如何确定一个类在C#中是不可变的？

public class ImmutableClass
{
    public readonly int Property1;
    public readonly string Property2;

    public ImmutableClass(int property1, string property2)
    {
        Property1 = property1;
        Property2 = property2;
    }
}

public class ImmutableClass
{
    private int property1;
    private string property2;

    public ImmutableClass(int property1, string property2)
    {
        this.property1 = property1;
        this.property2 = property2;
    }

    public int GetProperty1()
    {
        return property1;
    }

    public string GetProperty2()
    {
        return property2;
    }
}

public class ImmutableClass
{
    public int Property1 { get; }
    public string Property2 { get; }

    public ImmutableClass(int property1, string property2)
    {
        Property1 = property1;
        Property2 = property2;
    }
}

using System.Collections.Immutable;

public class ImmutableClass
{
    public ImmutableList<int> Property1 { get; }
    public ImmutableDictionary<string, string> Property2 { get; }

    public ImmutableClass(IEnumerable<int> property1, IDictionary<string, string> property2)
    {
        Property1 = property1.ToImmutableList();
        Property2 = property2.ToImmutableDictionary();
    }
}