diff --git a/src/vs/workbench/api/common/extHost.api.impl.ts b/src/vs/workbench/api/common/extHost.api.impl.ts
index 230d09a2903..2e8267a6b26 100644
--- a/src/vs/workbench/api/common/extHost.api.impl.ts
+++ b/src/vs/workbench/api/common/extHost.api.impl.ts
@@ -630,7 +630,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				const options = uriOrFileNameOrOptions as { language?: string; content?: string; };
 				if (typeof uriOrFileNameOrOptions === 'string') {
 					uriPromise = Promise.resolve(URI.file(uriOrFileNameOrOptions));
-				} else if (uriOrFileNameOrOptions instanceof URI) {
+				} else if (URI.isUri(uriOrFileNameOrOptions)) {
 					uriPromise = Promise.resolve(uriOrFileNameOrOptions);
 				} else if (!options || typeof options === 'object') {
 					uriPromise = extHostDocuments.createDocumentData(options);
diff --git a/src/vs/workbench/api/common/extHostApiCommands.ts b/src/vs/workbench/api/common/extHostApiCommands.ts
index e9022a259d5..1cf22a6fad6 100644
--- a/src/vs/workbench/api/common/extHostApiCommands.ts
+++ b/src/vs/workbench/api/common/extHostApiCommands.ts
@@ -239,7 +239,7 @@ export class ExtHostApiCommands {
 		this._register(OpenFolderAPICommand.ID, adjustHandler(OpenFolderAPICommand.execute), {
 			description: 'Open a folder or workspace in the current window or new window depending on the newWindow argument. Note that opening in the same window will shutdown the current extension host process and start a new one on the given folder/workspace unless the newWindow parameter is set to true.',
 			args: [
-				{ name: 'uri', description: '(optional) Uri of the folder or workspace file to open. If not provided, a native dialog will ask the user for the folder', constraint: (value: any) => value === undefined || value instanceof URI },
+				{ name: 'uri', description: '(optional) Uri of the folder or workspace file to open. If not provided, a native dialog will ask the user for the folder', constraint: (value: any) => value === undefined || URI.isUri(value) },
 				{ name: 'options', description: '(optional) Options. Object with the following properties: `forceNewWindow `: Whether to open the folder/workspace in a new window or the same. Defaults to opening in the same window. `noRecentEntry`: Whether the opened URI will appear in the \'Open Recent\' list. Defaults to true. Note, for backward compatibility, options can also be of type boolean, representing the `forceNewWindow` setting.', constraint: (value: any) => value === undefined || typeof value === 'object' || typeof value === 'boolean' }
 			]
 		});
diff --git a/src/vs/workbench/api/common/extHostDiagnostics.ts b/src/vs/workbench/api/common/extHostDiagnostics.ts
index d654408346b..1455bb030d8 100644
--- a/src/vs/workbench/api/common/extHostDiagnostics.ts
+++ b/src/vs/workbench/api/common/extHostDiagnostics.ts
@@ -64,7 +64,7 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 		this._checkDisposed();
 		let toSync: vscode.Uri[] = [];
 
-		if (first instanceof URI) {
+		if (URI.isUri(first)) {
 
 			if (!diagnostics) {
 				// remove this entry
diff --git a/src/vs/workbench/api/common/extHostQuickOpen.ts b/src/vs/workbench/api/common/extHostQuickOpen.ts
index 3d790f26276..c8587620994 100644
--- a/src/vs/workbench/api/common/extHostQuickOpen.ts
+++ b/src/vs/workbench/api/common/extHostQuickOpen.ts
@@ -455,7 +455,7 @@ function getIconUris(iconPath: QuickInputButton['iconPath']): { dark: URI, light
 function getLightIconUri(iconPath: QuickInputButton['iconPath']) {
 	if (iconPath && !(iconPath instanceof ThemeIcon)) {
 		if (typeof iconPath === 'string'
-			|| iconPath instanceof URI) {
+			|| URI.isUri(iconPath)) {
 			return getIconUri(iconPath);
 		}
 		return getIconUri((iconPath as any).light);
@@ -471,7 +471,7 @@ function getDarkIconUri(iconPath: QuickInputButton['iconPath']) {
 }
 
 function getIconUri(iconPath: string | URI) {
-	if (iconPath instanceof URI) {
+	if (URI.isUri(iconPath)) {
 		return iconPath;
 	}
 	return URI.file(iconPath);
diff --git a/src/vs/workbench/api/common/extHostTreeViews.ts b/src/vs/workbench/api/common/extHostTreeViews.ts
index 7c462183bc7..95dc89564a1 100644
--- a/src/vs/workbench/api/common/extHostTreeViews.ts
+++ b/src/vs/workbench/api/common/extHostTreeViews.ts
@@ -540,7 +540,7 @@ class ExtHostTreeView<T> extends Disposable {
 	private getLightIconPath(extensionTreeItem: vscode.TreeItem): URI | undefined {
 		if (extensionTreeItem.iconPath && !(extensionTreeItem.iconPath instanceof ThemeIcon)) {
 			if (typeof extensionTreeItem.iconPath === 'string'
-				|| extensionTreeItem.iconPath instanceof URI) {
+				|| URI.isUri(extensionTreeItem.iconPath)) {
 				return this.getIconPath(extensionTreeItem.iconPath);
 			}
 			return this.getIconPath((<{ light: string | URI; dark: string | URI }>extensionTreeItem.iconPath).light);
@@ -556,7 +556,7 @@ class ExtHostTreeView<T> extends Disposable {
 	}
 
 	private getIconPath(iconPath: string | URI): URI {
-		if (iconPath instanceof URI) {
+		if (URI.isUri(iconPath)) {
 			return iconPath;
 		}
 		return URI.file(iconPath);
diff --git a/src/vs/workbench/api/common/extHostTypeConverters.ts b/src/vs/workbench/api/common/extHostTypeConverters.ts
index dfce55ade92..0c9236b67af 100644
--- a/src/vs/workbench/api/common/extHostTypeConverters.ts
+++ b/src/vs/workbench/api/common/extHostTypeConverters.ts
@@ -291,7 +291,7 @@ export namespace MarkdownString {
 			return part;
 		}
 		data = cloneAndChange(data, value => {
-			if (value instanceof URI) {
+			if (URI.isUri(value)) {
 				const key = `__uri_${Math.random().toString(16).slice(2, 8)}`;
 				bucket[key] = value;
 				return key;
diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index 981050bbef7..2324162060a 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -1474,7 +1474,7 @@ export class DocumentLink {
 	tooltip?: string;
 
 	constructor(range: Range, target: URI | undefined) {
-		if (target && !(target instanceof URI)) {
+		if (target && !(URI.isUri(target))) {
 			throw illegalArgument('target');
 		}
 		if (!Range.isRange(range) || range.isEmpty) {
@@ -2061,7 +2061,7 @@ export class TreeItem {
 	constructor(label: string | vscode.TreeItemLabel, collapsibleState?: vscode.TreeItemCollapsibleState)
 	constructor(resourceUri: URI, collapsibleState?: vscode.TreeItemCollapsibleState)
 	constructor(arg1: string | vscode.TreeItemLabel | URI, public collapsibleState: vscode.TreeItemCollapsibleState = TreeItemCollapsibleState.None) {
-		if (arg1 instanceof URI) {
+		if (URI.isUri(arg1)) {
 			this.resourceUri = arg1;
 		} else {
 			this.label = arg1;