在C＃中将Blob数据从一个表复制到另一个表

Question

basically i have a service which looks at two tables - one resides on a remote server, the other locally. I am trying to write a program which will select any required files from the remote server and copy them locally. i can get this working for standard records but how do i handle the blob in c# - i am just starting out with the language so be gentle

a snippet of what i have is below

public static void BroadcastCheck(String ip_addr)
    {
        OdbcConnection local = new OdbcConnection("DSN=local");
        OdbcConnection cloud = new OdbcConnection("DSN=cloud");
        local.Open();
        cloud.Open();
        OdbcCommand update1 = new OdbcCommand("UPDATE exchange set status = '1' where `status`='0' and inp_date=chg_date and LEFT(filename,12)='" + ip_addr + "' and type='UPDATE'", cloud);
        update1.ExecuteNonQuery();
        OdbcCommand broadcastSelect = new OdbcCommand("select * from exchange where inp_date=chg_date and LEFT(filename,12)='" + ip_addr + "' and status='1' and type='UPDATE'", cloud);
        OdbcDataReader DbReader = broadcastSelect.ExecuteReader();
        int fCount = DbReader.FieldCount;
        byte[] outByte = new byte[500]; 
        while (DbReader.Read())
        {
           String type = DbReader.GetString(0);
           String filename = DbReader.GetString(1);
           String data = DbReader.GetBytes(1);
           OdbcCommand broadcastCopy = new OdbcCommand("INSERT INTO exchange(type,filename) VALUES('"+type+"','"+filename+"'"+data+")", local);
           broadcastCopy.ExecuteNonQuery();


        }
        itouchcloud.Close();
        itouchlocal.Close();
        Console.Write("Broadcast Check Completed \n");

    }

Basically the cloud db is queried and may return multiple results, i want to process each record returned and copy it to the local DB. i have looked around and cant seem to really get a decent solution, i can do this simply in Visual FoxPro 9 so im guessing there is a similar solution.

any help appreciated :)

Answer 1

The first part of the answer is, avoid dynamic SQL if you can. You're using "... VALUES ('"+type+"','"+filename+"'"+data+")" when you should be using "... VALUES (?, ?, ?)".

Then, add the parameters using, for instance,

// sample: the name of the parameter (here @Type) can be anything, and the type and length should match your schema.
broadcastCommand.Parameters.Add("@Type", OleDbType.VarChar, 10).Value = type; 

The question marks will be replaced by the parameters in the order you specify them, so you should add type, then filename, then data, in that order.

Now, the value you specify should ALSO correspond to the type of field you are inserting into. So instead of String, String, String, you might want your variables to be of type String, String, byte[].

There are about a million reasons not to construct your queries dynamically, so I would recommend studying up on how to use the Parameters collection on your OdbcCommand. Start here .

UPDATE

In general you can get DataReader values simply by using the indexer [], without needing to go through the GetXXX() methods. For byte arrays, that's usually simpler, because you don't need to know or try to guess the length beforehand.

You can convert your code to use indexers this way:

String type = (string)DbReader[0];
String filename = (string)DbReader[1];
byte[] data = (byte[])DbReader[2];

Note that your GetBytes() call originally had a 1 in there, but I assume you aren't trying to get the bytes of the filename field. So, if your byte[] data is in another field, use that instead. Be aware, however, that you could also use the string field names just as easily (and it might be clearer the next time you need to read the code):

String type = (string)DbReader["type"]; // replace with whatever your fields are actually called
String filename = (string)DbReader["filename"];
byte[] data = (byte[])DbReader["data"];

On the off-chance you had filename and data both using the same field because data isn't actually in the database and instead you want to take the filename and read that filesystem object in as your data for the insert query, you'll need to use a different method.

byte[] data = System.IO.File.ReadAllBytes(filename); // requires .NET 2.0+

Either way you fill your variables, insert them with a parameterized query as explained above.