Fix UnixEncoding.GetBytes() for empty strings (#18520)

Current versions of the mono return a null pointer as address of the first
element for empty arrays, i.e.

    fixed (byte* ptr = new byte[0]) Console.WriteLine (ptr == null);

will print "true". This causes a problem in the UnixEncoding class where it
triggers a check for null pointers. Fix this by allowing null pointers when
the number of elements is zero.

2 files changed, 19 insertions, 2 deletions

diff --git a/mcs/class/Mono.Posix/Mono.Unix/UnixEncoding.cs b/mcs/class/Mono.Posix/Mono.Unix/UnixEncoding.cs
index d869b1b90c6..26529629fe0 100644
--- a/mcs/class/Mono.Posix/Mono.Unix/UnixEncoding.cs
+++ b/mcs/class/Mono.Posix/Mono.Unix/UnixEncoding.cs
@@ -311,8 +311,8 @@ public class UnixEncoding : Encoding
 
 	public unsafe override int GetBytes(char* chars, int charCount, byte* bytes, int byteCount)
 	{
-		if (bytes == null || chars == null)
-			throw new ArgumentNullException (bytes == null ? "bytes" : "chars");
+		if ((bytes == null && byteCount != 0) || (chars == null && charCount != 0))
+			throw new ArgumentNullException ((bytes == null && byteCount != 0) ? "bytes" : "chars");
 
 		if (charCount < 0 || byteCount < 0)
 			throw new ArgumentOutOfRangeException (charCount < 0 ? "charCount" : "byteCount");
diff --git a/mcs/class/Mono.Posix/Test/Mono.Unix/UnixEncodingTest.cs b/mcs/class/Mono.Posix/Test/Mono.Unix/UnixEncodingTest.cs
index 2f0747294d3..042c8dacf4b 100644
--- a/mcs/class/Mono.Posix/Test/Mono.Unix/UnixEncodingTest.cs
+++ b/mcs/class/Mono.Posix/Test/Mono.Unix/UnixEncodingTest.cs
@@ -995,6 +995,23 @@ namespace MonoTests.Mono.Unix {
 			);
 		}
 
+		[Test]
+		public void TestEmptyString ()
+		{
+			byte[] data = new byte [] {};
+			Encoding enc = new UnixEncoding ();
+
+			string s = enc.GetString (data);
+			Assert.AreEqual (s, "", "#1");
+			char[] chars = enc.GetChars (data);
+			Assert.AreEqual (chars.Length, 0, "#2");
+
+			byte[] b1 = enc.GetBytes ("");
+			Assert.AreEqual (b1.Length, 0, "#3");
+			byte[] b2 = enc.GetBytes (new char[] {});
+			Assert.AreEqual (b2.Length, 0, "#3");
+		}
+
 		private void Compare (string prefix, string start, byte[] end)
 		{
 			byte[] bytes = unix.GetBytes (start);