Change the value of a property of a struct in C#

Question

I was reading a book and found out that structs are actually immutable objects. But they have getters and setters. I was wondering if a property of structs can be changed after it has been created.

public struct Test 
{
    public string str {get; set; }
    public int int1 {get; set; }
}

Can the values of 'str' and 'int1' be changed once they have been assigned a value?

Answer 1

Structs can be either mutable or immutable, but they should be immutable according to many people.

Your example is a mutable struct.

Example of use:

var t = new Test();
// t.str is null, and t.int1 is 0

t.str = "changed!"; // OK

var t2 = t;
t2.int1 = 42;
// t.int1 is still 0

var li = new List<Test> { t, t2, };
t.int1 = 666;  // OK, but copy in li is unaffected

li[0].int1 = 911;  // compile-time error, not a variable

var t3 = t2;
bool checkA = (t3 == t2);  // compile-time error, you did not overload operator ==
bool checkB = t3.Equals(t2);  // OK, true, ValueType class overrides Equals for you
bool checkC = t2.Equals(t);  // OK, false
bool checkD = object.ReferenceEquals(t, t);  // false, two distinct boxes
                                             // same as (object)t==(object)t

---

By request, here is one way to make that struct immutable:

public struct Test 
{
    public string str { get; private set; }
    public int int1 { get; private set; }

    public Test(string str, int int1) : this()
    {
        this.str = str;
        this.int1 = int1;
    }
}
// if you introduce methods (other than constructors) that use the private setters,
// the struct will no longer be immutable

and here is another one:

public struct Test 
{
    readonly string m_str;
    readonly int m_int1;

    public string str { get { return m_str; } }
    public int int1 { get { return m_int1; } }

    public Test(string str, int int1)
    {
        m_str = str;
        m_int1 = int1;
    }
}

---

Update:

Starting from 2015 (C# 6.0), you can write the latter as:

public struct Test 
{
    public string str { get; }
    public int int1 { get; }

    public Test(string str, int int1)
    {
        this.str = str;
        this.int1 = int1;
    }
}

In later versions there are many more possibilities. I shall only mention that you can write (since 2021 (C# 10)):

public readonly record struct Test(string str, int int1) 
{
}

The word record now means that the type automatically overrides or overloads stuff like Equals, GetHashCode, operator ==, ToString, Deconstruct, etc. And the constructor is now a primary constructor.