How to generate QR code in visual studio 2008,asp.net

You can use Free Spire.Barcode.

https://www.e-iceblue.com/Introduce/barcode-for-net-introduce.html

using Spire.Barcode;
using System.Drawing;


namespace QRCode
{
    class Program
    {
        static void Main(string[] args)
        {
            BarcodeSettings settings = new BarcodeSettings();
            settings.Type = BarCodeType.QRCode;

            settings.Data = "Hello 123456789";
            settings.Data2D = "Hello 123456789";
            settings.QRCodeDataMode = QRCodeDataMode.AlphaNumber;
            settings.X = 1.0f;
            settings.QRCodeECL = QRCodeECL.H;

            BarCodeGenerator generator = new BarCodeGenerator(settings);
            Image image = generator.GenerateImage();
            image.Save("QRCode.png");
        }
    }
}

Try to add attached dll in your project.

i have all ready done this still same error .net 3.5

The usage of QRCoder  first you must add refernece to the QRCoder.dll.

How to add reference:
First you right-click on the “references” label in the Solution Explorer and click in the context menu on “Add reference”.
Now click on the “Browse” button, select the .dll and confirm your selection by clicking on the “Ok” button.

Or check the link: https://www.c-sharpcorner.com/UploadFile/1e050f/creating-and-using-dll-class-library-in-C-Sharp/

When I Store In Bin Folder DLL FILE THEN ERRO SHOWING

Error    4    Could not load file or assembly 'QRCoder' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.        

You can use QRCoder, download QRCoder.dll add to Your project. Here is sample tutorial how use it.
https://www.aspsnippets.com/Articles/Dynamically-generate-and-display-QR-code-Image-in-ASPNet.aspx

string code = "Your text";
QRCodeGenerator qrGenerator = new QRCodeGenerator();
QRCodeGenerator.QRCode qrCode = qrGenerator.CreateQrCode(code, QRCodeGenerator.ECCLevel.Q);
System.Web.UI.WebControls.Image imgBarCode = new System.Web.UI.WebControls.Image();
imgBarCode.Height = 150;
imgBarCode.Width = 150;
using (Bitmap bitMap = qrCode.GetGraphic(20))
{
    using (MemoryStream ms = new MemoryStream())
    {
        bitMap.Save(ms, System.Drawing.Imaging.ImageFormat.Png);
        byte[] byteImage = ms.ToArray();
        imgBarCode.ImageUrl = "data:image/png;base64," + Convert.ToBase64String(byteImage);
    }
    plBarCode.Controls.Add(imgBarCode);
}