从 nvarchar 转换为 varbinary(max)

Question

I am currently generating a QR code image for records in a table through a button click function, and then the image generated would need to be saved into the table.

But I am only able to generate the QR code image only and I can't seem to store the image into the database. The error given to me is:

Implicit conversion from data type nvarchar to varbinary(max) is not allowed. Use the CONVERT function to run this query.

I have search various methods online yet I can't seem to understand how the CONVERT function actually works. Below is my code:

string toinsertcode = "Consignment Number: " + txtConsignmentNum.Text +
   "\n" + "Student ID: " + txtStudentID.Text;

QRCodeGenerator qrGenerator = new QRCodeGenerator();
QRCodeGenerator.QRCode qrCode = qrGenerator.CreateQrCode(toinsertcode, QRCodeGenerator.ECCLevel.Q);

System.Web.UI.WebControls.Image imgQRCode = new System.Web.UI.WebControls.Image();
imgQRCode.Height = 150;
imgQRCode.Width = 150;
using (Bitmap bitmap = qrCode.GetGraphic(80))
{
    using (MemoryStream ms = new MemoryStream())
    {
        bitmap.Save(ms, System.Drawing.Imaging.ImageFormat.Png);
        byte[] byteImage = ms.ToArray();
        imgQRCode.ImageUrl = "data:image/png;base64, " + Convert.ToBase64String(byteImage);
    }
    placeholderQR.Controls.Add(imgQRCode);
}      

conn.Open();
string insertqrcode = "INSERT INTO Parcel(QRCode) values(@qr)"; 
SqlCommand cmdupload = new SqlCommand(insertqrcode, conn);
cmdupload.Parameters.AddWithValue("@qr", imgQRCode); 
cmdupload.ExecuteNonQuery();
conn.Close();  

Answer 1

Instead of posting an object to the database, which is being sent as a string of some kind, you need to actually send the bytes.

Also it's a good idea to define the parameter with an explicit data type to avoid C# guessing (incorrectly) at your required data type as it has done to you here.

Replace:

    cmdupload.Parameters.AddWithValue("@qr", imgQRCode); 

With:

    cmdupload.Parameters.Add("@qr", SqlDbType.VarBinary, -1 ).Value = byteImage;

Obviously you will also need to move the database insert statement to a scope where your byteImage is defined for this to work.