Stop calling real IO.Path.GetTempPath() when creating a MockFileSystem

[m-wild]

Fixes: #350
Tried to do this with the smallest change possible.
Hard-coding to "C:\" (which will get translated to "/" on unix) as per discussion on the issue.

Let me know if this is okay.

[fgreinacher]

Thanks for your contribution, generally this seems like a safe and valuable change to make.

Could you please verify that a scenario like the one mentioned in #350 (comment) will work with your change?

[m-wild]

I'm not exactly sure how to prove it will fix that.
When i wrote a test like the one in the comment

void Test...() {
    var fs = new MockFileSystem();
    var dirInfo = fs.DirectoryInfo.FromDirectoryName(".");
}

However, no exception was thrown.

I think you would need to have the real System.IO.Path.GetTempPath() throw an exception.

The test MockDirectory_GetCurrentDirectory_ShouldReturnDefaultPathWhenNotSet already tests that calling fileSystem.Directory.GetCurrentDirectory(); succeeds.
We could test that fileSystem.DirectoryInfo.FromDirectoryName("."); also succeeds, but as i mentioned earlier, it was already succeeding! So I'm not sure the test adds any value.

What are your thoughts?

[fgreinacher]

I'd expect the linked snippet to blow up with our current release because the temp directory does not exist in the MockFileSystem. No need to automate the test though, I was just looking for a quick manual validation.

[fgreinacher]

@tehmantra What about adding a test like this:

public void MockFileSystem_DefaultState_CurrentDirectoryExists()
{
   var fs = new MockFileSystem();

   var currentDirectory = fs.DirectoryInfo.FromDirectoryName(".");

   Assert.IsTrue(currentDirectory.Exists);
}

That should ensure that we can use the current directory safely after constructing the MockFileSystem.

[m-wild]

@fgreinacher MockFileSystem will now create the current directory if it doesn't exist.
Added a couple of test cases around this too.

Cheers.

[fgreinacher]

Thanks for your work @tehmantra!