为什么FileShare无法按预期工作？

Question

While writing some code dealing with logs and files, I've discovered some baffling behaviour in windows file io. Does anyone know why this test would fail with "cannot read file" message?

[TestMethod]
public void SouldAllowReads()
{
    using (var file = File.Open(_path, FileMode.Create, FileAccess.Write, FileShare.Read))
    {
        using (var file2 = File.Open(_path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
        {
            //works ok, doesn't throw
        }

        try
        {
            using (var file3 = File.Open(_path, FileMode.Open, FileAccess.Read, FileShare.Read))
            { 
                //fails
            }
        }
        catch (IOException)
        {
            Assert.Fail("cannot read file");
        }
    }
}

PS. _path = Path.GetTempFileName();

EDIT:

I'll mark elevener answer as correct one, but there is one thing that bothers me in this design. .NET methods such as File.ReadAllText(_path) throw exceptions, which just shouldn't happen.

For example this snipped my test would also fail assertion:

        try
        {
            string text = File.ReadAllText(_path);
        }
        catch (IOException)
        {
            Assert.Fail("cannot read file");
        }

Answer 1

您有var file =用FileAccess.Write打开，并且同时试图以文件共享模式FileShare.Read打开var file3 =，不允许同时进行写访问。