diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -3917,6 +3917,8 @@
       }
     }
   }
+  if (Style.isCSharp() && Left.is(Keywords.kw_is) && Right.is(tok::l_square))
+    return true;
   const auto SpaceRequiredForArrayInitializerLSquare =
       [](const FormatToken &LSquareTok, const FormatStyle &Style) {
         return Style.SpacesInContainerLiterals ||
diff --git a/clang/unittests/Format/FormatTestCSharp.cpp b/clang/unittests/Format/FormatTestCSharp.cpp
--- a/clang/unittests/Format/FormatTestCSharp.cpp
+++ b/clang/unittests/Format/FormatTestCSharp.cpp
@@ -1176,6 +1176,14 @@
   verifyFormat(R"(override (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(async (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(unsafe (string name, int age) methodTuple() {})", Style);
+
+  Style.SpacesInSquareBrackets = false;
+  Style.SpaceBeforeSquareBrackets = true;
+  verifyFormat("return a is [1, 2, 3];", Style);
+  verifyFormat("return a is [..];", Style);
+  Style.SpaceBeforeSquareBrackets = false;
+  verifyFormat("return a is [1, 2, 3];", Style);
+  verifyFormat("return a is [..];", Style);
 }
 
 TEST_F(FormatTestCSharp, CSharpNullableTypes) {