在C#中，使用WebDAV傳輸大文件時，為了避免內存溢出，可以采用以下方法：

1. 分塊傳輸：將大文件分成較小的數據塊，然后逐個上傳。這樣可以避免一次性加載整個文件到內存中。

private const int ChunkSize = 4096; // 4KB

public async Task UploadLargeFileAsync(string filePath, string uploadUrl)
{
    using (var fileStream = File.OpenRead(filePath))
    {
        var buffer = new byte[ChunkSize];
        int bytesRead;
        long totalBytesRead = 0;

        while ((bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length)) > 0)
        {
            // 上傳數據塊到WebDAV服務器
            await UploadChunkAsync(uploadUrl, buffer, bytesRead, totalBytesRead);
            totalBytesRead += bytesRead;
        }
    }
}

private async Task UploadChunkAsync(string uploadUrl, byte[] chunk, int chunkSize, long startPosition)
{
    using (var client = new HttpClient())
    {
        var request = new HttpRequestMessage(HttpMethod.Put, uploadUrl);
        request.Headers.Add("Content-Range", $"bytes {startPosition}-{startPosition + chunkSize - 1}/{chunkSize}");
        request.Content = new ByteArrayContent(chunk, 0, chunkSize);

        var response = await client.SendAsync(request);
        response.EnsureSuccessStatusCode();
    }
}

2. 使用流式傳輸：使用流式傳輸可以在讀取文件的同時進行上傳，而不需要將整個文件加載到內存中。

public async Task UploadLargeFileAsync(string filePath, string uploadUrl)
{
    using (var fileStream = File.OpenRead(filePath))
    {
        using (var client = new HttpClient())
        {
            var content = new StreamContent(fileStream);
            var response = await client.PutAsync(uploadUrl, content);
            response.EnsureSuccessStatusCode();
        }
    }
}

3. 調整緩沖區大小：在使用HttpClient發送請求時，可以通過設置HttpClientHandler.MaxRequestContentBufferSize屬性來調整緩沖區大小。這將限制HttpClient在發送請求時所使用的內存。

public async Task UploadLargeFileAsync(string filePath, string uploadUrl)
{
    using (var fileStream = File.OpenRead(filePath))
    {
        using (var handler = new HttpClientHandler())
        {
            handler.MaxRequestContentBufferSize = ChunkSize;

            using (var client = new HttpClient(handler))
            {
                var content = new StreamContent(fileStream);
                var response = await client.PutAsync(uploadUrl, content);
                response.EnsureSuccessStatusCode();
            }
        }
    }
}

通過以上方法，可以有效地避免在使用C# WebDAV傳輸大文件時出現內存溢出的問題。