c# – 如何在DataReader上实现Peek()函数？

sealed public class PeekDataReader : IDataReader
{
    private IDataReader wrappedReader;
    private bool wasPeeked;
    private bool lastResult;

    public PeekDataReader(IDataReader wrappedReader)
    {
        this.wrappedReader = wrappedReader;
    }

    public bool Peek()
    {
        // If the previous operation was a peek,do not move...
        if (this.wasPeeked)
            return this.lastResult;

        // This is the first peek for the current position,so read and tag
        bool result = Read();
        this.wasPeeked = true;
        return result;
    }

    public bool Read()
    {
        // If last operation was a peek,do not actually read
        if (this.wasPeeked)
        {
            this.wasPeeked = false;
            return this.lastResult;
        }

        // Remember the result for any subsequent peeks
        this.lastResult = this.wrappedReader.Read();
        return this.lastResult;
    }

    public bool NextResult()
    {
        this.wasPeeked = false;
        return this.wrappedReader.NextResult();
    }

    // Add pass-through operations for all other IDataReader methods
    // that simply call on 'this.wrappedReader'
}

请注意,对于所有未受影响的属性,这确实需要相当多的传递代码,但好处是它是一个通用的抽象,可以“窥视”结果集中的任何位置,而无需继续前进的“读取”操作.

使用：

using (IDataReader reader = new PeekDataReader(/* actual reader */))
{
    if (reader.Peek())
    {
        // perform some operations on the first row if it exists...
    }

    while (reader.Read())
    {
        // re-use the first row,and then read the remainder...
    }
}

请注意,如果前一个操作不是’Peek()’,则每个’Peek()’调用实际上将移动到下一个记录.使用’Read()’操作保持这种对称性提供了更简单的实现和更优雅的API.