CSHARP-2582: Decimal128 to decimal conversion loses precision for zero.

Author: DmitryLukyanov

src/MongoDB.Bson/ObjectModel/Decimal128.cs

@@ -716,11 +716,7 @@ public static decimal ToDecimal(Decimal128 d)
         {
             if (Flags.IsFirstForm(d._highBits))
             {
-                if (Decimal128.IsZero(d))
-                {
-                    return decimal.Zero;
-                }
-                else if (Decimal128.Compare(d, __minDecimalValue) < 0 || Decimal128.Compare(d, __maxDecimalValue) > 0)
+                if (Decimal128.Compare(d, __minDecimalValue) < 0 || Decimal128.Compare(d, __maxDecimalValue) > 0)
                 {
                     throw new OverflowException("Value is too large or too small to be converted to a Decimal.");
                 }
@@ -830,7 +826,7 @@ public static short ToInt16(Decimal128 d)
                 ulong value;
                 if (Decimal128.TryTruncateToUInt64(d, maxNegativeValue, (ulong)short.MaxValue, out value))
                 {
-                    return Decimal128.IsNegative(d) ? (value == maxNegativeValue ? short.MinValue : (short )(-(short)value)) : (short)value;
+                    return Decimal128.IsNegative(d) ? (value == maxNegativeValue ? short.MinValue : (short)(-(short)value)) : (short)value;
                 }
                 else
                 {

tests/MongoDB.Bson.Tests/ObjectModel/Decimal128Tests.cs

@@ -14,11 +14,6 @@
 */
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using MongoDB.Bson;
 using FluentAssertions;
 using System.Globalization;
 using Xunit;
@@ -40,27 +35,30 @@ public void Default_value()
         [InlineData("-1.01", "-1.01")]
         [InlineData("-1", "-1")]
         [InlineData("0", "0")]
+        [InlineData("0.000", "0.000")]
         [InlineData("1", "1")]
         [InlineData("1.01", "1.01")]
+        [InlineData("1.000", "1.000")]
         [InlineData("79228162514264337593543950335", "79228162514264337593543950335")]
         [InlineData("-79228162514264337593543950335", "-79228162514264337593543950335")]
-        public void Decimal(string valueString, string s)
+        public void Decimal(string valueString, string expectedResult)
         {
             var value = decimal.Parse(valueString, CultureInfo.InvariantCulture);
             var subject = new Decimal128(value);
 
-            subject.ToString().Should().Be(s);
+            subject.ToString().Should().Be(expectedResult);
             AssertSpecialProperties(subject);
 
             var result = Decimal128.ToDecimal(subject);
-            result.Should().Be(value);
+            result.ToString().Should().Be(value.ToString());
 
             result = (decimal)subject;
-            result.Should().Be(value);
+            result.ToString().Should().Be(value.ToString());
         }
 
         [Theory]
         [InlineData("0", "0")]
+        [InlineData("0.00", "0.00")]
         [InlineData("79228162514264337593543950335", "79228162514264337593543950335")]
         [InlineData("1E1", "10")]
         [InlineData("1E2", "100")]
@@ -76,14 +74,26 @@ public void Decimal(string valueString, string s)
         [InlineData("1E-57", "0")]
         [InlineData("1E-99", "0")]
         [InlineData("1E-6111", "0")]
-        [InlineData("10000.0000000000000000000000001", "10000")] // see: CSHARP-2001
+        [InlineData("10000.0000000000000000000000001", "10000.000000000000000000000000")] // see: CSHARP-2001
         public void ToDecimal_should_return_expected_result(string valueString, string expectedResultString)
         {
             var subject = Decimal128.Parse(valueString);
             var expectedResult = decimal.Parse(expectedResultString);
 
             var result = Decimal128.ToDecimal(subject);
 
+            result.ToString().Should().Be(expectedResult.ToString());
+        }
+
+        [Theory]
+        [InlineData(0xE000_0000_0000_0000UL, 0)]
+        [InlineData(0xE000_2000_0000_0000UL, 0)]
+        public void ToDecimal_should_return_expected_result_for_second_form(ulong highBits, ulong expectedResult)
+        {
+            var subject = Decimal128.FromIEEEBits(highBits, 0x0);
+
+            var result = Decimal128.ToDecimal(subject);
+
             result.Should().Be(expectedResult);
         }