CSHARP-5916: ExpressionNotSupportedException when a $set stage expression references a member of a captured constant by sanych-sun · Pull Request #1908 · mongodb/mongo-csharp-driver

sanych-sun · 2026-03-10T17:13:30Z

The root cause of the issue was in the fact that for $set translation we postponed partial evaluator execution and used to run SerializerFinder actually before the partial evaluator. Because of that fact we could run into a situation when serializer finder resolved some serializers, but partial evaluator has rewritten the expression so part of the expression will not be covered by serializer map anymore.
In the PR I've rewritten $set translation to run the serializer finder just after the partial evaluator.

…sion references a member of a captured constant

sanych-sun · 2026-03-10T17:16:38Z

src/MongoDB.Driver/Linq/LinqProviderAdapter.cs

-            }
-            SerializerFinder.FindSerializers(expression, translationOptions, nodeSerializers);
-
-            var context = TranslationContext.Create(translationOptions, nodeSerializers); // do not partially evaluate expression


We should not run partial evaluator too early, it is postponed and basically executed per member (see ExpressionToSetStageTranslator). PartialEvaluator could rewrite part of the expression, so we should not run SerializerFinder here too.

And the constant returned from the partial evaluator gets the serializer finder run on it right?

Copilot

Pull request overview

Adjusts LINQ3 $set stage translation to avoid ExpressionNotSupportedException when a $set expression references a member of a captured constant, by ensuring serializer discovery happens after partial evaluation of the relevant expressions.

Changes:

Reworked $set stage translation flow to invoke serializer finding after preprocessing/partial evaluation (via new ExpressionToSetStageTranslator.Translate signature).
Updated TranslationContext.Create overload to accept IEnumerable<(Expression, IBsonSerializer)> and made the internal factory overload private.
Added an integration test covering conditional nested object assignment in a pipeline update.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5916Tests.cs	Adds regression test reproducing and validating the `$set` translation scenario.
src/MongoDB.Driver/Linq/LinqProviderAdapter.cs	Switches `$set` translation to new translator entrypoint (context construction moved/hidden).
src/MongoDB.Driver/Linq/Linq3Implementation/Translators/TranslationContext.cs	Tweaks `Create` overload signature to accept `IEnumerable` and narrows internal factory visibility.
src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs	Reworks computed-field translation to preprocess before serializer finding and to build translation context differently.

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...Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs

Copilot · 2026-03-10T17:19:45Z

...Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs

+            var context = TranslationContext.Create(valueExpression, initialSerializers, translationOptions);
+


CreateComputedField creates a new TranslationContext (and runs SerializerFinder) per assigned member. If a $set stage initializes many members this becomes repeated work. Consider preprocessing the relevant expressions and building a single TranslationContext for the whole $set lambda, then reusing it across fields to keep translation cost closer to O(1) per field.

src/MongoDB.Driver/Linq/LinqProviderAdapter.cs

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

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Copilot · 2026-03-10T17:47:39Z

...Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs

+            valueExpression = LinqExpressionPreprocessor.Preprocess(valueExpression);
+            var elementName = member.Name;
+
+            var rootDocumentParameter = rootExpression.Parameters.Single();
+            var initialSerializers = new List<(Expression Node, IBsonSerializer Serializer)> { (rootDocumentParameter, documentSerializer) };
            if (documentSerializer.TryGetMemberSerializationInfo(member.Name, out var serializationInfo))
            {
                elementName = serializationInfo.ElementName;
-                var memberSerializer = serializationInfo.Serializer;

                if (valueExpression is ConstantExpression constantValueExpression)
                {
                    var value = constantValueExpression.Value;
-                    var serializedValue = SerializationHelper.SerializeValue(memberSerializer, value);
-                    valueAst = AstExpression.Constant(serializedValue);
+                    var serializedValue = SerializationHelper.SerializeValue(serializationInfo.Serializer, value);
+
+                    return AstExpression.ComputedField(elementName, AstExpression.Constant(serializedValue));
                }
-                else
+
+                if (valueExpression.Type == serializationInfo.Serializer.ValueType)
                {
-                    var valueTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, valueExpression);
-                    ThrowIfMemberAndValueSerializersAreNotCompatible(valueExpression, memberSerializer, valueTranslation.Serializer);
-                    valueAst = valueTranslation.Ast;
+                    initialSerializers.Add((valueExpression, serializationInfo.Serializer));
                }
            }
-            else
+
+            var context = TranslationContext.Create(valueExpression, initialSerializers, translationOptions);
+            var symbol = context.CreateRootSymbol(rootDocumentParameter, documentSerializer);
+            context = context.WithSymbol(symbol);
+
+            var valueTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, valueExpression);


CreateComputedField preprocesses and then constructs a new TranslationContext (including SerializerFinder.FindSerializers) for every assigned member. Compared to the previous implementation (single serializer-finder pass for the whole $set lambda), this can become O(N) serializer-finder runs for N fields and may noticeably slow translation for updates that set many fields. Consider preprocessing the full $set lambda once up-front, then running SerializerFinder once and reusing a single TranslationContext (or at least reusing a shared serializer map) across all computed fields while still ensuring the serializer-finder runs after partial evaluation.

We cannot have the shared SerializerMap, because as a part of LinqExpressionPreprocessor (which is done for each member expression) we are doing partial evaluation which can rewrite the expression what means pre-created serializerMap could be outdated after the LinqExpressionPreprocessor.

Canb't we just pre-process without the serializer map and then run the serializer on the constant node resulting from the preprocessor/partial evaluator? The partial evaluator shouldn't need access.to serializers.

damieng · 2026-03-11T18:26:34Z

...Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs

+            valueExpression = LinqExpressionPreprocessor.Preprocess(valueExpression);
+            var elementName = member.Name;
+
+            var rootDocumentParameter = rootExpression.Parameters.Single();
+            var initialSerializers = new List<(Expression Node, IBsonSerializer Serializer)> { (rootDocumentParameter, documentSerializer) };
            if (documentSerializer.TryGetMemberSerializationInfo(member.Name, out var serializationInfo))
            {
                elementName = serializationInfo.ElementName;
-                var memberSerializer = serializationInfo.Serializer;

                if (valueExpression is ConstantExpression constantValueExpression)
                {
                    var value = constantValueExpression.Value;
-                    var serializedValue = SerializationHelper.SerializeValue(memberSerializer, value);
-                    valueAst = AstExpression.Constant(serializedValue);
+                    var serializedValue = SerializationHelper.SerializeValue(serializationInfo.Serializer, value);
+
+                    return AstExpression.ComputedField(elementName, AstExpression.Constant(serializedValue));
                }
-                else
+
+                if (valueExpression.Type == serializationInfo.Serializer.ValueType)
                {
-                    var valueTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, valueExpression);
-                    ThrowIfMemberAndValueSerializersAreNotCompatible(valueExpression, memberSerializer, valueTranslation.Serializer);
-                    valueAst = valueTranslation.Ast;
+                    initialSerializers.Add((valueExpression, serializationInfo.Serializer));
                }
            }
-            else
+
+            var context = TranslationContext.Create(valueExpression, initialSerializers, translationOptions);
+            var symbol = context.CreateRootSymbol(rootDocumentParameter, documentSerializer);
+            context = context.WithSymbol(symbol);
+
+            var valueTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, valueExpression);


Canb't we just pre-process without the serializer map and then run the serializer on the constant node resulting from the preprocessor/partial evaluator? The partial evaluator shouldn't need access.to serializers.

damieng · 2026-03-11T18:28:01Z

src/MongoDB.Driver/Linq/LinqProviderAdapter.cs

-            }
-            SerializerFinder.FindSerializers(expression, translationOptions, nodeSerializers);
-
-            var context = TranslationContext.Create(translationOptions, nodeSerializers); // do not partially evaluate expression


And the constant returned from the partial evaluator gets the serializer finder run on it right?

damieng · 2026-03-11T18:28:53Z

...Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs

                for (var i = 0; i < members.Count; i++)
                {
                    var member = members[i];
-                    var valueExpression = LinqExpressionPreprocessor.Preprocess(arguments[i]);


So the pre-process doesn't happen here any more, it happens up front before all this processing?

damieng · 2026-03-11T18:30:24Z

...Implementation/Translators/ExpressionToSetStageTranslators/ExpressionToSetStageTranslator.cs

        {
-            string elementName;
-            AstExpression valueAst;
+            valueExpression = LinqExpressionPreprocessor.Preprocess(valueExpression);


So this is now called before the other paths? Kind of hard to follow in a PR.

CSHARP-5916: ExpressionNotSupportedException when a $set stage expres…

7e146be

…sion references a member of a captured constant

sanych-sun requested a review from a team as a code owner March 10, 2026 17:13

sanych-sun added the bug Fixes issues or unintended behavior. label Mar 10, 2026

sanych-sun requested review from adelinowona, Copilot, damieng and papafe and removed request for papafe March 10, 2026 17:13

Copilot started reviewing on behalf of sanych-sun March 10, 2026 17:14 View session

sanych-sun commented Mar 10, 2026

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remove unused field in the model

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Copilot AI reviewed Mar 10, 2026

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sanych-sun added 2 commits March 10, 2026 10:31

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sanych-sun requested a review from Copilot March 10, 2026 17:42

Copilot started reviewing on behalf of sanych-sun March 10, 2026 17:43 View session

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damieng reviewed Mar 11, 2026

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		var context = TranslationContext.Create(valueExpression, initialSerializers, translationOptions);

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sanych-sun commented Mar 10, 2026

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