FlushAsync();
+
+				// Verify the file was created and contains the expected content
+				Assert.IsTrue(File.Exists(filePath), "File should have been created");
+				var content = await FileIO.ReadTextAsync(storageFile);
+				Assert.AreEqual("Test content", content);
+			}
+			finally
+			{
+				if (File.Exists(filePath))
+				{
+					File.Delete(filePath);
+				}
+				await CleanupRootFolderAsync();
+			}
+		}
+
 		private string GetRandomFolderName() => Guid.NewGuid().ToString();
 
 		private string GetRandomTextFileName() => Guid.NewGuid().ToString() + ".txt";

src/Uno.UWP/Storage/StorageFile.Local.cs

@@ -40,7 +40,7 @@ public override Task<IRandomAccessStreamWithContentType> OpenAsync(CancellationT
 				=> Task.FromResult<IRandomAccessStreamWithContentType>(new RandomAccessStreamWithContentType(FileRandomAccessStream.CreateLocal(Path, ToFileAccess(accessMode), ToFileShare(options)), ContentType));
 
 			public override Task<Stream> OpenStreamAsync(CancellationToken ct, FileAccessMode accessMode, StorageOpenOptions options)
-				=> Task.FromResult<Stream>(File.Open(Path, FileMode.Open, ToFileAccess(accessMode), ToFileShare(options)));
+				=> Task.FromResult<Stream>(File.Open(Path, ToFileMode(accessMode), ToFileAccess(accessMode), ToFileShare(options)));
 
 			public override Task<StorageStreamTransaction> OpenTransactedWriteAsync(CancellationToken ct, StorageOpenOptions option)
 				=> Task.FromResult(new StorageStreamTransaction(Owner, ToFileShare(option)));

src/Uno.UWP/Storage/StorageFile.cs

@@ -216,6 +216,22 @@ private static FileAccess ToFileAccess(FileAccessMode accessMode)
 				_ => throw new ArgumentOutOfRangeException(nameof(accessMode))
 			};
 
+		/// <summary>
+		/// Maps FileAccessMode to the appropriate FileMode for file operations.
+		/// </summary>
+		/// <param name="accessMode">The access mode requested for the file.</param>
+		/// <returns>
+		/// FileMode.Open for Read access (requires file to exist),
+		/// FileMode.OpenOrCreate for ReadWrite access (creates file if it doesn't exist).
+		/// </returns>
+		private static FileMode ToFileMode(FileAccessMode accessMode)
+			=> accessMode switch
+			{
+				FileAccessMode.Read => FileMode.Open,
+				FileAccessMode.ReadWrite => FileMode.OpenOrCreate,
+				_ => throw new ArgumentOutOfRangeException(nameof(accessMode))
+			};
+
 		private static FileShare ToFileShare(StorageOpenOptions options)
 			=> options switch
 			{

src/Uno.UWP/Storage/StorageFile.iOS.cs

@@ -84,7 +84,7 @@ public override Task<IRandomAccessStreamWithContentType> OpenAsync(CancellationT
 
 			public override Task<Stream> OpenStreamAsync(CancellationToken ct, FileAccessMode accessMode, StorageOpenOptions options)
 			{
-				Func<Stream> streamBuilder = () => File.Open(Path, FileMode.Open, ToFileAccess(accessMode), ToFileShare(options));
+				Func<Stream> streamBuilder = () => File.Open(Path, ToFileMode(accessMode), ToFileAccess(accessMode), ToFileShare(options));
 				var streamWrapper = new SecurityScopeStreamWrapper(_nsUrl, streamBuilder);
 				return Task.FromResult<Stream>(streamWrapper);
 			}