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

* Resolved a bug that would cause a struct to not match and throw a runtime 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

* Add tests for struct with default constructor

- 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.

* Fixed class constructor curly brace format

* Refactor default value handling logic

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

Author: Notallthatevil

src/NSubstitute/Core/DefaultForType.cs

@@ -46,6 +46,15 @@ private object DefaultInstanceOfValueType(Type returnType)
             return BoxedDouble;
         }
 
-        return Activator.CreateInstance(returnType)!;
+        // 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
     }
 }

src/NSubstitute/Routing/AutoValues/AutoObservableProvider.cs

@@ -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);
     }
 }

src/NSubstitute/Routing/AutoValues/AutoTaskProvider.cs

@@ -1,3 +1,4 @@
+using NSubstitute.Core;
 using System.Reflection;
 
 namespace NSubstitute.Routing.AutoValues;
@@ -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);
     }
 }

tests/NSubstitute.Acceptance.Specs/ArgumentMatching.cs

@@ -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()
     {