我正在尝试使用NpgSQL v3.0.4.0编写和读取PostgreSQL数据库V9.4.x中的大对象。所以我创建了一个方法来将数据库中的本地文件存储为大对象，如下所示： / p>
public static async Task<uint> InsertLargeObjectFileToDB(string theFilePath)
{
// connecting to DB
string connstring = MakeDatabaseConnectionString();
// make a connection object
NpgsqlConnection Conn = new NpgsqlConnection(connstring);
try
{
await OpenDatabaseConnection(Conn); //open database connection
}
catch (Exception Ex)
{
throw (Ex);
}
uint oid; // to store object ID number
try
{
// Reading and writing Large Objects requires the use of a transaction
using (FileStream fs = new FileStream(theFilePath, FileMode.Open))
{
using (var transaction = Conn.BeginTransaction())
{
// Retrieve a Large Object Manager for this connection
var manager = new NpgsqlLargeObjectManager(Conn);
// Create a new empty file, returning the identifier to later access it
oid = manager.Create();
using (var DbStream = manager.OpenReadWrite(oid))
{
long theFileSize = GetFileSizeInBytes(theFilePath);
StreamReader sr = new StreamReader(fs);
byte[] buffer = new byte[1024 * 1024];
while (sr.BaseStream.Position < theFileSize)
{
await fs.ReadAsync(buffer, 0, buffer.Length);
await DbStream.WriteAsync(buffer, 0, buffer.Length);
}
}
transaction.Commit();
return oid;
}
}
}
catch // any error
{
// exception
Exception ex = new Exception();
ex.Data.Add(ex.Data.Count, "some error message");
throw ex;
}
}
然后我创建了一个方法来读取一个大对象并将其存储在temp目录中随机命名的文件中，如下所示：
public static async Task<string> GetLargeObjectFileFromDB(uint oid)
{
// connecting to DB
string connstring = MakeDatabaseConnectionString();
// make a connection object
NpgsqlConnection Conn = new NpgsqlConnection(connstring);
try
{
await OpenDatabaseConnection(Conn); //open database connection
}
catch (Exception Ex)
{
throw (Ex);
}
// getting a temorary file name from the system to use it to store the fetched file
string TempFileName = GetRandomFileNameFromSystem();
try
{
using (FileStream LocalStream = new FileStream(TempFileName, FileMode.Create))
{
using (var transaction = Conn.BeginTransaction())
{
// create a Large Object Manager for this connection
var DbLargeObjectManager = new NpgsqlLargeObjectManager(Conn);
using (var DbStream = await DbLargeObjectManager.OpenReadAsync(oid))
{
byte[] buffer = new byte[1024 * 1024];
// get the length of the database object
long LengthOfDbObject = DbStream.Length;
while (DbStream.Position < LengthOfDbObject)
{
// read from the database to buffer
await DbStream.ReadAsync(buffer, 0, buffer.Length);
//write from buffer to local file
await LocalStream.WriteAsync(buffer, 0, buffer.Length);
}
}
transaction.Commit();
return TempFileName;
}
}
}
catch // any error
{
// exception
Exception ex = new Exception();
ex.Data.Add(ex.Data.Count, "Error inserting object in database");
throw ex;
}
}
你可以看到我一直在异步写作。问题是我对这2个方法进行了测试，这个测试将6MB文件写入数据库，但是当我从数据库中再次读取该文件时，它大约大400kb，当然（当然）MD5哈希值不匹配。不要忘记提及没有异常发生的事情。如果您关心它，这是测试：
private async void button3_Click(object sender, EventArgs e)
{
listBox1.Items.Clear();
// getting the MD5 hash of the source file
string FirstMd5Hash = GetMd5OfFile(tbSourceFile.Text);
// performance measurment ##########################################
DateTime dt1 = new DateTime(DateTime.Now.Ticks);
listBox1.Items.Add("Uploading file to database");
//storing that file into database
uint oid = await InsertLargeObjectFileToDB(tbSourceFile.Text);
// performance measurment #########################################################
DateTime dt2 = new DateTime(DateTime.Now.Ticks);
TimeSpan ts = new TimeSpan(dt2.Ticks - dt1.Ticks);
listBox1.Items.Add("Large object (oid = " + oid + ") inserted in " + ts.Seconds + "." + ts.Milliseconds + " seconds");
// performance measurment ##########################################
dt1 = new DateTime(DateTime.Now.Ticks);
listBox1.Items.Add("reading file back from the database");
// get that object back from the database into temporary file
string ReturnedFileName = await PostgresqlLargeObject.GetLargeObjectFileFromDB(oid);
// performance measurment #########################################################
dt2 = new DateTime(DateTime.Now.Ticks);
ts = new TimeSpan(dt2.Ticks - dt1.Ticks);
listBox1.Items.Add("reading done in " + ts.Seconds + "." + ts.Milliseconds + " seconds");
//calculate md5 of that file
string SecondMd5Hash = GetMd5OfFile(ReturnedFileName);
// compare the 2 hashes
if (FirstMd5Hash == SecondMd5Hash)
{
listBox1.Items.Add("the hashes are match . MD5 = " + FirstMd5Hash);
}
else
{
listBox1.Items.Add("failed with oid = " + oid);
tbFileBack.Text = ReturnedFileName;
}
}
有什么问题？
答案 0 :(得分：1)
好吧我已经找到了问题，事实证明（除了在帐户中接受Emil回答）你必须异步读取然后同步写入。我不知道为什么会这样。这段代码工作：
using (FileStream LocalStream = new FileStream(TempFileName, FileMode.Create))
{
using (var transaction = Conn.BeginTransaction())
{
// create a Large Object Manager for this connection
var DbLargeObjectManager = new NpgsqlLargeObjectManager(Conn);
using (var DbStream = await DbLargeObjectManager.OpenReadAsync(oid))
{
byte[] buffer = new byte[262144]; //256KB
// query the database stream length
long DatabaseStreamLength = DbStream.Length;
while (DbStream.Position < DatabaseStreamLength)
{
// read from the database to buffer (async)
int bufferByteCount = await DbStream.ReadAsync(buffer, 0, buffer.Length);
//write from buffer to local file (sync)
LocalStream.Write(buffer, 0, bufferByteCount);
}
}
transaction.Commit();
答案 1 :(得分：0)
当你执行await <stream>.WriteAsync(buffer, 0, buffer.Length);时，你应该写上一个读取方法实际读取的字节数（该值将被返回）。