Expose StringBuilder.GetChunks()

src/System.Runtime/ref/System.Runtime.cs

@@ -7584,6 +7584,12 @@ public StringBuilder(string value, int startIndex, int length, int capacity) { }
         public System.Text.StringBuilder AppendJoin<T>(string separator, System.Collections.Generic.IEnumerable<T> values) { throw null; }
         public System.Text.StringBuilder AppendLine() { throw null; }
         public System.Text.StringBuilder AppendLine(string value) { throw null; }
+        public struct ChunkEnumerator
+        {
+            public ChunkEnumerator GetEnumerator() { throw null; }
+            public bool MoveNext() { throw null; }
+            public ReadOnlyMemory<char> Current { get { throw null; } }
+        }
         public System.Text.StringBuilder Clear() { throw null; }
         public void CopyTo(int sourceIndex, char[] destination, int destinationIndex, int count) { }
         public void CopyTo(int sourceIndex, System.Span<char> destination, int count) { }
@@ -7613,6 +7619,7 @@ public void CopyTo(int sourceIndex, System.Span<char> destination, int count) {
         public System.Text.StringBuilder Insert(int index, uint value) { throw null; }
         [System.CLSCompliantAttribute(false)]
         public System.Text.StringBuilder Insert(int index, ulong value) { throw null; }
+        public ChunkEnumerator GetChunks() { throw null; }
         public System.Text.StringBuilder Remove(int startIndex, int length) { throw null; }
         public System.Text.StringBuilder Replace(char oldChar, char newChar) { throw null; }
         public System.Text.StringBuilder Replace(char oldChar, char newChar, int startIndex, int count) { throw null; }

src/System.Runtime/src/ApiCompatBaseline.uapaot.txt

@@ -0,0 +1 @@
+

src/System.Runtime/tests/System/Text/StringBuilderTests.netcoreapp.cs

@@ -388,5 +388,34 @@ public static void Equals(StringBuilder sb1, string value, bool expected)
         {
             Assert.Equal(expected, sb1.Equals(value.AsSpan()));
         }
+
+        [Fact]
+        public static void ForEach()
+        {
+            // Test on a variety of lengths, at least up to the point of 9 8K chunks = 72K because this is where
+            // we start using a different technique for creating the ChunkEnumerator.   200 * 500 = 100K which hits this.   
+            for (int i = 0; i < 200; i++)
+            {
+                StringBuilder inBuilder = new StringBuilder();
+                for (int j = 0; j < i; j++)
+                {
+                    // Make some unique strings that are at least 500 bytes long.  
+                    inBuilder.Append(j);
+                    inBuilder.Append("_abcdefghijklmnopqrstuvwxyz01234567890__Abcdefghijklmnopqrstuvwxyz01234567890__ABcdefghijklmnopqrstuvwxyz01_");
+                    inBuilder.Append("_abcdefghijklmnopqrstuvwxyz01234567890__Abcdefghijklmnopqrstuvwxyz01234567890__ABcdefghijklmnopqrstuvwxyz0123_");
+                    inBuilder.Append("_abcdefghijklmnopqrstuvwxyz01234567890__Abcdefghijklmnopqrstuvwxyz01234567890__ABcdefghijklmnopqrstuvwxyz012345_");
+                    inBuilder.Append("_abcdefghijklmnopqrstuvwxyz01234567890__Abcdefghijklmnopqrstuvwxyz01234567890__ABcdefghijklmnopqrstuvwxyz012345678_");
+                    inBuilder.Append("_abcdefghijklmnopqrstuvwxyz01234567890__Abcdefghijklmnopqrstuvwxyz01234567890__ABcdefghijklmnopqrstuvwxyz01234567890_");
+                }
+
+                // Copy the string out (not using StringBuilder).  
+                string outStr = "";
+                foreach (ReadOnlyMemory<char> chunk in inBuilder.GetChunks())
+                    outStr += new string(chunk.Span);
+
+                // The strings formed by concatenating the chunks should be the same as the value in the StringBuilder. 
+                Assert.Equal(outStr, inBuilder.ToString());
+            }
+        }
     }
 }