Question

I have a struct

struct myStruct {
    Dictionary<string, int> a;
    Dictionary<string, string> b;
    ......
}

I want to create a arraylist of that struct

ArrayList l = new ArrayList();
myStruct s;

s.a.Add("id",1);
s.b.Add("name","Tim");

l.Add(s);

However, I got the error “Object reference not set to an instance of an object.”

Anyone could tell me why?

Thanks.

Answer 1

Some suggestions to improve your code:

• Don’t use a struct, use a class instead. Structs in .NET are a little different and unless one understands those differences I doubt one will ever have a valid use for structs. A class is almost always what you want.

• ArrayList is more or less obsolete, it’s almost always better to use a generic List<T> instead. Even if you need to place mixed objects in the list, List<object> is a better choice than ArrayList.

• Make sure your members are properly initialized and not null before you access methods or properties of them.

• It is better to use properties instead of public fields.

Here is an example:

class Container
{
    Dictionary<string, int> A { get; set; }
    Dictionary<string, string> B { get; set; }

    public Container()
    {
         // initialize the dictionaries so they are not null
         // this can also be done at another place 
         // do it wherever it makes sense
         this.A = new Dictionary<string, int>();
         this.B = new Dictionary<string, string>();
    }
}

...
List<Container> l = new List<Container>();
Container c = new Container();
c.A.Add("id", 1);
c.B.Add("name", "Tim");

l.Add(c);
...