Path.Combine Method

https://docs.microsoft.com/en-us/dotnet/api/system.io.path.combine?view=netframework-4.8#System_IO_Path_Combine_System_String_System_String_

public static string Combine (string path1, string path2);

Returns

String

The combined paths.

If one of the specified paths is a zero-length string, this method returns the other path.

If path2 contains an absolute path, this method returns path2.

如果不希望path2是绝对路径的话，可以用Path.IsPathRooted函数检查

A rooted path is file path that is fixed to a specific drive or UNIC path; it contrasts with a path that is relative to the current drive or working directory.

For example, on Windows systems, a rooted path begins with a backslash (for example, "Documents") or a drive letter and colon (for example, "C:Documents").

Note that rooted paths can be either absolute (that is, fully qualified) or relative.

An absolute rooted path is a fully qualified path from the root of a drive to a specific directory.

A relative rooted path specifies a drive, but its fully qualified path is resolved against the current directory.

The following example illustrates the difference.

using System;
using System.IO;

class Program
{
    static void Main()
    {
        string relative1 = "C:Documents"; 
        ShowPathInfo(relative1);

        string relative2 = "/Documents";
        ShowPathInfo(relative2);

        string absolute = "C:/Documents";
        ShowPathInfo(absolute);
    }

    private static void ShowPathInfo(string path)
    {
        Console.WriteLine($"Path: {path}");
        Console.WriteLine($"   Rooted: {Path.IsPathRooted(path)}");
        Console.WriteLine($"   Fully qualified: {Path.IsPathFullyQualified(path)}");
        Console.WriteLine($"   Full path: {Path.GetFullPath(path)}");
        Console.WriteLine();
    }
}
// The example displays the following output when run on a Windows system:
//    Path: C:Documents
//        Rooted: True
//        Fully qualified: False
//        Full path: c:Usersuser1DocumentsprojectspathispathrootedDocuments
//
//    Path: /Documents
//       Rooted: True
//       Fully qualified: False
//       Full path: c:Documents
//
//    Path: C:/Documents
//       Rooted: True
//       Fully qualified: True
//       Full path: C:Documents