static class variable scope issue in C# winforms - why doesn't this bit of code work?

Question

Hi,

I've got two different forms in my WinForms app (MainForm and Form2 say). They both ask for an access of MyDataSet, via a "getInstance" static method. The issue is after MainForm has got an instance, when Form2 needs to get an instance the static "myDataSet" variable is null, whereas I expect to have been already set? Any ideas?

public class MyDataSet
{
    public static MyDataSet myDataSet;   
       // This was null 2nd call to getInstance

    private DataSet myData = new DataSet();

    public static MyDataSet GetInstance()
    {
        if (myDataSet == null)
        {
            return new MyDataSet();
        }
        else
        {
            return myDataSet;
        }
    }

So it almost seems the static "myDataSet" variable isn't working in terms of only having once instance?

Answer 1

you forgot to assign the newly create instance to myDataset

if(myDataSet == null)
{
    myDataSet = new MyDataSet();
}
return myDataSet

Answer 2

You didn't set myDataSet

This is the correct code:

public class MyDataSet
{
    public static MyDataSet myDataSet;   
       // This was null 2nd call to getInstance

    private DataSet myData = new DataSet();

    public static MyDataSet GetInstance()
    {
        if (myDataSet == null)
        {
            myDataSet =new MyDataSet();
        }

            return myDataSet;

    }

Answer 3

public class MyDataSet
{
    public static MyDataSet myDataSet;   
       // This was null 2nd call to getInstance

    private DataSet myData = new DataSet();

    public static MyDataSet GetInstance()
    {
        if (myDataSet == null)
        {
            myDataSet = new MyDataSet();      // Changed at this point
            return myDataSet;
        }
        else
        {
            return myDataSet;
        }
    }

Answer 4

Take a look at this article from Jon Skeet. As others have said you are not setting the variable but you might also want to implement a more robust pattern, or perhaps get rid of the singleton. As it is you could end up multiple instances being created of MyDataSet.

I would go with the fourth or fifth version, if you need a singleton.