How do I get .NET's Path.Combine to convert forward slashes to backslashes?

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.

Because your "test1/test2" is already a string literal, Path.Combine will not change the '/' for you to a '\'.

Path.Combine will only concat the 2 string literals with the appropriate path delimiter used by the OS, in this case Windows, which is '\', from there your output

test1/test2\test3\test4

Your best bet would be the string.Replace.

Try using the Uri class. It will create valid Uris for the correct target machine (/ -> \).