#CSHARP-5792: Fix Decimal.MaxValue and Decimal.MinValue serialization behavior in RepresentationConverter, add decimal test cases to RepresentationConverterTests

Author: lukasvosyka

src/MongoDB.Bson/ObjectModel/Decimal128.cs

@@ -42,17 +42,29 @@ public struct Decimal128 : IConvertible, IComparable<Decimal128>, IEquatable<Dec
 
         // public static properties
         /// <summary>
-        /// Gets the maximum value.
+        /// Gets the maximum Decimal128 value.
         /// </summary>
         public static Decimal128 MaxValue =>
             __maxValue;
 
         /// <summary>
-        /// Gets the minimum value.
+        /// Gets the minimum Decimal128 value.
         /// </summary>
         public static Decimal128 MinValue =>
             __minValue;
 
+        /// <summary>
+        /// Gets the maximum System.Decimal value.
+        /// </summary>
+        public static Decimal128 MaxDecimalValue =>
+            __maxDecimalValue;
+
+        /// <summary>
+        /// Gets the minimum System.Decimal value.
+        /// </summary>
+        public static Decimal128 MinDecimalValue =>
+            __minDecimalValue;
+
         /// <summary>
         /// Represents negative infinity.
         /// </summary>

src/MongoDB.Bson/Serialization/Options/RepresentationConverter.cs

@@ -78,11 +78,13 @@ obj is RepresentationConverter other &&
         /// <returns>A Decimal.</returns>
         public decimal ToDecimal(Decimal128 value)
         {
-            if (value == Decimal128.MaxValue)
+            // comparison against Decimal128.MaxValue remains valid for backwards compat.
+            if (value == Decimal128.MaxValue || value == Decimal128.MaxDecimalValue)
             {
                 return decimal.MaxValue;
             }
-            else if (value == Decimal128.MinValue)
+            // comparison against Decimal128.MinValue remains valid for backwards compat.
+            else if (value == Decimal128.MinValue || value == Decimal128.MinDecimalValue)
             {
                 return decimal.MinValue;
             }
@@ -166,11 +168,11 @@ public Decimal128 ToDecimal128(decimal value)
         {
             if (value == decimal.MaxValue)
             {
-                return Decimal128.MaxValue;
+                return Decimal128.MaxDecimalValue;
             }
             else if (value == decimal.MinValue)
             {
-                return Decimal128.MinValue;
+                return Decimal128.MinDecimalValue;
             }
 
             // conversion from decimal to Decimal128 is lossless so need to check for overflow or truncation

tests/MongoDB.Bson.Tests/Serialization/Options/RepresentationConverterTests.cs

@@ -119,6 +119,16 @@ public void TestConversions()
             Assert.Equal((long)(ulong)int.MaxValue, converter.ToInt64(int.MaxValue));
             Assert.Equal(1UL, converter.ToUInt64((long)1));
             Assert.Equal((long)(ulong)long.MaxValue, converter.ToInt64(long.MaxValue));
+
+            // general decimal <-> Decimal128 checks
+            Assert.Equal(123.45m, converter.ToDecimal(new Decimal128(123.45m)));
+            Assert.Equal(-123.45m, converter.ToDecimal(new Decimal128(-123.45m)));
+            Assert.Equal(Decimal128.MaxDecimalValue, converter.ToDecimal128(decimal.MaxValue));
+            Assert.Equal(Decimal128.MinDecimalValue, converter.ToDecimal128(decimal.MinValue));
+            Assert.Equal(decimal.MaxValue, converter.ToDecimal(Decimal128.MaxDecimalValue));
+            Assert.Equal(decimal.MinValue, converter.ToDecimal(Decimal128.MinDecimalValue));
+            Assert.Equal(decimal.MaxValue, converter.ToDecimal(Decimal128.MaxValue));
+            Assert.Equal(decimal.MinValue, converter.ToDecimal(Decimal128.MinValue));
         }
 
         [Theory]