AVRO-4161: [C#] Fix System.Decimal to Avro decimal conversion (#3423)

* AVRO-4161: [C#] Fix System.Decimal to Avro decimal conversion

* Fix test

* Add test for negative high precision value

Author: rayokota

lang/csharp/src/apache/main/AvroDecimal.cs

@@ -53,8 +53,11 @@ public AvroDecimal(decimal value)
         {
             var bytes = GetBytesFromDecimal(value);
 
-            var unscaledValueBytes = new byte[12];
-            Array.Copy(bytes, unscaledValueBytes, unscaledValueBytes.Length);
+            // Copy the first 12 bytes of the decimal into an array of size 13
+            // so that the last byte is 0, which is required by the
+            // BigInteger constructor to ensure the unscaled value is positive.
+            var unscaledValueBytes = new byte[13];
+            Array.Copy(bytes, unscaledValueBytes, 12);
 
             var unscaledValue = new BigInteger(unscaledValueBytes);
             var scale = bytes[14];

lang/csharp/src/apache/test/AvroDecimalTest.cs

@@ -38,10 +38,30 @@ public void TestAvroDecimalToString(decimal value)
         {
             var valueString = value.ToString();
 
-            var avroDecimal = new AvroDecimal(value);            
+            var avroDecimal = new AvroDecimal(value);
             var avroDecimalString = avroDecimal.ToString();
 
             Assert.AreEqual(valueString, avroDecimalString);
         }
+
+        [Test]
+        public void TestHighPrecisionAvroDecimalToString()
+        {
+            var value = 4.1748330066797328106875724512m; // High precision decimal value
+            var valueString = value.ToString();
+
+            var avroDecimal = new AvroDecimal(value);
+            var avroDecimalString = avroDecimal.ToString();
+
+            Assert.AreEqual(valueString, avroDecimalString);
+
+            value = -4.1748330066797328106875724512m; // High precision decimal value
+            valueString = value.ToString();
+
+            avroDecimal = new AvroDecimal(value);
+            avroDecimalString = avroDecimal.ToString();
+
+            Assert.AreEqual(valueString, avroDecimalString);
+        }
     }
 }