Why does the "in" keyword allow mutation of properties C#?

Question

I am currently working on a function where an object (of self written type Catalog) is passed and shall not be mutated / changed in the called function. Here a quick example.

public override bool Migrate(in Catalog fromCatalog)
{
    fromCatalog.SupplierId = 1; //works, but why
}

I want to strictly undermine this behaviour and disallow mutating any properties of the passed object.

Answer 1

Because the in modifier applies only to the variable (the parameter in this case). It prevents the parameter from having its value changed to refer to a different object.

public bool Migrate(in Catalog fromCatalog)
{
/* Error CS8331  Cannot assign to variable 'fromCatalog' 
or use it as the right hand side of a ref assignment 
because it is a readonly variable */
    fromCatalog = new Catalog(); 

    fromCatalog.SupplierId = 1;
}

and create a shallow copy of the object so that you don't change anything in the object you passed (within the class)

example:

public Catalog CreateACopy()
{
    Catalog obj = (Catalog)this.MemberwiseClone();
    // and other...
    return obj;
}
        

utilization:

public class Program 
{
    private static void Main()
    {
        Catalog catalog = new Catalog();
        new Program().Migrate(catalog.CreateACopy());
    }
    public bool Migrate(in Catalog fromCatalog)
    {
        fromCatalog.SupplierId = 1;
    }
}