System.IO.Path.GetPathRoot string.Empty or whitespace character string should throw ArgumentException (#11387)

* GetPathRoot string.Empty and string whitespace throws ArgumentException

* throw in next line

* Refined method description

* Unix impl adjusted

* Indentation

* pr feedback

* Delete Path.Unix.cs

* Adding param to ArgumentException

2 files changed, 12 insertions, 3 deletions

diff --git a/src/mscorlib/shared/System/IO/Path.Unix.cs b/src/mscorlib/shared/System/IO/Path.Unix.cs
index 500c60aa8c..68c5f70036 100644
--- a/src/mscorlib/shared/System/IO/Path.Unix.cs
+++ b/src/mscorlib/shared/System/IO/Path.Unix.cs
@@ -23,7 +23,7 @@ namespace System.IO
                 throw new ArgumentNullException(nameof(path));
 
             if (path.Length == 0)
-                throw new ArgumentException(SR.Arg_PathIllegal);
+                throw new ArgumentException(SR.Arg_PathIllegal, nameof(path));
 
             PathInternal.CheckInvalidPathChars(path);
 
@@ -193,10 +193,15 @@ namespace System.IO
             return path.Length > 0 && path[0] == PathInternal.DirectorySeparatorChar;
         }
 
+        // The resulting string is null if path is null. If the path is empty or
+        // only contains whitespace characters an ArgumentException gets thrown.
         public static string GetPathRoot(string path)
         {
             if (path == null) return null;
-            return IsPathRooted(path) ? PathInternal.DirectorySeparatorCharAsString : String.Empty;
+			if (string.IsNullOrWhiteSpace(path))
+                throw new ArgumentException(SR.Arg_PathIllegal, nameof(path));
+
+			return IsPathRooted(path) ? PathInternal.DirectorySeparatorCharAsString : String.Empty;
         }
 
         /// <summary>Gets whether the system is case-sensitive.</summary>
diff --git a/src/mscorlib/shared/System/IO/Path.Windows.cs b/src/mscorlib/shared/System/IO/Path.Windows.cs
index d6f0c628c3..1e573cd95d 100644
--- a/src/mscorlib/shared/System/IO/Path.Windows.cs
+++ b/src/mscorlib/shared/System/IO/Path.Windows.cs
@@ -136,10 +136,14 @@ namespace System.IO
         // path on the current drive), "X:" (a relative path on a given drive,
         // where X is the drive letter), "X:\" (an absolute path on a given drive),
         // and "\\server\share" (a UNC path for a given server and share name).
-        // The resulting string is null if path is null.
+        // The resulting string is null if path is null. If the path is empty or
+        // only contains whitespace characters an ArgumentException gets thrown.
         public static string GetPathRoot(string path)
         {
             if (path == null) return null;
+            if (string.IsNullOrWhiteSpace(path)) 
+                throw new ArgumentException(SR.Arg_PathIllegal, nameof(path));
+
             PathInternal.CheckInvalidPathChars(path);
 
             // Need to return the normalized directory separator