diff --git a/extensions/vscode-api-tests/src/singlefolder-tests/rpc.test.ts b/extensions/vscode-api-tests/src/singlefolder-tests/rpc.test.ts
index 9e595da621d..6f8ddc7a212 100644
--- a/extensions/vscode-api-tests/src/singlefolder-tests/rpc.test.ts
+++ b/extensions/vscode-api-tests/src/singlefolder-tests/rpc.test.ts
@@ -19,6 +19,12 @@ suite('vscode', function () {
 		assertNoRpc();
 	});
 
+	test('no rpc, createDiagnosticCollection()', function () {
+		const item = vscode.languages.createDiagnosticCollection();
+		dispo.push(item);
+		assertNoRpcFromEntry([item, 'DiagnosticCollection']);
+	});
+
 	test('no rpc, createTextEditorDecorationType(...)', function () {
 		const item = vscode.window.createTextEditorDecorationType({});
 		dispo.push(item);
diff --git a/src/vs/workbench/api/common/extHostDiagnostics.ts b/src/vs/workbench/api/common/extHostDiagnostics.ts
index 5326da678e7..bacfe4bdd80 100644
--- a/src/vs/workbench/api/common/extHostDiagnostics.ts
+++ b/src/vs/workbench/api/common/extHostDiagnostics.ts
@@ -19,7 +19,10 @@ import { IExtUri } from 'vs/base/common/resources';
 
 export class DiagnosticCollection implements vscode.DiagnosticCollection {
 
-	private readonly _data: ResourceMap<vscode.Diagnostic[]>;
+	readonly #proxy: MainThreadDiagnosticsShape | undefined;
+	readonly #onDidChangeDiagnostics: Emitter<vscode.Uri[]>;
+	readonly #data: ResourceMap<vscode.Diagnostic[]>;
+
 	private _isDisposed = false;
 
 	constructor(
@@ -27,19 +30,21 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 		private readonly _owner: string,
 		private readonly _maxDiagnosticsPerFile: number,
 		extUri: IExtUri,
-		private readonly _proxy: MainThreadDiagnosticsShape | undefined,
-		private readonly _onDidChangeDiagnostics: Emitter<vscode.Uri[]>
+		proxy: MainThreadDiagnosticsShape | undefined,
+		onDidChangeDiagnostics: Emitter<vscode.Uri[]>
 	) {
-		this._data = new ResourceMap(uri => extUri.getComparisonKey(uri));
+		this.#data = new ResourceMap(uri => extUri.getComparisonKey(uri));
+		this.#proxy = proxy;
+		this.#onDidChangeDiagnostics = onDidChangeDiagnostics;
 	}
 
 	dispose(): void {
 		if (!this._isDisposed) {
-			this._onDidChangeDiagnostics.fire([...this._data.keys()]);
-			if (this._proxy) {
-				this._proxy.$clear(this._owner);
+			this.#onDidChangeDiagnostics.fire([...this.#data.keys()]);
+			if (this.#proxy) {
+				this.#proxy.$clear(this._owner);
 			}
-			this._data.clear();
+			this.#data.clear();
 			this._isDisposed = true;
 		}
 	}
@@ -73,7 +78,7 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 			}
 
 			// update single row
-			this._data.set(first, diagnostics.slice());
+			this.#data.set(first, diagnostics.slice());
 			toSync = [first];
 
 		} else if (Array.isArray(first)) {
@@ -87,22 +92,22 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 			for (const tuple of first) {
 				const [uri, diagnostics] = tuple;
 				if (!lastUri || uri.toString() !== lastUri.toString()) {
-					if (lastUri && this._data.get(lastUri)!.length === 0) {
-						this._data.delete(lastUri);
+					if (lastUri && this.#data.get(lastUri)!.length === 0) {
+						this.#data.delete(lastUri);
 					}
 					lastUri = uri;
 					toSync.push(uri);
-					this._data.set(uri, []);
+					this.#data.set(uri, []);
 				}
 
 				if (!diagnostics) {
 					// [Uri, undefined] means clear this
-					const currentDiagnostics = this._data.get(uri);
+					const currentDiagnostics = this.#data.get(uri);
 					if (currentDiagnostics) {
 						currentDiagnostics.length = 0;
 					}
 				} else {
-					const currentDiagnostics = this._data.get(uri);
+					const currentDiagnostics = this.#data.get(uri);
 					if (currentDiagnostics) {
 						currentDiagnostics.push(...diagnostics);
 					}
@@ -111,16 +116,16 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 		}
 
 		// send event for extensions
-		this._onDidChangeDiagnostics.fire(toSync);
+		this.#onDidChangeDiagnostics.fire(toSync);
 
 		// compute change and send to main side
-		if (!this._proxy) {
+		if (!this.#proxy) {
 			return;
 		}
 		const entries: [URI, IMarkerData[]][] = [];
 		for (let uri of toSync) {
 			let marker: IMarkerData[] = [];
-			const diagnostics = this._data.get(uri);
+			const diagnostics = this.#data.get(uri);
 			if (diagnostics) {
 
 				// no more than N diagnostics per file
@@ -154,37 +159,37 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 
 			entries.push([uri, marker]);
 		}
-		this._proxy.$changeMany(this._owner, entries);
+		this.#proxy.$changeMany(this._owner, entries);
 	}
 
 	delete(uri: vscode.Uri): void {
 		this._checkDisposed();
-		this._onDidChangeDiagnostics.fire([uri]);
-		this._data.delete(uri);
-		if (this._proxy) {
-			this._proxy.$changeMany(this._owner, [[uri, undefined]]);
+		this.#onDidChangeDiagnostics.fire([uri]);
+		this.#data.delete(uri);
+		if (this.#proxy) {
+			this.#proxy.$changeMany(this._owner, [[uri, undefined]]);
 		}
 	}
 
 	clear(): void {
 		this._checkDisposed();
-		this._onDidChangeDiagnostics.fire([...this._data.keys()]);
-		this._data.clear();
-		if (this._proxy) {
-			this._proxy.$clear(this._owner);
+		this.#onDidChangeDiagnostics.fire([...this.#data.keys()]);
+		this.#data.clear();
+		if (this.#proxy) {
+			this.#proxy.$clear(this._owner);
 		}
 	}
 
 	forEach(callback: (uri: URI, diagnostics: ReadonlyArray<vscode.Diagnostic>, collection: DiagnosticCollection) => any, thisArg?: any): void {
 		this._checkDisposed();
-		for (let uri of this._data.keys()) {
+		for (let uri of this.#data.keys()) {
 			callback.apply(thisArg, [uri, this.get(uri), this]);
 		}
 	}
 
 	get(uri: URI): ReadonlyArray<vscode.Diagnostic> {
 		this._checkDisposed();
-		const result = this._data.get(uri);
+		const result = this.#data.get(uri);
 		if (Array.isArray(result)) {
 			return <ReadonlyArray<vscode.Diagnostic>>Object.freeze(result.slice(0));
 		}
@@ -193,7 +198,7 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 
 	has(uri: URI): boolean {
 		this._checkDisposed();
-		return Array.isArray(this._data.get(uri));
+		return Array.isArray(this.#data.get(uri));
 	}
 
 	private _checkDisposed() {