Fixed bug that would cause a struct to not match and throw a runtime error. by Notallthatevil · Pull Request #895 · nsubstitute/NSubstitute

Notallthatevil · 2025-08-15T16:11:22Z

When a struct had an empty constructor that modified a field/property to a non-default value. The matcher would fail and throw a runtime error. This bug fix attempts to fix that issue by essentially zero initializing the struct the same way that default(T) does through the use of a runtime helper method. Simply, we replace Activator.CreateInstance with GetUninitializedObject.

The idea came from this post

Which contains and deprecated method as of .NET8. Microsoft has a recommended fix for that here.

This resolves the issue in #766

…time error. When a struct had an empty constructor that modified a field/property to a non-default value. The matcher would fail and throw a runtime error. This bug fix attempts to fix that issue by essentially zero initializing the struct the same way that default(T) does through the use of a runtime helper method. Simply, we replace Activator.CreateInstance with GetUninitializedObject. The idea came from this post https://stackoverflow.com/questions/1005336/c-sharp-using-reflection-to-create-a-struct Which contains and deprecated method as of .NET8. Microsoft has a recommended fix for that here. https://learn.microsoft.com/en-us/dotnet/fundamentals/syslib-diagnostics/syslib0050

304NotModified · 2025-08-15T16:24:53Z

Thanks for the pull request!

Could you please add some unit tests?

- Added test method `Any_on_struct_with_default_constructor_should_work` to ensure method calls with struct arguments do not throw exceptions. - Introduced `StructWithDefaultConstructor` struct: - Contains a property `Value`. - Has a default constructor initializing `Value` to 42. - Created `IWithStructWithDefaultConstructor` interface: - Declares `MethodWithStruct` that accepts `StructWithDefaultConstructor` as an argument.

Notallthatevil · 2025-08-15T17:12:51Z

I added a simple test that does fail on the main branch. Not sure there is a whole lot to test here. I did ensure that all tests pass, especially ones that take in nullable primitive types like int? as they need special handling with the new DefaultInstanceOfValueType functionality. If you have any specific scenarios you would like to see let me know.

Notallthatevil · 2025-09-04T14:39:30Z

Is there anything still blocking this from being merged?

Notallthatevil · 2025-09-04T15:30:59Z

I fixed the format issue.

304NotModified · 2025-09-04T15:40:10Z

Is there anything still blocking this from being merged?

Only a review I guess 😅

304NotModified · 2025-09-04T19:38:21Z

src/NSubstitute/Core/DefaultForType.cs

            return BoxedDouble;
        }

-        return Activator.CreateInstance(returnType)!;


I'm doubting if we should use Activator.CreateInstance(returnType)!; for non-stucts, to keep things 100% the same for the previous cases.

Although all tests works, so not sure.

@nsubstitute/core-team any opinion on this?

I'd feel more comfortable if we keep the existing behaviour for non-structs. Never sure how reflection stuff is going to break 😅

Is this fixed? This isn't marked as resolved?

So after some reading, this is large change?

Activator.CreateInstance(returnType)!;

Calls the (default) constructor - that is indeed an issue with structs

System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject

Won't call the constructor.

I'm not sure if this is a good idea.

I'm not sure how DefaultValue is used exactly and if this poses issues.

Anyway, without resolving this discussion, I cannot approve this PR.

The method itself is attempting to get the default value for the type. i.e. 0 for int. For reference types, as you know, this would return null when you try to do object o = default;.

For structs, when you set them to default, for example, when the parameter list has something along the lines of void MyMethod(CancellationToken cancellationToken = default) and then you try to create a matcher against it using Arg.Any<CancellationToken>(), it results in two instances of a struct that will not match, because Arg.Any<CancellationToken>() does not equal default(CancellationToken), which is why GetUninitializedObject is needed.

Copilot

Pull Request Overview

This PR fixes a runtime error that occurred when using NSubstitute's Arg.Any<T>() matcher with structs that have non-default constructors. The issue arose because Activator.CreateInstance would invoke the constructor, causing the matcher to fail when comparing against the modified struct values.

Replaces Activator.CreateInstance with GetUninitializedObject to create zero-initialized structs
Adds special handling for Nullable<T> types to return null
Implements conditional compilation for .NET 5+ vs older frameworks

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/NSubstitute/Core/DefaultForType.cs	Updates struct instantiation logic to use `GetUninitializedObject` instead of `Activator.CreateInstance`
tests/NSubstitute.Acceptance.Specs/ArgumentMatching.cs	Adds regression test for struct with default constructor scenario

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Updated `GetDefault` methods in `AutoObservableProvider.cs` and `AutoTaskProvider.cs` to use the `DefaultForType` class for determining default values, improving code modularity and reusability.

Notallthatevil · 2025-10-07T14:03:20Z

@304NotModified Anything still holding this back that I can help with? I would love to not have to maintain my own branch anymore and it seems that others have faced problems similar to this one.

dtchepak

LGTM, thanks!

@Romfos, @304NotModified , if this looks ok to use please approve and merge 🙏

Romfos · 2025-11-13T21:02:59Z

cc @304NotModified

Notallthatevil · 2025-12-08T18:14:30Z

@304NotModified I am sure you are busy, is there anything I can help with that is holding you back from approving these changes?

304NotModified · 2025-12-08T23:13:34Z

Hey!

I'm not that active anymore on NSubstitute 😅
And I got hunderds of notifications/tags recently, so dropped a lot of those notifications 😅

I could try to check this PR this week, but I'm not sure if this needs an approval of me?

304NotModified · 2025-12-12T22:31:58Z

reviewed. There is still an open discussion in this PR. I don't know the exact side effects of this change, so unfortunately I cannot approve this.

@dtchepak maybe enable the check of the discussions so we could not forget this

Under settings -> rules -> rulesets

zvirja

Looks reasonable to me! I think the usage of Activator was indeed meant to get uninitialized zeroed struct. Especially giving that it's exactly what default() is doing and is not invoking struct constructor.

304NotModified · 2025-12-26T00:08:20Z

Ok let's merge then! :)

dtchepak · 2026-01-09T05:10:59Z

@dtchepak maybe enable the check of the discussions so we could not forget this

Under settings -> rules -> rulesets

Thanks @304NotModified . Previously PR rules were applied via branch protection. I've added a ruleset for default branch as well so hopefully this will be sorted now. 🤞

Fixed class constructor curly brace format

1c12153

304NotModified reviewed Sep 4, 2025

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304NotModified requested a review from Copilot September 4, 2025 19:38

Copilot AI reviewed Sep 4, 2025

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Refactor default value handling logic

2885899

Updated `GetDefault` methods in `AutoObservableProvider.cs` and `AutoTaskProvider.cs` to use the `DefaultForType` class for determining default values, improving code modularity and reusability.

dtchepak requested review from 304NotModified and Romfos October 12, 2025 06:44

dtchepak approved these changes Oct 12, 2025

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304NotModified self-assigned this Dec 8, 2025

Merge branch 'nsubstitute:main' into 766-Argument-Matcher-For-A-Struct

e0aeb81

nsubstitute deleted a comment from Copilot AI Dec 12, 2025

304NotModified removed their assignment Dec 12, 2025

zvirja approved these changes Dec 25, 2025

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304NotModified merged commit 6f75ebc into nsubstitute:main Dec 26, 2025
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Notallthatevil deleted the 766-Argument-Matcher-For-A-Struct branch December 26, 2025 00:20

Romfos mentioned this pull request Dec 26, 2025

NSubstitute 6.0.0 roadmap #830

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moerwald mentioned this pull request Jan 16, 2026

Argument matcher for a struct with a field with non-default value #766

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Pull Request Overview

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