dotnet · eiriktsarpalis · Sep 24, 2023 · Sep 23, 2023 · Sep 24, 2023
diff --git a/src/libraries/System.Formats.Cbor/ref/System.Formats.Cbor.cs b/src/libraries/System.Formats.Cbor/ref/System.Formats.Cbor.cs
@@ -124,7 +124,9 @@ public enum CborTag : ulong
     }
     public partial class CborWriter
     {
-        public CborWriter(System.Formats.Cbor.CborConformanceMode conformanceMode = System.Formats.Cbor.CborConformanceMode.Strict, bool convertIndefiniteLengthEncodings = false, bool allowMultipleRootLevelValues = false) { }
+        [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
+        public CborWriter(System.Formats.Cbor.CborConformanceMode conformanceMode, bool convertIndefiniteLengthEncodings, bool allowMultipleRootLevelValues) { }
+        public CborWriter(System.Formats.Cbor.CborConformanceMode conformanceMode = System.Formats.Cbor.CborConformanceMode.Strict, bool convertIndefiniteLengthEncodings = false, bool allowMultipleRootLevelValues = false, int initialCapacity = -1) { }
         public bool AllowMultipleRootLevelValues { get { throw null; } }
         public int BytesWritten { get { throw null; } }
         public System.Formats.Cbor.CborConformanceMode ConformanceMode { get { throw null; } }

diff --git a/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Writer/CborWriter.cs b/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Writer/CborWriter.cs
@@ -3,6 +3,7 @@
 
 using System.Buffers;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Diagnostics;
 using System.Runtime.InteropServices;
 
@@ -11,9 +12,10 @@ namespace System.Formats.Cbor
     /// <summary>A writer for Concise Binary Object Representation (CBOR) encoded data.</summary>
     public partial class CborWriter
     {
+        private const int DefaultCapacitySentinel = -1;
         private static readonly ArrayPool<byte> s_bufferPool = ArrayPool<byte>.Create();
 
-        private byte[] _buffer = null!;
+        private byte[] _buffer;
         private int _offset;
 
         private Stack<StackFrame>? _nestedDataItems;
@@ -60,14 +62,42 @@ public partial class CborWriter
         /// <param name="convertIndefiniteLengthEncodings"><see langword="true" /> to enable automatically converting indefinite-length encodings into definite-length equivalents and allow use of indefinite-length write APIs in conformance modes that otherwise do not permit it; otherwise, <see langword="false" />.</param>
         /// <param name="allowMultipleRootLevelValues"><see langword="true" /> to allow multiple root-level values to be written by the writer; otherwise, <see langword="false" />.</param>
         /// <exception cref="ArgumentOutOfRangeException"><paramref name="conformanceMode" /> is not a defined <see cref="CborConformanceMode" />.</exception>
-        public CborWriter(CborConformanceMode conformanceMode = CborConformanceMode.Strict, bool convertIndefiniteLengthEncodings = false, bool allowMultipleRootLevelValues = false)
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        public CborWriter(CborConformanceMode conformanceMode, bool convertIndefiniteLengthEncodings, bool allowMultipleRootLevelValues)
+            : this(conformanceMode, convertIndefiniteLengthEncodings, allowMultipleRootLevelValues, DefaultCapacitySentinel)
+        {
+        }
+
+        /// <summary>Initializes a new instance of <see cref="CborWriter" /> class using the specified configuration.</summary>
+        /// <param name="conformanceMode">One of the enumeration values that specifies the guidance on the conformance checks performed on the encoded data.
+        /// Defaults to <see cref="CborConformanceMode.Strict" /> conformance mode.</param>
+        /// <param name="convertIndefiniteLengthEncodings"><see langword="true" /> to enable automatically converting indefinite-length encodings into definite-length equivalents and allow use of indefinite-length write APIs in conformance modes that otherwise do not permit it; otherwise, <see langword="false" />.</param>
+        /// <param name="allowMultipleRootLevelValues"><see langword="true" /> to allow multiple root-level values to be written by the writer; otherwise, <see langword="false" />.</param>
+        /// <param name="initialCapacity">The initial capacity of the underlying buffer. The value -1 can be used to use the default capacity.</param>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// <para><paramref name="conformanceMode" /> is not a defined <see cref="CborConformanceMode" />.</para>
+        /// <para>-or-</para>
+        /// <para><paramref name="initialCapacity"/> is not zero, positive, or the default value indicator -1.</para>
+        /// </exception>
+        public CborWriter(
+            CborConformanceMode conformanceMode = CborConformanceMode.Strict,
+            bool convertIndefiniteLengthEncodings = false,
+            bool allowMultipleRootLevelValues = false,
+            int initialCapacity = DefaultCapacitySentinel)
         {
             CborConformanceModeHelpers.Validate(conformanceMode);
 
             ConformanceMode = conformanceMode;
             ConvertIndefiniteLengthEncodings = convertIndefiniteLengthEncodings;
             AllowMultipleRootLevelValues = allowMultipleRootLevelValues;
             _definiteLength = allowMultipleRootLevelValues ? null : (int?)1;
+
+            _buffer = initialCapacity switch
+            {
+                DefaultCapacitySentinel or 0 => Array.Empty<byte>(),
+                < -1 => throw new ArgumentOutOfRangeException(nameof(initialCapacity)),
+                _ => new byte[initialCapacity],
+            };
         }
 
         /// <summary>Resets the writer to have no data, without releasing resources.</summary>
@@ -205,7 +235,7 @@ private void EnsureWriteCapacity(int pendingCount)
                 throw new OverflowException();
             }
 
-            if (_buffer is null || _buffer.Length - _offset < pendingCount)
+            if (_buffer.Length - _offset < pendingCount)
             {
                 const int BlockSize = 1024;
                 int blocks = checked(_offset + pendingCount + (BlockSize - 1)) / BlockSize;

diff --git a/src/libraries/System.Formats.Cbor/tests/Writer/CborWriterTests.cs b/src/libraries/System.Formats.Cbor/tests/Writer/CborWriterTests.cs
@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Security.Cryptography;
 using Test.Cryptography;
 using Xunit;
@@ -332,7 +333,43 @@ public static void WriteEncodedValue_ValidPayloadWithTrailingBytes_ShouldThrowAr
         [InlineData((CborConformanceMode)(-1))]
         public static void InvalidConformanceMode_ShouldThrowArgumentOutOfRangeException(CborConformanceMode mode)
         {
-            Assert.Throws<ArgumentOutOfRangeException>(() => new CborWriter(conformanceMode: mode));
+            Assert.Throws<ArgumentOutOfRangeException>("conformanceMode", () => new CborWriter(conformanceMode: mode));
+        }
+
+        [Theory]
+        [InlineData(-2)]
+        [InlineData(int.MinValue)]
+        public static void InvalidInitialCapacity_ShouldThrowArgumentOutOfRangeException(int capacity)
+        {
+            Assert.Throws<ArgumentOutOfRangeException>("initialCapacity", () => new CborWriter(initialCapacity: capacity));
+        }
+
+        [Theory]
+        [InlineData(-1, 0)]
+        [InlineData(0, 0)]
+        [InlineData(1, 1)]
+        [InlineData(1023, 1023)]
+        public static void InitialCapacity_ShouldSetInitialBuffer(int capacity, int expectedBufferLength)
+        {
+            CborWriter writer = new CborWriter(initialCapacity: capacity);
+            byte[]? buffer = (byte[]?)typeof(CborWriter).GetField("_buffer", BindingFlags.Instance | BindingFlags.NonPublic)?.GetValue(writer);
+
+            Assert.NotNull(buffer);
+            Assert.Equal(expectedBufferLength, buffer.Length);
+        }
+
+        [Theory]
+        [InlineData(0)]
+        [InlineData(-1)]
+        [InlineData(1)]
+        public static void Encode_InitialCapacity_Grows(int capacity)
+        {
+            CborWriter writer = new CborWriter(initialCapacity: capacity);
+            writer.WriteByteString((ReadOnlySpan<byte>)new byte[] { 1, 2, 3, 4, 5, 6 });
+            byte[] encoded = writer.Encode();
+
+            ReadOnlySpan<byte> expected = new byte[] { (2 << 5) | 6, 1, 2, 3, 4, 5, 6 };
+            AssertExtensions.SequenceEqual(expected, encoded);
         }
 
         public static IEnumerable<object[]> EncodedValueInputs => CborReaderTests.SampleCborValues.Select(x => new [] { x });