diff --git a/src/vs/workbench/api/common/extHostTypeConverters.ts b/src/vs/workbench/api/common/extHostTypeConverters.ts
index ef8b23b5c7..ef18a8d506 100644
--- a/src/vs/workbench/api/common/extHostTypeConverters.ts
+++ b/src/vs/workbench/api/common/extHostTypeConverters.ts
@@ -274,8 +274,8 @@ export namespace MarkdownString {
 		if (isCodeblock(markup)) {
 			const { language, value } = markup;
 			res = { value: '```' + language + '\n' + value + '\n```\n' };
-		} else if (htmlContent.isMarkdownString(markup)) {
-			res = markup;
+		} else if (types.MarkdownString.isMarkdownString(markup)) {
+			res = { value: markup.value, isTrusted: markup.isTrusted, supportThemeIcons: markup.supportThemeIcons };
 		} else if (typeof markup === 'string') {
 			res = { value: markup };
 		} else {
@@ -343,7 +343,7 @@ export namespace MarkdownString {
 		return result;
 	}
 
-	export function fromStrict(value: string | types.MarkdownString): undefined | string | htmlContent.IMarkdownString {
+	export function fromStrict(value: string | vscode.MarkdownString): undefined | string | htmlContent.IMarkdownString {
 		if (!value) {
 			return undefined;
 		}
diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index 092b676301..1d205603f4 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -1276,7 +1276,9 @@ export class CodeLens {
 }
 
 @es5ClassCompat
-export class MarkdownString extends BaseMarkdownString implements vscode.MarkdownString {
+export class MarkdownString implements vscode.MarkdownString {
+
+	readonly #delegate: BaseMarkdownString;
 
 	static isMarkdownString(thing: any): thing is vscode.MarkdownString {
 		if (thing instanceof MarkdownString) {
@@ -1286,18 +1288,53 @@ export class MarkdownString extends BaseMarkdownString implements vscode.Markdow
 	}
 
 	constructor(value?: string, supportThemeIcons: boolean = false) {
-		super(value ?? '', { supportThemeIcons });
+		this.#delegate = new BaseMarkdownString(value, { supportThemeIcons });
 	}
 
+	get value(): string {
+		return this.#delegate.value;
+	}
+	set value(value: string) {
+		this.#delegate.value = value;
+	}
+
+	get isTrusted(): boolean | undefined {
+		return this.#delegate.isTrusted;
+	}
+
+	set isTrusted(value: boolean | undefined) {
+		this.#delegate.isTrusted = value;
+	}
+
+	get supportThemeIcons(): boolean | undefined {
+		return this.#delegate.supportThemeIcons;
+	}
+
+	appendText(value: string): vscode.MarkdownString {
+		this.#delegate.appendText(value);
+		return this;
+	}
+
+	appendMarkdown(value: string): vscode.MarkdownString {
+		this.#delegate.appendMarkdown(value);
+		return this;
+	}
+
+	appendCodeblock(value: string, language?: string): vscode.MarkdownString {
+		this.#delegate.appendCodeblock(language ?? '', value);
+		return this;
+	}
+
+
 }
 
 @es5ClassCompat
 export class ParameterInformation {
 
 	label: string | [number, number];
-	documentation?: string | MarkdownString;
+	documentation?: string | vscode.MarkdownString;
 
-	constructor(label: string | [number, number], documentation?: string | MarkdownString) {
+	constructor(label: string | [number, number], documentation?: string | vscode.MarkdownString) {
 		this.label = label;
 		this.documentation = documentation;
 	}
@@ -1307,11 +1344,11 @@ export class ParameterInformation {
 export class SignatureInformation {
 
 	label: string;
-	documentation?: string | MarkdownString;
+	documentation?: string | vscode.MarkdownString;
 	parameters: ParameterInformation[];
 	activeParameter?: number;
 
-	constructor(label: string, documentation?: string | MarkdownString) {
+	constructor(label: string, documentation?: string | vscode.MarkdownString) {
 		this.label = label;
 		this.documentation = documentation;
 		this.parameters = [];
@@ -1397,7 +1434,7 @@ export class CompletionItem implements vscode.CompletionItem {
 	kind?: CompletionItemKind;
 	tags?: CompletionItemTag[];
 	detail?: string;
-	documentation?: string | MarkdownString;
+	documentation?: string | vscode.MarkdownString;
 	sortText?: string;
 	filterText?: string;
 	preselect?: boolean;
diff --git a/src/vs/workbench/test/browser/api/extHostTypes.test.ts b/src/vs/workbench/test/browser/api/extHostTypes.test.ts
index 897bff0498..301a650d51 100644
--- a/src/vs/workbench/test/browser/api/extHostTypes.test.ts
+++ b/src/vs/workbench/test/browser/api/extHostTypes.test.ts
@@ -642,4 +642,10 @@ suite('ExtHostTypes', function () {
 			1, 0, 3, 3, (1 << 2) | (1 << 4)
 		]);
 	});
+
+	test('Markdown codeblock rendering is swapped #111604', function () {
+		const md = new types.MarkdownString().appendCodeblock('<img src=0 onerror="alert(1)">', 'html');
+		assert.deepEqual(md.value, '\n```html\n<img src=0 onerror="alert(1)">\n```\n');
+	});
+
 });