CSHARP-4040: Use AppContext switch to disable CSHARP-4040 validation #1825

rstam · 2025-11-24T21:27:58Z

No description provided.

rstam · 2025-11-24T21:28:29Z

src/MongoDB.Bson/Serialization/BsonClassMap.cs


            void EnsureNoMemberMapConflicts(string elementName)
            {
+                if (AppContext.TryGetSwitch("DisableCSharp4040Validation", out bool disableCSharp4040Validation) && disableCSharp4040Validation)


If this switch is not set this change has no effect so should be very safe.

rstam · 2025-11-24T21:29:28Z

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4040SwitchTests.cs

+
+    public void Dispose()
+    {
+        AppContext.SetSwitch("DisableCSharp4040Validation", false);


I'm not sure this is safe if any tests run in parallel.

We might choose not to push these tests to main, just the change to BsonClassMap.cs.

Good question,
@sanych-sun any creative ideas?

Also if we are turning off the switch in Dispose, should we set turn it on in the constructor?

We do not run tests in parallel, at least for now. We should make MongoClient not rely on statics before we can really run them in parallel. I suppose we either remove this workaround earlier, or will find a better way to configure this somehow via MongoClientSettings.

For now: setting this in constructor and removing in Dispose is good enough.

Also if we are turning off the switch in Dispose, should we set turn it on in the constructor?

That might be too late.

What we really want to do is turn it off after ALL the tests in this file have run, but I don't know a way to do that. Turning it off after EACH test has run is the closest I can come to that.

We do not run tests in parallel, at least for now

So that means we could push this test file to main if we want to. We might remove the switch long before we are able to run tests in parallel.

or will find a better way to configure this somehow via MongoClientSettings

To do it via MongoClientSettings would require adding to the public API. Since this is for one customer only a global switch should be adequate.

What we really want to do is turn it off after ALL the tests in this file have run, but I don't know a way to do that. Turning it off after EACH test has run is the closest I can come to that.

If it's turned off after each test, but set once, does that mean that just the first test case will see the switch turned on?
I think per-class disposing can be achieved by overriding dispose in ClassFixture.

It is a bit odd... we are turning the switch on once in the static constructor and turning it off 6 times in the Dispose method (because there are 6 tests).

The reason it works OK is that by the time the first test has finished running the class map has been frozen and the switch is not consulted again.

I wouldn't worry too much about any of this since we will before long remove this switch and these tests.

we are turning the switch on once in the static constructor and turning it off 6 times in the Dispose method (because there are 6 tests).

We can either set it in constructor and remove in Dispose of the test class - because test class is being created for each test, or we can use Fixture for that. Fixture is being created once for the whole test collection (test class in this case) and will be disposed only once - when all tests are done.

rstam · 2025-11-24T21:31:00Z

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4040SwitchTests.cs

+            cm.SetIsRootClass(true);
+            cm.SetDiscriminatorIsRequired(true);
+            cm.MapMember(x => x.TypeNames).SetShouldSerializeMethod(_ => false);
+            cm.SetDiscriminatorConvention(discriminatorConvention);


This is only slightly different than what the user was doing in their sample code, but the end result is the same.

We can't modify the global serialization configuration the way they do, but we can simulate closely on this one class what they are doing globally.

rstam · 2025-11-24T21:31:32Z

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4040SwitchTests.cs

+{
+    static CSharp4040SwitchTests()
+    {
+        AppContext.SetSwitch("DisableCSharp4040Validation", true);


@ajcvickers thank you for suggesting this approach. I was unaware of it.

Second that, nice trick.

sanych-sun · 2025-11-24T21:40:07Z

src/MongoDB.Bson/Serialization/BsonClassMap.cs


            void EnsureNoMemberMapConflicts(string elementName)
            {
+                if (AppContext.TryGetSwitch("DisableCSharp4040Validation", out bool disableCSharp4040Validation) && disableCSharp4040Validation)


Should we use some more meaningful name? DisableDiscriminatorFieldUniqueValidation for example.

Thanks for thinking of that. I meant to ask if anyone wanted to suggest a different name.

What do others think?

In some ways the name is not very important because we plan to remove this anyway.

I hesitate to give it too good a name that sounds like we want people to use!

I would also suggest adding a namespace and a 'Switch' prefix:
Switch.MongoDB.Driver.DisableDiscriminatorFieldUniqueValidation

Looks like MS has the convention of prefixing the switches with 'Switch' string.

I used Switch.MongoDB.Driver.DisableDiscriminatorFieldConflictCheck.

The discriminator by definition is unique because there is only one. What we are checking is that it does not conflict with any members of the POCO.

BorisDog · 2025-11-24T21:45:23Z

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4040SwitchTests.cs

+{
+    static CSharp4040SwitchTests()
+    {
+        AppContext.SetSwitch("DisableCSharp4040Validation", true);


Second that, nice trick.

BorisDog · 2025-11-24T21:48:35Z

src/MongoDB.Bson/Serialization/BsonClassMap.cs


            void EnsureNoMemberMapConflicts(string elementName)
            {
+                if (AppContext.TryGetSwitch("DisableCSharp4040Validation", out bool disableCSharp4040Validation) && disableCSharp4040Validation)


I would also suggest adding a namespace and a 'Switch' prefix:
Switch.MongoDB.Driver.DisableDiscriminatorFieldUniqueValidation

Looks like MS has the convention of prefixing the switches with 'Switch' string.

BorisDog · 2025-11-24T21:51:56Z

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4040SwitchTests.cs

+
+    public void Dispose()
+    {
+        AppContext.SetSwitch("DisableCSharp4040Validation", false);


Good question,
@sanych-sun any creative ideas?

Also if we are turning off the switch in Dispose, should we set turn it on in the constructor?

BorisDog

LGTM

papafe

LGTM!

sanych-sun

LGTM

BorisDog

LGTM

CSHARP-4040: Use AppContext switch to disable CSHARP-4040 validation

5f08046

rstam requested review from BorisDog, ajcvickers and papafe November 24, 2025 21:27

rstam requested a review from a team as a code owner November 24, 2025 21:27

rstam commented Nov 24, 2025

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sanych-sun requested changes Nov 24, 2025

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rstam added the feature Adds new user-facing functionality. label Nov 24, 2025

BorisDog requested changes Nov 24, 2025

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CSHARP-4040: Rename switch

726afb8

rstam requested review from BorisDog and sanych-sun November 24, 2025 22:17

BorisDog previously approved these changes Nov 24, 2025

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CSHARP-4040: Set and clear switch in ClassFixture

bab6d3b

rstam dismissed BorisDog’s stale review via bab6d3b November 25, 2025 02:04

papafe approved these changes Nov 25, 2025

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sanych-sun approved these changes Nov 25, 2025

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BorisDog approved these changes Nov 25, 2025

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rstam merged commit d1cf901 into mongodb:main Nov 26, 2025
34 of 36 checks passed

CSHARP-4040: Use AppContext switch to disable CSHARP-4040 validation #1825

CSHARP-4040: Use AppContext switch to disable CSHARP-4040 validation #1825

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rstam commented Nov 24, 2025

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sanych-sun Nov 24, 2025 •

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