I can't initialize the internal structure with local variables C#

Question

I honestly tried googling the answer but couldn't. Ok, i have a structure like this:

struct TestStruct 
        {   public int n;
            public struct _InnerStruct  {   public int m;   }   
            public  _InnerStruct InnerStruct;   
        }

And I can assign the value to some local variable in the n field:

static void  Main ()    
    {       
        int SomeLOCALvar = 1;
        TestStruct TEST_STRUCT = new TestStruct () { n = SomeLOCALvar };
    }

My problem is that I don't understand how to do exactly the same with the m field. Why doesn't code like this compile:

TestStruct TEST_STRUCT = new TestStruct () { n = SomeLOCALvar, .InnerStruct.m = SomeLOCALvar };

What would be the correct syntax?

Answer 1

Well, the following should work:

TestStruct TEST_STRUCT = new TestStruct { n = SomeLOCALvar, InnerStruct = new _InnerStruct { m = SomeLOCALvar } };

However, frankly it would be a very bad idea to do that. Value types (structs) should almost always be immutable; a struct with public mutable fields like this is incredibly brittle, and it will cause confusion and broken code.

IMO, these should be readonly struct, with get-only properties, and constructors. For example:

readonly struct InnerStruct {
    public int M { get; }
    public InnerStruct(int m) => M = m;
}
readonly struct TestStruct 
{
    public int N { get; }
    public InnerStruct InnerStruct { get; }
    public TestStruct(int n, int m)
    {
        N = n;
        InnerStruct = new InnerStruct(m);           
    }
}

(they should also ideally both override Equals, GetHashCode and ToString, to prevent boxing)