[release/2.0] Port dictionary hang detection from 2.1

src/mscorlib/Resources/Strings.resx

@@ -2474,6 +2474,9 @@
   <data name="InvalidOperation_CollectionCorrupted" xml:space="preserve">
     <value>A prior operation on this collection was interrupted by an exception. Collection's state is no longer trusted.</value>
   </data>
+  <data name="InvalidOperation_ConcurrentOperationsNotSupported" xml:space="preserve">
+    <value>Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.</value>
+  </data>
   <data name="InvalidOperation_ConstructorNotAllowedOnInterface" xml:space="preserve">
     <value>Interface cannot have constructors.</value>
   </data>

src/mscorlib/src/System/Collections/Generic/Dictionary.cs

@@ -380,10 +380,19 @@ private int FindEntry(TKey key)
 
             if (buckets != null)
             {
+                int collisionCount = 0;
                 int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
                 for (int i = buckets[hashCode % buckets.Length]; i >= 0; i = entries[i].next)
                 {
                     if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) return i;
+
+                    if (collisionCount >= entries.Length)
+                    {
+                        // The chain of entries forms a loop; which means a concurrent update has happened.
+                        // Break out of the loop and throw, rather than looping forever.
+                        ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
+                    }
+                    collisionCount++;
                 }
             }
             return -1;
@@ -431,6 +440,12 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
                     return false;
                 }
 #if FEATURE_RANDOMIZED_STRING_HASHING
+                if (collisionCount >= entries.Length)
+                {
+                    // The chain of entries forms a loop; which means a concurrent update has happened.
+                    // Break out of the loop and throw, rather than looping forever.
+                    ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
+                }
                 collisionCount++;
 #endif
             }

src/mscorlib/src/System/ThrowHelper.cs

@@ -251,6 +251,11 @@ internal static void ThrowInvalidOperationException_InvalidOperation_EnumOpCantH
             throw GetInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);
         }
 
+        internal static void ThrowInvalidOperationException_ConcurrentOperationsNotSupported()
+        {
+            throw GetInvalidOperationException(ExceptionResource.InvalidOperation_ConcurrentOperationsNotSupported);
+        }
+
         internal static void ThrowArraySegmentCtorValidationFailedExceptions(Array array, int offset, int count)
         {
             throw GetArraySegmentCtorValidationFailedException(array, offset, count);
@@ -525,6 +530,7 @@ internal enum ExceptionResource
         InvalidOperation_HandleIsNotInitialized,
         AsyncMethodBuilder_InstanceNotInitialized,
         ArgumentNull_SafeHandle,
+        InvalidOperation_ConcurrentOperationsNotSupported,
     }
 }