NullReferenceException Was Unhandled by user code in c#

Question

When we create a connection using connection string NullReferenceException is occurred. The error is

NullReferenceException is unhandled by user code.

My code is given below:

 protected void upload_Click(object sender, EventArgs e)
    {

        string filename = Path.GetFileName(FileUpload1.PostedFile.FileName);

        string contentType = FileUpload1.PostedFile.ContentType;

        using (Stream fs = FileUpload1.PostedFile.InputStream)
        {
              using(BinaryReader br = new BinaryReader(fs))
              {
                  byte[] bytes = br.ReadBytes((Int32)fs.Length);


                      string constr =ConfigurationManager.ConnectionStrings["constr"].ConnectionString; 

                      using (SqlConnection con = new SqlConnection(constr))
                      {
                          string query = "insert into tblFiles values(@Name, @ContentType, @Data)";

                          using (SqlCommand cmd = new SqlCommand(query))
                          {
                              cmd.Connection = con;
                              cmd.Parameters.AddWithValue("@Name", filename);
                              cmd.Parameters.AddWithValue("@contentType", contentType);
                              cmd.Parameters.AddWithValue("Data", bytes);
                              con.Open();
                              cmd.ExecuteNonQuery();
                              con.Close();
                          }
                      }


              }
        }
        Response.Redirect(Request.Url.AbsoluteUri);
    }

Answer 1

You are using Data instead of @Data in

cmd.Parameters.AddWithValue("Data", bytes)

so this is the correct way to send

cmd.Parameters.AddWithValue("@Data", bytes);

Answer 2

Your are inserting the value in the table using @Data variable while you are sending the variable Data only

cmd.Parameters.AddWithValue("Data", bytes);  // error prompt code

cmd.Parameters.AddWithValue("@Data", bytes);  // correct Code

Answer 3

Most probably your code throwing NullReferenceException because of FileUpload1.PostedFile is null.

You may have added FileUpload inside UpdatePanel. If yes, then you need use PostBackTrigger.