What is difference between File.Exists(“”) and Fil

问题:

I have an *.exe file in \ProgramFiles(x86)\MyAppFolder.

In x86 application I check if the file exists (64 bit system). simple:

bool fileExists = File.Exists(@"\ProgramFiles(x86)\MyAppFolder\Manager.exe");

The result is: "fileExists == false" (the file is really there). It's Virtualization as I understand.That issue described here Its ok. But next code:

bool fileExists = new FileInfo("\\Path").Exists;

"fileExists == true"

Why is the result different in 1st and 2nd cases?

var controller = new ServiceController(Product.ServiceName);
_manager.Enabled = controller.Status == ServiceControllerStatus.Running;

var info = new DirectoryInfo(Assembly.GetExecutingAssembly().Location);

var s = File.Exists(@"D:\TFS\GL_SOURCES\Teklynx_LPM\Dev\Server\Debug\Manager.exe");

string pathToManager = string.Empty;

if (info.Parent != null)
{
    var pathToModule = info.Parent.FullName;
    pathToManager = Path.Combine(pathToModule,"Manager.exe").Replace(" ",string.Empty);
}

//works good

var fileInfo = new FileInfo(pathToManager);
var managerSeparator = new ToolStripSeparator()
{
    Visible = _manager.Visible = fileInfo.Exists // true
};

//Does not work

var managerSeparator = new ToolStripSeparator()
{
    Visible = _manager.Visible = File.Exists(pathToManager ) // false
};

Thanks!

回答1:

There is no difference, these methods use the exact same internal helper method inside the .NET Framework. Something you can see with a decompiler or the Reference Source source code, the helper method name is File.FillAttributeInfo().

Having duplication like this in the .NET Framework is pretty unusual, not exactly a Good Thing to have more than one way to accomplish the same thing. The File class is however special, it got added after a usability study conducted before .NET 1.0 shipped. The test subjects just had the basic BCL classes to work with, like FileStream and FileInfo, and otherwise only had MSDN documentation available. The test results were not very good, the File class got added to help programmers fall in the pit of success writing very basic file manipulation code. Like File.Exists() and File.ReadAllLines().

So it doesn't have anything to do with the classes, you are just using them wrong. Like not actually using the same path. Do go easy on the forward slashes, the mapping to backward slashes happens inside Windows and is inconsistently implemented in other code. Using // certainly doesn't do what you hope it does.

回答2:

This is about the only difference and it has more to do with the nature of FileInfo:

FileInfo fileInfo = new FileInfo("myFile.txt"); // non-existent file
Console.WriteLine(fileInfo.Exists);             // false
File.Create("myFile.txt");
Console.WriteLine(File.Exists("myFile.txt"));   // true
Console.WriteLine(fileInfo.Exists);             // false

So as you can see the value of fileInfo.Exists is cached the first time you use it.

Other than that, they do the same thing behind the scenes.

回答3:

in your first case, the file path is incorrect, you need spaces in "Program Files (x86)".

Secondly, the Path.Combine will return a Directory path so you'll end up with something like "C:\Program Files (x86)\MyAppFolder\Manager.exe\" so it's a bad idea.

Both methods work the same way, so make sure you check that the path is correct.

回答4:

The difference between File.Exists() and new FileInfo().Exists on it's behavior when full path (directory name + file name) is long:

var f = @"C:\Program Files (x86)\MyAppFolder\many_subfolders\manager.exe";

//f.length > 260 characters

bool fileExists = File.Exists(f); //return false, even if the file exists

// Throw exception: "The specified path, file name, or both are too long. The fully qualified file name must be less than 260 characters, and the directory name must be less than 248 characters."
bool fileInfoExists = new FileInfo(f).Exists;

回答5:

I've replicated your scenario using the below Linqpad script

var f = @"C:\Program Files (x86)\MyAppFolder\manager.exe";

bool fileExists = File.Exists(f);
bool fileInfoExists = new FileInfo(f).Exists;

fileExists.Dump();
fileInfoExists.Dump();

Ran this both when the file existed and when it did not and both produced the same output each time. Maybe try this on your system and see if you still see differences.