How to solve value cannot be null

Question

0.00/5 (No votes)

See more:

this is my code and it work normaly i have 10 contacts and when i press button the lbl19
increase to show the next ID but when the lbl19 is 11 above contacts ID the form stop and get an error Value cannot be null i try solve it like when the project get a null value it show message say this is max of ur contacts, but didnt' work can u help me? and im getting error on ImageByte.value ik it wrong but this what i get to

What I have tried:

if (ImageByte.Value = null)
           {
               MessageBox.Show("This is max of ur contacts");
           }
           else
           {
               string sql = "select Attachments.FileData from Contacts where ID =" + lbl19.Text;//Attachments.FileData
               OleDbCommand vcom = new OleDbCommand(sql, cn);
               ImageByte = (byte[])vcom.ExecuteScalar(); //contains 20 extra bytes
               MemoryStream MemStream = new MemoryStream(ImageByte.Skip(20).ToArray()); //Read bytes starting at position 20
               Image image = Image.FromStream(MemStream); //Will work now
               pictureBox1.Image = image;
           }

Posted 3-Nov-19 1:04am

Member 14630006

Updated 3-Nov-19 1:40am

v2

Add a Solution

Comments

Richard Deeming 5-Nov-19 15:55pm

string sql = "select Attachments.FileData from Contacts where ID =" + lbl19.Text;

Don't do it like that!

While you're probably safe in this case, since the user (presumably) can't change the value of your label, using string concatenation to build SQL queries can and will lead to SQL Injection[^] vulnerabilities.

NEVER use string concatenation to build a SQL query. ALWAYS use a parameterized query.

const string sql = "select Attachments.FileData from Contacts where ID = ?";
using (OleDbCommand vcom = new OleDbCommand(sql, cn))
{
    vcom.Parameters.AddWithValue("ID", lbl19.Text);
    ImageByte = (byte[])vcom.ExecuteScalar();
}

Everything you wanted to know about SQL injection (but were afraid to ask) | Troy Hunt[^]
How can I explain SQL injection without technical jargon? | Information Security Stack Exchange[^]
Query Parameterization Cheat Sheet | OWASP[^]

Richard Deeming 5-Nov-19 15:57pm

Also, do yourself a favour and give your controls meaningful names, rather than accepting Visual Studio's default names.

Sure, you might remember what lbl42 represents today; but in six weeks time, when you come back to edit your code, you'll have forgotten. Whereas if it's called something like lblTheAnswer, you'll at least have a clue. :)

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Richard MacCutchan · Answer 1 · 2019-11-03T01:40:00

Solution 1

C#

if (ImageByte.Value = null) // *** this sets ImageByte.Value to null.
// it should be 
if (ImageByte.Value == null) // equality test, not assignment

Posted 3-Nov-19 1:40am

Richard MacCutchan

Comments

Member 14630006 3-Nov-19 8:13am

the second one the test one i got error on value

[no name] 3-Nov-19 8:15am

But if (ImageByte.Value = null) would even not compile with c#. Anyway a 5.

Richard MacCutchan 3-Nov-19 9:31am

I often wonder how some people manage to build code that we know the compiler would normally reject.

Member 14630006 3-Nov-19 8:17am

how reach to the value null that error said

Richard MacCutchan 3-Nov-19 9:23am

Change the operator to the double == as I already showed you. You really need to get these operators fixed in your head, so you avoid simple mistakes like this. It is something we have all done in the past, but reading through the code carefully is a useful method of finding the obvious errors.

Member 14630006 3-Nov-19 9:37am

i did i told u the second one i mean the double one but error on value still

Richard MacCutchan 3-Nov-19 9:42am

What error? Please remember we cannot see you screen, so saying "I got error" tells us nothing. You need to explain clearly what the error message is, and exactly which line it occurs on.

Member 14630006 3-Nov-19 9:48am

the error is on (ImageByte.Value == null) ERROR = 'System.Array' does not contain a definition for 'Value' and no extension method 'Value' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

Richard MacCutchan 3-Nov-19 10:26am

Why are you trying to refer to a property that does not exist? Go to the documentation for SystemArray, and see what actual properties there are available.

Member 14630006 3-Nov-19 10:37am

ok thanks