How to fetch a binary data from database

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I've a list of hexadecimal number with a data type varbinary. But when I run a query, nothing shown as a result.

What I have tried:

Currently, I'm getting an error:
System.InvalidOperationException: 'Invalid attempt to read when no data is present.'

Can I know what is the best way to fetch the binary data from the database?

Posted 8-Oct-20 21:45pm

AMK16

Updated 9-Oct-20 22:19pm

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Richard Deeming 9-Oct-20 5:01am

Your code is vulnerable to SQL Injection[^]. NEVER use string concatenation to build a SQL query. ALWAYS use a parameterized query.

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[^]

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Richard Deeming · Answer 1 · 2020-10-08T23:07:00

Use parameters to fix the SQL Injection[^] vulnerabilities in your code.

Wrap all disposable objects in using blocks.

Don't share connection, command, dataadapter, or datareader objects between invocations of your method.

There's no need to use a dataadapter to execute a single update command.

C#

string SNE = data["End_Rg"];

using (var command = new SqlCommand("UPDATE Range SET status = 'Used' WHERE End_SerialNum = @SNE", cnn))
{
    command.Parameters.AddWithValue("@SNE", SNE);
    command.ExecuteNonQuery();
}

using (var command = new SqlCommand("SELECT Start_Range, End_Range, Status FROM SDCard_SNRange WHERE End_SerialNum = @SNE", cnn))
{
    command.Parameters.AddWithValue("@SNE", SNE.Substring(2));
    
    using (var reader = command.ExecuteReader())
    {
        if (reader.HasRows)
        {
            using (var tw = File.AppendText(logPath))
            {
                while (reader.Read())
                {
                    tw.WriteLine("Update status");
                    tw.WriteLine("Start Range: {0}", dataReader["Start_Range"]);
                    tw.WriteLine("End Range: {0}", dataReader["End_Range"]);
                    tw.WriteLine("Status: {0}", dataReader["Status"]);
                }
            }
        }
    }
}

Richard MacCutchan · Answer 2 · 2020-10-08T23:12:00

The error message is clearly telling you that your dataReader does not contain any records. That tells you that the previous query did not find any matching rows. You should check the value you are using as the search criteria in your SELECT clause. But you also need to add checks in all your database code to see whether the requests were successful or not. As it stands your code could run for weeks without ever doing anything correctly and you would never know.

Also take note of the comment above from Richard Deemimg, which is very important.

How to fetch a binary data from database

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