diff --git a/src/vs/base/common/errors.ts b/src/vs/base/common/errors.ts
index a558a0b06d8..f0d9296057b 100644
--- a/src/vs/base/common/errors.ts
+++ b/src/vs/base/common/errors.ts
@@ -201,16 +201,10 @@ export function illegalState(name?: string): Error {
 	}
 }
 
-export function readonly(name?: string): Error {
-	return name
-		? new Error(`readonly property '${name} cannot be changed'`)
-		: new Error('readonly property cannot be changed');
-}
-
-export function disposed(what: string): Error {
-	const result = new Error(`${what} has been disposed`);
-	result.name = 'DISPOSED';
-	return result;
+export class ReadonlyError extends TypeError {
+	constructor(name?: string) {
+		super(name ? `${name} is read-only and cannot be changed` : 'Cannot change read-only property');
+	}
 }
 
 export function getErrorMessage(err: any): string {
diff --git a/src/vs/workbench/api/browser/mainThreadEditors.ts b/src/vs/workbench/api/browser/mainThreadEditors.ts
index b5e8bb0ef56..84bc6f34f1e 100644
--- a/src/vs/workbench/api/browser/mainThreadEditors.ts
+++ b/src/vs/workbench/api/browser/mainThreadEditors.ts
@@ -3,7 +3,7 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
-import { disposed } from 'vs/base/common/errors';
+import { illegalArgument } from 'vs/base/common/errors';
 import { IDisposable, dispose, DisposableStore } from 'vs/base/common/lifecycle';
 import { equals as objectEquals } from 'vs/base/common/objects';
 import { URI, UriComponents } from 'vs/base/common/uri';
@@ -174,7 +174,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 	$trySetSelections(id: string, selections: ISelection[]): Promise<void> {
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		editor.setSelections(selections);
 		return Promise.resolve(undefined);
@@ -184,7 +184,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 		key = `${this._instanceId}-${key}`;
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		editor.setDecorations(key, ranges);
 		return Promise.resolve(undefined);
@@ -194,7 +194,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 		key = `${this._instanceId}-${key}`;
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		editor.setDecorationsFast(key, ranges);
 		return Promise.resolve(undefined);
@@ -203,7 +203,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 	$tryRevealRange(id: string, range: IRange, revealType: TextEditorRevealType): Promise<void> {
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		editor.revealRange(range, revealType);
 		return Promise.resolve();
@@ -212,7 +212,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 	$trySetOptions(id: string, options: ITextEditorConfigurationUpdate): Promise<void> {
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		editor.setConfiguration(options);
 		return Promise.resolve(undefined);
@@ -221,7 +221,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 	$tryApplyEdits(id: string, modelVersionId: number, edits: ISingleEditOperation[], opts: IApplyEditsOptions): Promise<boolean> {
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		return Promise.resolve(editor.applyEdits(modelVersionId, edits, opts));
 	}
@@ -229,7 +229,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 	$tryInsertSnippet(id: string, modelVersionId: number, template: string, ranges: readonly IRange[], opts: IUndoStopOptions): Promise<boolean> {
 		const editor = this._editorLocator.getEditor(id);
 		if (!editor) {
-			return Promise.reject(disposed(`TextEditor(${id})`));
+			return Promise.reject(illegalArgument(`TextEditor(${id})`));
 		}
 		return Promise.resolve(editor.insertSnippet(modelVersionId, template, ranges, opts));
 	}
diff --git a/src/vs/workbench/api/common/extHost.api.impl.ts b/src/vs/workbench/api/common/extHost.api.impl.ts
index 5eadd12f6dd..95d016287a2 100644
--- a/src/vs/workbench/api/common/extHost.api.impl.ts
+++ b/src/vs/workbench/api/common/extHost.api.impl.ts
@@ -876,7 +876,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				return extHostWorkspace.getPath();
 			},
 			set rootPath(value) {
-				throw errors.readonly();
+				throw new errors.ReadonlyError('rootPath');
 			},
 			getWorkspaceFolder(resource) {
 				return extHostWorkspace.getWorkspaceFolder(resource);
@@ -888,13 +888,13 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				return extHostWorkspace.name;
 			},
 			set name(value) {
-				throw errors.readonly();
+				throw new errors.ReadonlyError('name');
 			},
 			get workspaceFile() {
 				return extHostWorkspace.workspaceFile;
 			},
 			set workspaceFile(value) {
-				throw errors.readonly();
+				throw new errors.ReadonlyError('workspaceFile');
 			},
 			updateWorkspaceFolders: (index, deleteCount, ...workspaceFoldersToAdd) => {
 				return extHostWorkspace.updateWorkspaceFolders(extension, index, deleteCount || 0, ...workspaceFoldersToAdd);
@@ -946,7 +946,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				return extHostDocuments.getAllDocumentData().map(data => data.document);
 			},
 			set textDocuments(value) {
-				throw errors.readonly();
+				throw new errors.ReadonlyError('textDocuments');
 			},
 			openTextDocument(uriOrFileNameOrOptions?: vscode.Uri | string | { language?: string; content?: string }) {
 				let uriPromise: Thenable<URI>;
diff --git a/src/vs/workbench/api/common/extHostTextEditor.ts b/src/vs/workbench/api/common/extHostTextEditor.ts
index c1317c709d9..8ab93b18b21 100644
--- a/src/vs/workbench/api/common/extHostTextEditor.ts
+++ b/src/vs/workbench/api/common/extHostTextEditor.ts
@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import { ok } from 'vs/base/common/assert';
-import { illegalArgument, readonly } from 'vs/base/common/errors';
+import { ReadonlyError, illegalArgument } from 'vs/base/common/errors';
 import { IdGenerator } from 'vs/base/common/idGenerator';
 import { TextEditorCursorStyle } from 'vs/editor/common/config/editorOptions';
 import { IRange } from 'vs/editor/common/core/range';
@@ -431,7 +431,7 @@ export class ExtHostTextEditor {
 				return document.value;
 			},
 			set document(_value) {
-				throw readonly('document');
+				throw new ReadonlyError('document');
 			},
 			// --- selection
 			get selection(): Selection {
@@ -459,7 +459,7 @@ export class ExtHostTextEditor {
 				return that._visibleRanges;
 			},
 			set visibleRanges(_value: Range[]) {
-				throw readonly('visibleRanges');
+				throw new ReadonlyError('visibleRanges');
 			},
 			// --- options
 			get options(): vscode.TextEditorOptions {
@@ -475,7 +475,7 @@ export class ExtHostTextEditor {
 				return that._viewColumn;
 			},
 			set viewColumn(_value) {
-				throw readonly('viewColumn');
+				throw new ReadonlyError('viewColumn');
 			},
 			// --- edit
 			edit(callback: (edit: TextEditorEdit) => void, options: { undoStopBefore: boolean; undoStopAfter: boolean } = { undoStopBefore: true, undoStopAfter: true }): Promise<boolean> {