Methods of structs cannot be used as delegates. The struct is copied in the process of delegate creation, and therefore there is no way to get mutuable struct updated in this case. https://replit.com/@Konard/StructMethodCannotBeUsedAsDelegate

output.txt

$ dotnet run
StructSetter result before: 0
StructSetter result after: 0
ClassSetter result before: 0
ClassSetter result after: 42

StructMethodCannotBeUsedAsDelegate.cs

using System;

struct StructSetter
{
    public int Result;

    public void Set(int value) => Result = value;
}

class ClassSetter
{
    public int Result;

    public void Set(int value) => Result = value;
}

class Program
{
    static void UseSetter(Action<int> action)
    {
        action(42);
    }

    public static void Main (string[] args)
    {
        StructSetter structSetter = new StructSetter();

        Console.WriteLine($"StructSetter result before: {structSetter.Result}");
        UseSetter(structSetter.Set);
        Console.WriteLine($"StructSetter result after: {structSetter.Result}");

        ClassSetter сlassSetter = new ClassSetter();

        Console.WriteLine($"ClassSetter result before: {сlassSetter.Result}");
        UseSetter(сlassSetter.Set);
        Console.WriteLine($"ClassSetter result after: {сlassSetter.Result}");
    }
}