StringWriter未沿所有异常路径放置

Question

我正在使用一个StringWriter我传递给foreach循环中写入值的方法.我相信这导致产生两个警告:

CA2000:Microsoft.Reliability:在方法'ToCsvService.ToCsv()'中,对象'sw'不是沿所有异常路径放置的.在对所有引用超出范围之前,调用System.IDisposable.Dispose对象'sw'.

和

CA2202:Microsoft.Usage:对象'sw'可以在方法'ToCsvService.ToCsv()'中多次处理.为避免生成System.ObjectDisposedException,不应在对象上多次调用Dispose.

public string ToCsv()
{
    IEnumerable<string> props = GetProperties();
    StringWriter sw = new StringWriter(); // first warning here
    sw.WriteLine(GetHeadings(props));
    WriteValues(props, sw);

    sw.Close();
    string returnCsv = sw.ToString();
    sw.Dispose(); // second warning here

    return returnCsv;
}

我GetProperties()从被称为方法的列表中省略了,因为它似乎并不相关.

private string GetHeadings(IEnumerable<string> props)
{
    string headings = String.Join(",",
        props.Select(prop =>
            _headings.ContainsKey(prop) ? _headings[prop] : prop));

    return headings;
}

private void WriteValues(IEnumerable<string> props, StringWriter sw)
{
    foreach (object obj in _collection)
    {
        var x = obj.GetType().GetProperties()
            .Where(pi => props.Contains(pi.Name))
            .Select(pi =>
                _format.ContainsKey(pi.Name)
                ? String.Format("{0:" + _format[pi.Name] + "}",
                                pi.GetGetMethod().Invoke(obj, null))
                : pi.GetGetMethod().Invoke(obj, null).ToString());

        string values = String.Join<string>(",", x);

        sw.WriteLine(values);
    }
}

为什么要生成这些警告？

Answer 1

您的代码允许抛出异常导致执行跳过关闭您的语句的可能性StringWriter.您希望确保在异常导致执行离开之前ToCSV关闭sw.

处理此问题的最简单方法是使用using块.using保证在退出块的范围之前处理在子句中构造的对象:

public string ToCsv()
{
    IEnumerable<string> props = GetProperties();
    using (StringWriter sw = new StringWriter())
    {
        sw.WriteLine(GetHeadings(props));
        WriteValues(props, sw);
        return sw.ToString();
    }
}

请注意,您并不需要通话双方Close并Dispose在StringWriter的.仅仅Dispose是不够的.

通常,您需要using围绕创建和使用所有实现的对象IDisposable(如同StringWriter)来包装块.这将确保无论抛出什么异常,对象总是被正确处理掉.