public void BitmapService_Should_SaveBitmapAsPngImage()
{
//Arrange
var mockedStream = Mock.Of <Stream>();
var fileSystemMock = new Mock <IFileSystem>();
fileSystemMock
.Setup(_ => _.OpenOrCreateFileStream(It.IsAny <string>()))
.Returns(mockedStream);
fileSystemMock.SetupAllProperties();
var sut = new BitmapService(fileSystemMock.Object);
var renderBitmap = new Canvas();
var path = new Uri("//A_valid_path");
//Act
sut.SaveBitmapAsPngImage(path, renderBitmap);
//Assert
Mock.Get(mockedStream).Verify(_ => _.Write(It.IsAny <byte[]>(), It.IsAny <int>(), It.IsAny <int>()));
}