gistfile1.cs

async void Main()
{
    var content = "Hello, World!";
    var size = QRCodeSize.Large;
    var errorCorrection = QRErrorCorrection.H;
    
    var qrUrl = GenerateQRCodeUrl(content, size, errorCorrection);
    qrUrl.Dump();
    
    var image = await GetQRCodeImage(qrUrl);
    image.Dump();
}

public enum QRCodeSize
{
    Small = 120,
    Medium = 230,
    Large = 350
}

public enum QRErrorCorrection
{
    L,
    M,
    Q,
    H
}

private string GenerateQRCodeUrl(string content, QRCodeSize size, QRErrorCorrection errorCorrection)
{
    var baseUrl = "http://chart.apis.google.com/chart?cht=qr&chs={0}x{0}&chld={1}&choe=UTF-8&chl={2}";
    
    return string.Format(baseUrl, (int)size, errorCorrection.ToString(), Uri.EscapeDataString(content));
}

private async Task<Image> GetQRCodeImage(string url)
{
    using (var client = new HttpClient())
    {
        var msg = await client.GetAsync(url);
        if (msg.IsSuccessStatusCode)
        {
            using (var stream = await msg.Content.ReadAsStreamAsync())
            {
                var image = Image.FromStream(stream);
                return image;
            }
        }
    }
    
    return null;
}