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WHY?

In .NET, structs are value types, and when you mark a struct as readonly, it promises that the struct's internal state cannot change after it’s created. However, structs can have methods that modify their fields or internal state.

When a readonly struct is accessed, .NET needs to make sure this immutability promise is kept. If you call a method on a readonly struct that could modify its internal state, the runtime creates a defensive copy to protect the original struct.

Immutability Guarantee: Without the defensive copy, calling a method like MutatingMethod could break the immutability contract of a readonly struct.

Safety for Consumers: It protects the developer from unintended side effects caused by struct methods.

For example:

readonly struct MyStruct
{
    public int X { get; }

    public MyStruct(int x) => X = x;

    public void MutatingMethod()
    {
        // Hypothetically mutating the state
        Console.WriteLine("Mutating...");
    }
}

readonly MyStruct myStruct = new MyStruct(10);
myStruct.MutatingMethod(); // Defensive copy happens here

When myStruct.MutatingMethod() is called, a defensive copy of myStruct is made before invoking MutatingMethod. This ensures that the original myStruct stays unchanged, preserving its immutability.

Readonly Fields Accessing methods or properties on a readonly struct field triggers a copy:

class Example { private readonly MyStruct _myStruct;
```
 public void DoSomething()
 {
     _myStruct.MutatingMethod(); // Defensive copy is created
 }
```
}
Readonly Parameters Passing a readonly struct as an in parameter can also result in defensive copies if the struct’s method or property mutates the struct:
```
 void ExampleMethod(in MyStruct myStruct)
```
{ myStruct.MutatingMethod(); // Creates a copy of myStruct }

Read more about on Microsoft Docs on readonly structs which explains how readonly enforces immutability and when defensive copies occur -> https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct#readonly-struct

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