CleanFileCleaner.cs

using System.IO.Abstractions;

public class CleanFileCopier
{
    private readonly IFileSystem _fileSystem;

    public CleanFileCopier(): this(new FileSystem())
    {
    }

    internal CleanFileCopier(IFileSystem fileSystem)
    {
        _fileSystem = fileSystem;
    }

    public string CopyFile(string sourceFilePath, string destinationDirectoryPath)
    {
        if (!_fileSystem.Directory.Exists(destinationDirectoryPath))
        {
            _fileSystem.Directory.CreateDirectory(destinationDirectoryPath);
        }

        var fileName = _fileSystem.Path.GetFileName(sourceFilePath);
        var destFileName = _fileSystem.Path.Combine(destinationDirectoryPath, fileName);
        _fileSystem.File.Copy(sourceFilePath, destFileName);
        return destFileName;
    }
}

CleanFileCleanerTests.cs

    using System.IO.Abstractions.TestingHelpers;
    
    [TestFixture]
    public class FileCopierTests
    {
        [Test]
        public void CopyFile_SourceFileExistsDestinationDirDoesNotExist_FileCopiedToDestination()
        {
            // Arrange
            var mockFileSystem = new MockFileSystem();
            mockFileSystem.AddFile(@"c:\file.txt", MockFileData.NullObject);

            var cleanFileCopier = new CleanFileCopier(mockFileSystem);

            // Act
            var destFilePath = cleanFileCopier.CopyFile(@"c:\file.txt", @"c:\users\borismod.net");

            // Assert
            Assert.AreEqual(@"c:\users\borismod.net\file.txt", destFilePath);
            Assert.IsTrue(mockFileSystem.File.Exists(destFilePath));
        }
    }

LegacyFileCopier.cs

using System.IO;

public class LegacyFileCopier
{
    public string CopyFile(string sourceFilePath, string destinationDirectoryPath)
    {
        if (!Directory.Exists(destinationDirectoryPath))
        {
            Directory.CreateDirectory(destinationDirectoryPath);
        }

        var fileName = Path.GetFileName(sourceFilePath);
        var destFileName = Path.Combine(destinationDirectoryPath, fileName);
        File.Copy(sourceFilePath, destFileName);
        return destFileName;
    }
}