System.Net.Http.WinHttpResponseParser.GetResponseHeaderCharBufferLength C# (CSharp) Method

        private unsafe static int GetResponseHeaderCharBufferLength(SafeWinHttpHandle requestHandle, uint infoLevel)
        {
            char* buffer = null;
            int bufferLength = 0;
            uint index = 0;

            if (!QueryHeaders(requestHandle, infoLevel, buffer, ref bufferLength, ref index))
            {
                int lastError = Marshal.GetLastWin32Error();

                Debug.Assert(lastError != Interop.WinHttp.ERROR_WINHTTP_HEADER_NOT_FOUND);

                if (lastError != Interop.WinHttp.ERROR_INSUFFICIENT_BUFFER)
                {
                    throw WinHttpException.CreateExceptionUsingError(lastError);
                }
            }

            return bufferLength;
        }

        private void ReadResponseTrailers()
        {
            // Only load response trailers if:
            // 1. WINHTTP_QUERY_FLAG_TRAILERS is supported by the OS
            // 2. HTTP/2 or later (WINHTTP_QUERY_FLAG_TRAILERS does not work with HTTP/1.1)
            // 3. Response trailers not already loaded
            if (!WinHttpTrailersHelper.OsSupportsTrailers || _responseMessage.Version < WinHttpHandler.HttpVersion20 || _readTrailingHeaders)
            {
                return;
            }

            _readTrailingHeaders = true;

            var bufferLength = WinHttpResponseParser.GetResponseHeaderCharBufferLength(_requestHandle, isTrailingHeaders: true);

            if (bufferLength != 0)
            {
                char[] trailersBuffer = ArrayPool <char> .Shared.Rent(bufferLength);

                try
                {
                    WinHttpResponseParser.ParseResponseTrailers(_requestHandle, _responseMessage, trailersBuffer);
                }
                finally
                {
                    ArrayPool <char> .Shared.Return(trailersBuffer);
                }
            }
        }