CSHARP-4993: Avoid TruncationException when deserializing Decimal from Decimal128.

Author: rstam

src/MongoDB.Bson/Serialization/Attributes/BsonRepresentationAttribute.cs

@@ -25,7 +25,7 @@ namespace MongoDB.Bson.Serialization.Attributes
     public class BsonRepresentationAttribute : BsonSerializationOptionsAttribute
     {
         // private fields
-        private BsonType _representation;
+        private readonly BsonType _representation;
         private bool _allowOverflow;
         private bool _allowTruncation;
 
@@ -37,6 +37,8 @@ public class BsonRepresentationAttribute : BsonSerializationOptionsAttribute
         public BsonRepresentationAttribute(BsonType representation)
         {
             _representation = representation;
+            _allowOverflow = false;
+            _allowTruncation = representation == BsonType.Decimal128;
         }
 
         // public properties

src/MongoDB.Bson/Serialization/Serializers/DecimalSerializer.cs

@@ -51,7 +51,7 @@ public DecimalSerializer()
         /// </summary>
         /// <param name="representation">The representation.</param>
         public DecimalSerializer(BsonType representation)
-            : this(representation, new RepresentationConverter(false, false))
+            : this(representation, new RepresentationConverter(allowOverflow: false, allowTruncation: true))
         {
         }
 

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/BinaryExpressionToAggregationExpressionTranslator.cs

@@ -119,7 +119,7 @@ public static AggregationExpression Translate(TranslationContext context, Binary
                 Type t when t == typeof(ulong) => new UInt64Serializer(),
                 Type t when t == typeof(float) => new SingleSerializer(),
                 Type t when t == typeof(double) => new DoubleSerializer(),
-                Type t when t == typeof(decimal) => new DecimalSerializer(),
+                Type t when t == typeof(decimal) => DecimalSerializer.Decimal128Instance,
                 Type { IsConstructedGenericType: true } t when t.GetGenericTypeDefinition() == typeof(Nullable<>) => (IBsonSerializer)Activator.CreateInstance(typeof(NullableSerializer<>).MakeGenericType(t.GenericTypeArguments[0])),
                 Type { IsArray: true } t => (IBsonSerializer)Activator.CreateInstance(typeof(ArraySerializer<>).MakeGenericType(t.GetElementType())),
                 _ => context.KnownSerializersRegistry.GetSerializer(expression) // Required for Coalesce

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4993Tests.cs

@@ -0,0 +1,67 @@
+/* Copyright 2010-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Bson;
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Driver.Linq;
+using MongoDB.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp4993Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [ParameterAttributeData]
+        public void Select_decimal_divide_should_work(
+            [Values(LinqProvider.V2, LinqProvider.V3)] LinqProvider linqProvider)
+        {
+            var collection = GetCollection(linqProvider);
+
+            var queryable = collection.AsQueryable()
+                .Select(x => x.D / 1.3M);
+
+            var stages = Translate(collection, queryable);
+            if (linqProvider == LinqProvider.V2)
+            {
+                AssertStages(stages, "{ $project : { __fld0 : { $divide : ['$D', NumberDecimal('1.3')] }, _id : 0 } }");
+            }
+            else
+            {
+                AssertStages(stages, "{ $project : { _v : { $divide : ['$D', NumberDecimal('1.3')] }, _id : 0 } }");
+            }
+
+            var result = queryable.First();
+            result.Should().Be(769.23076923076923076923076923M);
+        }
+
+        private IMongoCollection<C> GetCollection(LinqProvider linqProvider)
+        {
+            var collection = GetCollection<C>("test", linqProvider);
+            CreateCollection(
+                collection,
+                new C { Id = 1, D = 1000.0M });
+            return collection;
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            [BsonRepresentation(BsonType.Decimal128)] public decimal D { get; set; }
+        }
+    }
+}