Background and motivation

The CborWriter class currently has an internal buffer that grows as data is written to the buffer.

Callers that write large CBOR documents may end up spending a significant amount time constantly re-allocating and copying the buffer as it grows. This was observed in #91969. In this scenario, 98% of the time spent was in Array.Resize.

Callers that know before hand that they are going to write a large CBOR document currently don't have an easy way to give a hint that the internal buffer should be larger than the default.

Conversely, callers that know they are going to create small CBOR documents may give a smaller-than-the-default hint to avoid over-allocating the internal buffer.

This is similar to what we did for AsnWriter except now we are doing it for CborWriter.

API Proposal

namespace System.Formats.Cbor;

public partial class CborWriter {
    public CborWriter(int initialCapacity, CborConformanceMode conformanceMode = CborConformanceMode.Strict, bool convertIndefiniteLengthEncodings = false, bool allowMultipleRootLevelValues = false);
}

API Usage

CborWriter writer = new CborWriter(initialCapacity: 10000000);
// write 1000000 bytes of stuff to the writer.

Alternative Designs

No response

Risks

No response