List inside a struct

[Islorvat]

Hello, Can anyone tell me how to use a List of strings inside a struct? I get the 'Object reference not set to an instance of an object.' error.

    public Form1()
    {
        InitializeComponent();
    }

    struct testStruct
    {
        public List<string> sList
        {
            get;
            set;
        }

    }

    private void Form1\_Load(object sender, EventArgs e)
    {
        testStruct aTestStruct = new testStruct();
        aTestStruct.sList.Add("test");
    }

Thank you!

[J4amieC]

You've not instantiated the List anywhere. This has nothing to do with it being inside a struct - the same would have happened if your list was inside a class. You can fix this by first creating the List.

testStruct aTestStruct = new testStruct();
aTestStruct.sList = new List<string>();
aTestStruct.sList.Add("test");

However, a few pointers 0.1) Naming convention for C# would instruct that your property is called List instead of sList. 0.2) Be careful using structs, they have some hard to understand facets - if in doubt use a class. 0.3) Strictly speaking, a List should generally not be exposed by a public property. Consider whether the struct/class should instead expose just the interface, such as an Add method to add to an internal List.

[DaveyM69]

structs are meant for small value types. Both List<T> and string are reference types, and their size could potentially be very large, so I would never use them inside a struct.

Dave
Binging is like googling, it just feels dirtier. Please take your VB.NET out of our nice case sensitive forum. Astonish us. Be exceptional. (Pete O'Hanlon)
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)

[Lost User]

But since they are reference types, the struct would only contain the reference.. "The actual thing" is included in the struct if it's marked ByValArray or ByValTStr or when it is a fixed size buffer[^] (I've never actually seen one of those)

[DaveyM69]

True, I hadn't actually considered that! I still don't like it due to the mutability implied by using a reference type inside a struct, especially with List<T> as the contents would be mutable even if the exposed list wasn't. A ReadOnlyCollection<T> would be OK I suppose, something like this wrapper...

public struct ExampleStruct
{
private List<string> list;

public ExampleStruct(IEnumerable<string> strings)
{
    if (strings == null)
        list = null;
    else
        list = new List<string>(strings);
}

public ReadOnlyCollection<string> List
{
    get
    {
        if (list == null)
            return null;
        return list.AsReadOnly();
    }
}

}

Dave
Binging is like googling, it just feels dirtier. Please take your VB.NET out of our nice case sensitive forum. Astonish us. Be exceptional. (Pete O'Hanlon)
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)

[Lost User]

A field in a struct is mutable though, so I'm not sure what you mean..?

[DaveyM69]

Of course, but they are generally exposed only by a read-only property. It's rare and generally not recommended to have a property with a setter or to expose the field directly in a struct.

Dave
Binging is like googling, it just feels dirtier. Please take your VB.NET out of our nice case sensitive forum. Astonish us. Be exceptional. (Pete O'Hanlon)
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)