c# - 从 protobuf-net 读取时跳过数据

Question

我有一个文件，其中有 n 个对象实例被序列化并存储在其中。 在某些情况下，我需要跳过存储的记录并以 k 2k 3k ... n 序列读取，而不是正常的 1 2 3 ... n 序列。由于这些实例的长度不同，我编写了这段代码来跳过记录，但它抛出了无效的线类型异常。 (异常有一个指向 this question 的链接，但没有帮助)

这只是一个简单的错误还是我以错误的方式这样做？

long position = stream.Position;
int length = 0;
for (int i = 0; i < skipRate; ++i)
{
    Serializer.TryReadLengthPrefix(stream, PrefixStyle.Fixed32, out length);
    position += length;
    stream.Position = position;
    Console.WriteLine("Skipped " + length + " bytes");
}

MYCLASS retval = Serializer.DeserializeWithLengthPrefix<MYCLASS>(stream, PrefixStyle.Fixed32, 1);

编辑:

long position = stream.Position;
int length;
for (int i = 0; i < skipRate; ++i)
{
    Serializer.TryReadLengthPrefix(stream, PrefixStyle.Fixed32, out length);
    length += (int)(stream.Position - position); // add number of bytes that TryReadLengthPrefix moves stream

    stream.Position = position; // Rewind
    Serializer.DeserializeWithLengthPrefix<SimulationOutputData>(stream, PrefixStyle.Fixed32, 1);
    Console.WriteLine("Try read returns " + length + ", but deserialize goes on " + (stream.Position-position));
}

输出:

Try read returns 1209, but deserialize goes on 1209
Try read returns 1186, but deserialize goes on 1186
Try read returns 1186, but deserialize goes on 1186
Try read returns 1186, but deserialize goes on 1186
Try read returns 1186, but deserialize goes on 1186
Try read returns 1209, but deserialize goes on 1209
Try read returns 1167, but deserialize goes on 1167
.
.
.

这段代码有效(为什么？!!有什么区别？):

Serializer.TryReadLengthPrefix(stream, PrefixStyle.Fixed32, out length);
length += (int)(stream.Position - position);
stream.Position = position;
position += length;
stream.Position = position;

Answer 1

athabaska(评论)有其要点。您的 position 增量不考虑标题。一个简洁的实现可能是:

for (int i = 0; i < skipRate; ++i)
{
    int length;
    if(!Serializer.TryReadLengthPrefix(stream, PrefixStyle.Fixed32, out length))
    {
        throw new EndOfStreamException(); // not enough records
    }
    s.Seek(length, SeekOrigin.Current);
}

MYCLASS retval = Serializer.DeserializeWithLengthPrefix<MYCLASS>(
    stream, PrefixStyle.Fixed32, 0);