如何在没有代码分析警告的情况下同时使用StringWriter和HtmlWriter

Question

我使用的是.net，需要获取一些html文本，所以我想我应该结合使用HtmlTextWriter和StringWriter来获得格式良好的html。但是，尽管我以各种不同的方式编写代码，我仍然会收到来自静态代码分析器的警告(使用Microsoft all Rules)。在下面的代码示例中，我在注释中显示了代码分析器警告。为了简化代码，我实际上没有对HtmlTextWriter进行任何调用(您将在每个函数中看到相应的注释)。如何正确编写代码以避免出现警告？

// CA2000 : Microsoft.Reliability : In method 'Default.Func1()', object 'stringWriter' is not disposed along all exception paths. Call System.IDisposable.Dispose on object 'stringWriter' before all references to it are out of scope.
public static string Func1()
{
    string html;
    StringWriter stringWriter;
    using (var writer = new HtmlTextWriter(stringWriter = new StringWriter()))
    {
        // You would do some stuff with the writer here, but not for this example.

        html = stringWriter.ToString();
    }
    return html;
}

// CA2202 : Microsoft.Usage : Object 'stringWriter' can be disposed more than once in method 'Default.Func2()'. To avoid generating a System.ObjectDisposedException you should not call Dispose more than one time on an object.: Lines: 45
public static string Func2()
{
    string html;
    StringWriter stringWriter = null;
    try
    {
        using (var writer = new HtmlTextWriter(stringWriter = new StringWriter()))
        {
            // You would do some stuff with the writer here, but not for this example.

            html = stringWriter.ToString();
        }
    }
    finally
    {
        if (stringWriter != null)
            stringWriter.Dispose();
    }
    return html;
}

// CA2202 : Microsoft.Usage : Object 'stringWriter' can be disposed more than once in
// method 'Default.Func3()'. To avoid generating a System.ObjectDisposedException 
// you should not call Dispose more than one time on an object.: Lines: 61
public static string Func3()
{
    string html;
    using (var stringWriter = new StringWriter())
    {
        using (var writer = new HtmlTextWriter(stringWriter))
        {
            // You would do some stuff with the writer here, but not for this example.

            html = stringWriter.ToString();
        }
    }
    return html;
}

// CA2202 : Microsoft.Usage : Object 'stringWriter' can be disposed more than once in 
// method 'Default.Func4()'. To avoid generating a System.ObjectDisposedException you 
// should not call Dispose more than one time on an object.: Lines: 77
public static string Func4()
{
    string html;
    using (StringWriter stringWriter = new StringWriter())
    {
        using (HtmlTextWriter writer = new HtmlTextWriter(stringWriter))
        {
            // You would do some stuff with the writer here, but not for this example.

            html = stringWriter.ToString();
        }
    }
    return html;
}

// CA2202 : Microsoft.Usage : Object 'stringWriter' can be disposed more than once in 
// method 'Default.Func5()'. To avoid generating a System.ObjectDisposedException you 
// should not call Dispose more than one time on an object.: Lines: 100
public static string Func5()
{
    string html;
    StringWriter stringWriter = null;
    try
    {
        stringWriter = new StringWriter();
        using (HtmlTextWriter htmlTextWriter = new HtmlTextWriter(stringWriter))
        {
            // You would do some stuff with the writer here, but not for this example.

            html = stringWriter.ToString();
        }
    }
    finally
    {
        if (stringWriter != null)
            stringWriter.Dispose();
    }
    return html;
}

Answer 1

修改您的Func5如下：

public static string Func5()
{
    string html;
    StringWriter stringWriter = null;
    try
    {
        stringWriter = new StringWriter();
        using (HtmlTextWriter htmlTextWriter = new HtmlTextWriter(stringWriter))
        {
            stringWriter = null;

            // You would do some stuff with the writer here, but not for this example.

            html = htmlTextWriter.InnerWriter.ToString();
        }
    }
    finally
    {
        if (stringWriter != null)
            stringWriter.Dispose();
    }
    return html;
}

关键是将stringWriter变量设置为null (这不会影响HtmlTextWriter实例的InnerWriter )，然后使用InnerWriter.ToString()获取HtmlTextWriter。

这实际上只是MSDN文章中示例的修改版本，在前面的评论中引用，但仅适用于您的用法。

Answer 2

实际上，没有办法使此代码避免警告，因为在这种特殊情况下，代码分析是错误的。

正确的代码是添加CodeAnalysis.SuppressMessage属性的Func3：

// Code Analysis is incorrectly assuming that HtmlTextWriter.Dispose will dispose of the InnerWriter, but it actually does not.
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2202:Do not dispose objects multiple times")]
public static string Func3()
{
    string html;
    using (var stringWriter = new StringWriter())
    {
        using (var writer = new HtmlTextWriter(stringWriter))
        {
            // You would do some stuff with the writer here, but not for this example.

            // I prefer to use writer.InnerWriter as opposed to stringWriter for clarity.
            html = writer.InnerWriter.ToString();
        }
    }
    return html;
}

CA2202的文档使用了一个StreamWriter处理其流的示例，这是正确的，但是HtmlTextWriter没有处理它的内部TextWriter (可以通过对StringWriter进行子类化并在override dispose中设置断点来验证)。这有点让人困惑，因为HtmlTextWriter派生自TextWriter，而StringWriter也派生自TextWriter (而不是StreamWriter，它的Stream是两个完全不同的类)，那么为什么HtmlTextWriter需要InnerWriter呢？…但不管怎样，这就是它的工作方式。

此外，文档还说不要取消显示此警告，因为“即使已知对象的Dispose可以多次安全地调用，实现将来也可能会发生变化。”但是，在这种情况下，Dispose是而不是被多次调用，因此可以安全地取消该警告。

但是不要相信我的话！这就是证据：

using System;
using System.IO;
using System.Web.UI;

namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            StreamWillBeDisposed();
            TextWriterWillNotBeDisposed();
        }

        public static void StreamWillBeDisposed()
        {
            Stream stream = new DebugMemoryStream();
            using (StreamWriter writer = new StreamWriter(stream))
            {
                // Use the writer object...
            }// Underlying Stream will be disposed here by the StreamWriter
        }

        public static void TextWriterWillNotBeDisposed()
        {
            TextWriter stringWriter = new DebugStringWriter();
            using (HtmlTextWriter writer = new HtmlTextWriter(stringWriter))
            {
                // Use the writer object...
            }// Underlying TextWriter will NOT be disposed here by the HtmlTextWriter
        }
    }


    public class DebugMemoryStream : MemoryStream
    {
        protected override void Dispose(bool disposing)
        {
            // This Stream will be disposed when the StreamWriter is disposed
            System.Diagnostics.Debugger.Break();
            base.Dispose(disposing);
        }
    }

    public class DebugStringWriter : StringWriter
    {
        protected override void Dispose(bool disposing)
        {
            // This code will never see the light of day
            System.Diagnostics.Debugger.Break();
            base.Dispose(disposing);
        }
    }

}

Answer 3

因为StringWriter是可丢弃的，所以你可以用另一个来包装你的内部写入器。

using (StringWriter stringWriter = new StringWriter())
{
    using (var writer = new HtmlTextWriter(stringWriter))
    {
         html = stringWriter.ToString();  
    }
}
return html;