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Fixed bug that would cause a struct to not match and throw a runtime error. #895

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Merged
304NotModified merged 5 commits into from
Dec 26, 2025
+36 −3
Merged

Fixed bug that would cause a struct to not match and throw a runtime error. #895

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5edb54a
Resolved a bug that would cause a struct to not match and throw a run…
Notallthatevil Aug 15, 2025
276df59
Add tests for struct with default constructor
Notallthatevil Aug 15, 2025
1c12153
Fixed class constructor curly brace format
Notallthatevil Sep 4, 2025
2885899
Refactor default value handling logic
Notallthatevil Sep 17, 2025
e0aeb81
Merge branch 'nsubstitute:main' into 766-Argument-Matcher-For-A-Struct
Notallthatevil Dec 10, 2025
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11 changes: 10 additions & 1 deletion src/NSubstitute/Core/DefaultForType.cs
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Expand Up @@ -46,6 +46,15 @@ private object DefaultInstanceOfValueType(Type returnType)
return BoxedDouble;
}

return Activator.CreateInstance(returnType)!;
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@304NotModified 304NotModified Sep 4, 2025 •
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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?

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@dtchepak dtchepak Oct 12, 2025

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

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@304NotModified 304NotModified Dec 8, 2025

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

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@304NotModified 304NotModified Dec 12, 2025 •
edited
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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.

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@Notallthatevil Notallthatevil Dec 12, 2025

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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.

// Need to have a special case for Nullable<T> to return null.
if (returnType.IsGenericType && returnType.GetGenericTypeDefinition() == typeof(Nullable<>))
{
return null!;
}
#if NET5_0_OR_GREATER
return System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(returnType);
#else
return System.Runtime.Serialization.FormatterServices.GetUninitializedObject(returnType);
#endif
}
}
3 changes: 2 additions & 1 deletion src/NSubstitute/Routing/AutoValues/AutoObservableProvider.cs
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Expand Up @@ -21,6 +21,7 @@ public bool CanProvideValueFor(Type type) =>

private static object? GetDefault(Type type)
{
return type.GetTypeInfo().IsValueType ? Activator.CreateInstance(type) : null;
var defaultForType = new DefaultForType();
return defaultForType.GetDefaultFor(type);
}
}
4 changes: 3 additions & 1 deletion src/NSubstitute/Routing/AutoValues/AutoTaskProvider.cs
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@@ -1,3 +1,4 @@
using NSubstitute.Core;
using System.Reflection;

namespace NSubstitute.Routing.AutoValues;
Expand Down Expand Up @@ -32,6 +33,7 @@ public object GetValue(Type type)

private static object? GetDefault(Type type)
{
return type.GetTypeInfo().IsValueType ? Activator.CreateInstance(type) : null;
var defaultForType = new DefaultForType();
return defaultForType.GetDefaultFor(type);
}
}
21 changes: 21 additions & 0 deletions tests/NSubstitute.Acceptance.Specs/ArgumentMatching.cs
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Expand Up @@ -866,6 +866,27 @@ public void Does_support_out_method_with_base_override()
Assert.That(outArg, Is.EqualTo(4));
}

[Test]
public void Any_on_struct_with_default_constructor_should_work()
{
var something = Substitute.For<IWithStructWithDefaultConstructor>();
Assert.DoesNotThrow(() => something.MethodWithStruct(Arg.Any<StructWithDefaultConstructor>()));
}

public struct StructWithDefaultConstructor
{
public int Value { get; set; }
public StructWithDefaultConstructor()
{
Value = 42;
}
}

public interface IWithStructWithDefaultConstructor
{
void MethodWithStruct(StructWithDefaultConstructor arg);
}

[SetUp]
public void SetUp()
{
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