Movatterモバイル変換

After a file is loaded into thefile table, we can use theMySqlDataReader class to retrieve it.

The following code retrieves a row from thefile table, then loads the data into aFileStream object to be written to disk:

C# Code Example

MySql.Data.MySqlClient.MySqlConnection conn;MySql.Data.MySqlClient.MySqlCommand cmd;MySql.Data.MySqlClient.MySqlDataReader myData;conn = new MySql.Data.MySqlClient.MySqlConnection();cmd = new MySql.Data.MySqlClient.MySqlCommand();string SQL;UInt32 FileSize;byte[] rawData;FileStream fs;conn.ConnectionString = "server=127.0.0.1;uid=root;" +    "pwd=12345;database=test";SQL = "SELECT file_name, file_size, file FROM file";try{    conn.Open();    cmd.Connection = conn;    cmd.CommandText = SQL;    myData = cmd.ExecuteReader();    if (! myData.HasRows)        throw new Exception("There are no BLOBs to save");    myData.Read();    FileSize = myData.GetUInt32(myData.GetOrdinal("file_size"));    rawData = new byte[FileSize];    myData.GetBytes(myData.GetOrdinal("file"), 0, rawData, 0, (int)FileSize);    fs = new FileStream(@"C:\newfile.png", FileMode.OpenOrCreate, FileAccess.Write);    fs.Write(rawData, 0, (int)FileSize);    fs.Close();    MessageBox.Show("File successfully written to disk!",        "Success!", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);    myData.Close();    conn.Close();}catch (MySql.Data.MySqlClient.MySqlException ex){    MessageBox.Show("Error " + ex.Number + " has occurred: " + ex.Message,        "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);}

Visual Basic Code Example

Dim conn As New MySqlConnectionDim cmd As New MySqlCommandDim myData As MySqlDataReaderDim SQL As StringDim rawData() As ByteDim FileSize As UInt32Dim fs As FileStreamconn.ConnectionString = "server=127.0.0.1;" _    & "uid=root;" _    & "pwd=12345;" _    & "database=test"SQL = "SELECT file_name, file_size, file FROM file"Try    conn.Open()    cmd.Connection = conn    cmd.CommandText = SQL    myData = cmd.ExecuteReader    If Not myData.HasRows Then Throw New Exception("There are no BLOBs to save")    myData.Read()    FileSize = myData.GetUInt32(myData.GetOrdinal("file_size"))    rawData = New Byte(FileSize) {}    myData.GetBytes(myData.GetOrdinal("file"), 0, rawData, 0, FileSize)    fs = New FileStream("C:\newfile.png", FileMode.OpenOrCreate, FileAccess.Write)    fs.Write(rawData, 0, FileSize)    fs.Close()    MessageBox.Show("File successfully written to disk!", "Success!", MessageBoxButtons.OK, MessageBoxIcon.Asterisk)    myData.Close()    conn.Close()Catch ex As Exception    MessageBox.Show("There was an error: " & ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)End Try

After connecting, the contents of thefile table are loaded into aMySqlDataReader object. TheGetBytes method of theMySqlDataReader is used to load theBLOB into a byte array, which is then written to disk using aFileStream object.

TheGetOrdinal method of the MySqlDataReader can be used to determine the integer index of a named column. Use of the GetOrdinal method prevents errors if the column order of theSELECT query is changed.