As others have said, Path.Combine doesn’t change the separator.
However if you convert it to a full path:
Path.GetFullPath(Path.Combine("test1/test2", "test3\\test4"))
the resulting fully qualified path will use the standard directory separator (backslash for Windows).
Note that this works on Windows because both \ and / are legal path separators:
Path.DirectorySeparatorChar = \
Path.AltDirectorySeparatorChar = /
If you run on, say, .NET Core 2.0 on Linux, only the forward slash is a legal path separator:
Path.DirectorySeparatorChar = /
Path.AltDirectorySeparatorChar = /
and in this case it won’t convert backslash to forward slash, because backslash is not a legal alternate path separator.