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
@@ -3280,13 +3280,6 @@
       return true;
   }
 
-  // Put multiple C# attributes on a new line.
-  if (Style.isCSharp() &&
-      ((Left.is(TT_AttributeSquare) && Left.is(tok::r_square)) ||
-       (Left.is(tok::r_square) && Right.is(TT_AttributeSquare) &&
-        Right.is(tok::l_square))))
-    return true;
-
   // Put multiple Java annotation on a new line.
   if ((Style.Language == FormatStyle::LK_Java ||
        Style.Language == FormatStyle::LK_JavaScript) &&
diff --git a/clang/lib/Format/UnwrappedLineParser.h b/clang/lib/Format/UnwrappedLineParser.h
--- a/clang/lib/Format/UnwrappedLineParser.h
+++ b/clang/lib/Format/UnwrappedLineParser.h
@@ -125,6 +125,7 @@
   bool parseObjCProtocol();
   void parseJavaScriptEs6ImportExport();
   void parseStatementMacro();
+  void parseCSharpAttribute();
   bool tryToParseLambda();
   bool tryToParseLambdaIntroducer();
   void tryToParseJSFunction();
diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -323,6 +323,20 @@
   addUnwrappedLine();
 }
 
+void UnwrappedLineParser::parseCSharpAttribute() {
+  do {
+    switch (FormatTok->Tok.getKind()) {
+    case tok::r_square:
+      nextToken();
+      addUnwrappedLine();
+      return;
+    default:
+      nextToken();
+      break;
+    }
+  } while (!eof());
+}
+
 void UnwrappedLineParser::parseLevel(bool HasOpeningBrace) {
   bool SwitchLabelEncountered = false;
   do {
@@ -381,6 +395,13 @@
       SwitchLabelEncountered = true;
       parseStructuralElement();
       break;
+    case tok::l_square:
+      if (Style.isCSharp()) {
+        nextToken();
+        parseCSharpAttribute();
+        break;
+      }
+      LLVM_FALLTHROUGH;
     default:
       parseStructuralElement();
       break;
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
@@ -258,6 +258,21 @@
       "// The const char* returned by hello_world must not be deleted.\n"
       "private static extern IntPtr HelloFromCpp();)");
 
+  // Class attributes go on their own line and do not affect layout of
+  // interfaces. Line wrapping decisions previously caused each interface to be
+  // on its own line.
+  verifyFormat("[SomeAttribute]\n"
+               "[SomeOtherAttribute]\n"
+               "public class A : IShape, IAnimal, IVehicle\n"
+               "{\n"
+               "    int X;\n"
+               "}");
+
+  // Attributes in a method declaration do not cause line wrapping.
+  verifyFormat("void MethodA([In][Out] ref double x)\n"
+               "{\n"
+               "}");
+
   //  Unwrappable lines go on a line of their own.
   // 'target:' is not treated as a label.
   // Modify Style to enforce a column limit.