C＃未处理的异常：对象引用未设置为对象的实例

Question

I have a problem with an array of classes. basically I create an array of the class 'Student', but I cannot assign values to any properties of the instances in the array because I get the unhandled exception in the title. here's the code:

class ControllerQueue
{
    model.ModelStudent q = new model.ModelStudent();

    static int capacity = 15;
    model.ModelStudent[] arr = new model.ModelStudent[capacity];
    int top = -1, rear = 0;

    public ControllerQueue()
    {
        arr[0].name = "a";
        arr[0].grade = 0;
    }
}

I tried assigning values out of the constructor, but I still get the same result. now, the exception itself obviously sais I didn't instantiate the Student class but I don't understand why it would say that, I've already instantiated it. thanks in advance.

Answer 1

You need

arr[0] = new ModelStudent();
arr[0].name = "a";
arr[0].grade = 0;

You need this because you have to new up an instance to put into the array at index 0

model.ModelStudent[] arr = new model.ModelStudent[capacity];

Will just allocate the array, but each entry is by default the default value of ModelStudent (null)

Answer 2

you need to instantiate the members of your array

add the item to your array like this

arr[0] = new ModelStudent();
arr[0].name = "a";
arr[0].grade = 0;

Answer 3

Your item 0 is not set.

Try.

model.ModelStudent q = new model.ModelStudent();

        static int capacity = 15;
        model.ModelStudent[] arr = new model.ModelStudent[capacity];
        int top = -1, rear = 0;


        public ControllerQueue()
        {
            arr[0] = new model.ModelStudent();
            arr[0].name = "a";
            arr[0].grade = 0;
        }