4 files changed, 46 insertions, 4 deletions

diff --git a/mcs/class/corlib/System.Text/Decoder.cs b/mcs/class/corlib/System.Text/Decoder.cs
index fe715c33796..cf2e6f23efc 100644
--- a/mcs/class/corlib/System.Text/Decoder.cs
+++ b/mcs/class/corlib/System.Text/Decoder.cs
@@ -149,7 +149,10 @@ public abstract class Decoder
 		out int bytesUsed, out int charsUsed, out bool completed)
 	{
 		CheckArguments (bytes, byteIndex, byteCount);
-		CheckArguments (chars, charIndex);
+		if (chars == null)
+			throw new ArgumentNullException ("chars");
+		if (charIndex < 0)
+			throw new ArgumentOutOfRangeException ("charIndex");
 		if (charCount < 0 || chars.Length < charIndex + charCount)
 			throw new ArgumentOutOfRangeException ("charCount");
 
@@ -169,7 +172,7 @@ public abstract class Decoder
 	{
 		if (chars == null)
 			throw new ArgumentNullException ("chars");
-		if (charIndex < 0 || chars.Length <= charIndex)
+		if (charIndex < 0 || chars.Length < charIndex)
 			throw new ArgumentOutOfRangeException ("charIndex");
 	}
 
@@ -177,7 +180,7 @@ public abstract class Decoder
 	{
 		if (bytes == null)
 			throw new ArgumentNullException ("bytes");
-		if (byteIndex < 0 || bytes.Length <= byteIndex)
+		if (byteIndex < 0)
 			throw new ArgumentOutOfRangeException ("byteIndex");
 		if (byteCount < 0 || bytes.Length < byteIndex + byteCount)
 			throw new ArgumentOutOfRangeException ("byteCount");
diff --git a/mcs/class/corlib/System.Text/UTF8Encoding.cs b/mcs/class/corlib/System.Text/UTF8Encoding.cs
index 3a1c2bd680f..bdd80e8a626 100644
--- a/mcs/class/corlib/System.Text/UTF8Encoding.cs
+++ b/mcs/class/corlib/System.Text/UTF8Encoding.cs
@@ -663,7 +663,7 @@ fail_no_space:
 			throw new ArgumentOutOfRangeException ("charIndex", _("ArgRange_Array"));
 		}
 
-		if (charIndex == chars.Length)
+		if (charIndex == chars.Length && byteCount == 0)
 			return 0;
 
 		fixed (char* cptr = chars) {
diff --git a/mcs/class/corlib/Test/System.Text/DecoderTest.cs b/mcs/class/corlib/Test/System.Text/DecoderTest.cs
index 3da0846360b..6cf0306d823 100644
--- a/mcs/class/corlib/Test/System.Text/DecoderTest.cs
+++ b/mcs/class/corlib/Test/System.Text/DecoderTest.cs
@@ -98,5 +98,22 @@ namespace MonoTests.System.Text
 			}
 		}
 #endif
+
+		[Test]
+		public void Bug10789 ()
+		{
+			byte[] bytes = new byte[100];
+			char[] chars = new char[100];  
+
+			Decoder conv = Encoding.UTF8.GetDecoder ();
+			int charsUsed, bytesUsed;
+			bool completed;
+			
+			conv.Convert (bytes, 0, 0, chars, 100, 0, false, out bytesUsed, out charsUsed, out completed);
+
+			Assert.IsTrue (completed, "#1");
+			Assert.AreEqual (0, charsUsed, "#2");
+			Assert.AreEqual (0, bytesUsed, "#3");
+		}
 	}
 }
diff --git a/mcs/class/corlib/Test/System.Text/UTF8EncodingTest.cs b/mcs/class/corlib/Test/System.Text/UTF8EncodingTest.cs
index 022e8d7db7c..1f53af7e3eb 100644
--- a/mcs/class/corlib/Test/System.Text/UTF8EncodingTest.cs
+++ b/mcs/class/corlib/Test/System.Text/UTF8EncodingTest.cs
@@ -1174,5 +1174,27 @@ namespace MonoTests.System.Text
 
 			Encoding.UTF8.GetDecoder ().GetChars (bytes, 0, 4096, chars, 9, false);
 		}
+
+		[Test]
+		public void Bug10789()
+		{
+			byte[] bytes = new byte[4096];
+			char[] chars = new char[10];
+
+			try {
+				Encoding.UTF8.GetDecoder ().GetChars (bytes, 0, 1, chars, 10, false);
+				Assert.Fail ("ArgumentException is expected #1");
+			} catch (ArgumentException) {
+			}
+
+			try {
+				Encoding.UTF8.GetDecoder ().GetChars (bytes, 0, 1, chars, 11, false);
+				Assert.Fail ("ArgumentOutOfRangeException is expected #2");
+			} catch (ArgumentOutOfRangeException) {
+			}
+
+			int charactersWritten = Encoding.UTF8.GetDecoder ().GetChars (bytes, 0, 0, chars, 10, false);
+			Assert.AreEqual (0, charactersWritten, "#3");
+		}
 	}
 }