From a4fd21618635b891fefe34d5e440d6d9464e659f Mon Sep 17 00:00:00 2001
From: Jon Skeet <jonskeet@google.com>
Date: Wed, 7 Sep 2022 09:38:29 +0100
Subject: [PATCH] Retain existing array in RepeatedField.Clear

Fixes #7828.

(Also tweaks the comment for Capacity.)
---
 .../Collections/RepeatedFieldTest.cs                 | 12 ++++++++++++
 .../src/Google.Protobuf/Collections/RepeatedField.cs | 10 +++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/csharp/src/Google.Protobuf.Test/Collections/RepeatedFieldTest.cs b/csharp/src/Google.Protobuf.Test/Collections/RepeatedFieldTest.cs
index 766b4cb5db7a8..5a8977d611cba 100644
--- a/csharp/src/Google.Protobuf.Test/Collections/RepeatedFieldTest.cs
+++ b/csharp/src/Google.Protobuf.Test/Collections/RepeatedFieldTest.cs
@@ -888,5 +888,17 @@ public void Capacity_Zero()
             Assert.DoesNotThrow(() => list.Capacity = 0, "Can set Capacity to 0");
             Assert.AreEqual(0, list.Capacity);
         }
+
+        [Test]
+        public void Clear_CapacityUnaffected()
+        {
+            var list = new RepeatedField<int> { 1 };
+            Assert.AreEqual(1, list.Count);
+            Assert.AreEqual(8, list.Capacity);
+
+            list.Clear();
+            Assert.AreEqual(0, list.Count);
+            Assert.AreEqual(8, list.Capacity);
+        }
     }
 }
diff --git a/csharp/src/Google.Protobuf/Collections/RepeatedField.cs b/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
index 832e1669430eb..56117fd325d98 100644
--- a/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
+++ b/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
@@ -279,8 +279,9 @@ public void WriteTo(ref WriteContext ctx, FieldCodec<T> codec)
         }
 
         /// <summary>
-        /// Gets and sets the capacity of the RepeatedField's internal array.  WHen set, the internal array is reallocated to the given capacity.
-        /// <exception cref="ArgumentOutOfRangeException">The new value is less than Count -or- when Count is less than 0.</exception>
+        /// Gets and sets the capacity of the RepeatedField's internal array.
+        /// When set, the internal array is reallocated to the given capacity.
+        /// <exception cref="ArgumentOutOfRangeException">The new value is less than <see cref="Count"/>.</exception>
         /// </summary>
         public int Capacity
         {
@@ -338,7 +339,10 @@ public void Add(T item)
         /// </summary>
         public void Clear()
         {
-            array = EmptyArray;
+            // Clear the content of the array (so that any objects it referred to can be garbage collected)
+            // but keep the capacity the same. This allows large repeated fields to be reused without
+            // array reallocation.
+            Array.Clear(array, 0, count);
             count = 0;
         }