C# How can I declare a Dictionary inside a struct that is being marshalled to get the sizeof the struct?

Question

I have this following struct

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct CombinedDS
{
    public HeaderStruct Header;
    public StructA a;
    public StructB b;
    public StructC c;
    public StructD d;
    public Dictionary<string, KiteClass> KiteObjDict;
}

where StructA StructB StructC StructD all are structures with [StructLayout(LayoutKind.Sequential, Pack = 1)] and KiteClass is a normal class.

I am getting this ArgumentException : Type 'MyApp.CombinedDS' cannot be marshaled as an unmanaged structure; no meaningful size or offset can be computed when am trying to get the size of the struct CombinedDS using

int varsize = Marshal.SizeOf(typeof(CombinedDS));

I would like to know the theoretical reason behind this and also the correct methodology or any suggestion to achieve what i want.

Answer 1

A Dictionary is a collection of any number of items. It's size is not known to the compiler as it will change at runtime when items are added/removed.

You may want to add information to the question if you want suggestions as it doesn't really specify what you're trying to achieve.