diff --git a/src/client/lang.ts b/src/client/lang.ts
index e9460ca..c477232 100644
--- a/src/client/lang.ts
+++ b/src/client/lang.ts
@@ -23,6 +23,7 @@ import { toml } from '@codemirror/legacy-modes/mode/toml'
 import { nginx } from '@codemirror/legacy-modes/mode/nginx'
 import { dockerFile } from '@codemirror/legacy-modes/mode/dockerfile'
 import { properties } from '@codemirror/legacy-modes/mode/properties'
+import { csharp } from '@codemirror/legacy-modes/mode/clike'
 
 /** The lowercased file extension of a path ('' when none). */
 export function extOf(path: string): string {
@@ -48,6 +49,7 @@ export function languageKeyForExt(ext: string): string | null {
     case 'yaml': case 'yml': return 'yaml'
     case 'sql': return 'sql'
     case 'java': return 'java'
+    case 'cs': return 'csharp'
     case 'c': case 'h': return 'c'
     case 'cc': case 'cpp': case 'cxx': case 'hpp': case 'hh': case 'hxx': return 'cpp'
     case 'rs': return 'rust'
@@ -76,6 +78,7 @@ const FACTORIES: Record<string, () => Language | LanguageSupport> = {
   yaml: () => yaml(),
   sql: () => sql(),
   java: () => java(),
+  csharp: () => StreamLanguage.define(csharp),
   c: () => cpp(),
   cpp: () => cpp(),
   rust: () => rust(),
diff --git a/tests/unit.spec.ts b/tests/unit.spec.ts
index 1cccf71..3512998 100644
--- a/tests/unit.spec.ts
+++ b/tests/unit.spec.ts
@@ -1102,6 +1102,7 @@ describe('editor language mapping', () => {
     expect(languageKeyForExt('yaml')).toBe('yaml')
     expect(languageKeyForExt('sh')).toBe('shell')
     expect(languageKeyForExt('md')).toBe('md')
+    expect(languageKeyForExt('cs')).toBe('csharp')
     expect(languageKeyForExt('txt')).toBeNull()
     expect(languageKeyForExt('log')).toBeNull()
     expect(languageKeyForExt('')).toBeNull()