unoplatform/uno · error · Error

A DragDropExtension has already been enabled

Error message

A DragDropExtension has already been enabled

What it means

Thrown by DragDropExtension.enable when the static _current field is already non-null, meaning a DragDropExtension instance is already active. This is a singleton-enforcement guard: only one drag-drop extension can own the DOM event listeners at a time.

Source

Thrown at src/Uno.UI/ts/Windows/ApplicationModel/DataTransfer/DragAndDropExtension.ts:26

		private static _current: DragDropExtension;
		private static _nextDropId: number;

		private _dropHandler: any;

		private _pendingDropId: number;
		private _pendingDropData: DataTransfer;

		public static enable(pArgs: number): void {
			if (!DragDropExtension._dispatchDropEventMethod) {
				if ((<any>globalThis).DotnetExports !== undefined) {
					DragDropExtension._dispatchDropEventMethod = (<any>globalThis).DotnetExports.UnoUI.Windows.ApplicationModel.DataTransfer.DragDrop.Core.DragDropExtension.OnNativeDropEvent;
				} else {
					throw `DragDropExtension: Unable to find dotnet exports`;
				}
			}

			if (DragDropExtension._current) {
				throw new Error("A DragDropExtension has already been enabled");
			}

			DragDropExtension._dispatchDragDropArgs = pArgs;
			DragDropExtension._nextDropId = 1;
			DragDropExtension._current = new DragDropExtension();
		}

		public static disable(pArgs: number): void {
			if (DragDropExtension._dispatchDragDropArgs != pArgs) {
				throw new Error("The current DragDropExtension does not match the provided args");
			}
			
			DragDropExtension._current.dispose();
			DragDropExtension._current = null;
			DragDropExtension._dispatchDragDropArgs = null;
		}

		constructor() {

View on GitHub (pinned to 0418340488)

Solutions

1. Call disable(pArgs) to tear down the previous DragDropExtension before calling enable again.
2. Ensure only one DragDropExtension is enabled at a time in the WASM app; share a single extension across drop targets.
3. Verify that the previous enable/disable lifecycle completed (disable sets _current = null) before re-enabling.

Example fix

// before: enable called twice → throws on second
DragDropExtension.enable(args1);
DragDropExtension.enable(args2); // _current already set

// after: disable first
DragDropExtension.enable(args1);
// ... use it ...
DragDropExtension.disable(args1);
DragDropExtension.enable(args2);

// Check if an extension is already active before enabling
if (!(<any>DragDropExtension)._current) {
    DragDropExtension.enable(pArgs);
} else {
    // already enabled — disable first or skip
}

try {
    DragDropExtension.enable(pArgs);
} catch (e) {
    if (e.message === 'A DragDropExtension has already been enabled') {
        DragDropExtension.disable(existingArgs);
        DragDropExtension.enable(pArgs);
    } else { throw e; }
}

When it happens

Trigger: C# calls enable() twice without a matching disable() in between — e.g. two UI elements both registered a DragDropExtension, or enable was called from overlapping async paths without proper disable.

Common situations: Multiple DropBox/ListView controls each initializing their own DragDropExtension; a re-enable without disable after a page navigation or visual-tree rebuild; a race where enable is called concurrently.

Related errors

• The current DragDropExtension does not match the provided ar
• No pending drag and drop data.
• retrieveFiles failed to find pending drag and drop data for
• Item ${guid} is a directory handle. You cannot use it as a F
• The staged content was released when the download was trigge

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/197bee2e760f0b95. Report an issue: GitHub.