Question

I’m using this pattern for the first time, and using C#.

I just wanted to check that this is the correct implementation.
I have a button on a Winform, and when clicked on that will output some data in a particular format, defined by choosing from a drop down box. Now this may change in the future, hence me using the Strategy pattern to encapsulate what changes.

I have a ‘Strategy Interface’ that simply exposes a single method: “Display Data()”.

On my button click I use the following code:

private void ConfirmButton_Click(object sender, EventArgs e)
    {
        IViewData viewData;

        switch (outputMedia)
        {
            case "Excel":
                viewData = new ExcelOutput(operation, study);
                viewData.DisplayData();
                break;
            case "Spotfire":
                viewData = new SpotfireOutput(operation, study);
                viewData.DisplayData();
                break;
        }


    }

Is this an acceptable way to use this pattern? Obviously if additional output media is identified then I will simply create a new subclass and add an extra ‘case’ on the switch statement.

Thanks.

Answer 1

The proper way to use Strategy would be to separate the creation of the IViewData object from its usage. The creation itself might be handled by a Factory Method. Then you can use the created IViewData in a separate location, which is totally unaware of the object’s concrete class.

E.g.

private IViewData CreateViewData()
{
    IViewData viewData;

    switch (outputMedia)
    {
        case "Excel":
            viewData = new ExcelOutput(operation, study);
            break;
        case "Spotfire":
            viewData = new SpotfireOutput(operation, study);
            break;
    }
    return viewData;
}

...

private void ConfirmButton_Click(object sender, EventArgs e)
{
    IViewData viewData = CreateViewData();

    viewData.DisplayData();
}

Now, from this, you can improve the solution further by refactoring the Factory Method. You may decide to use a Dictionary instead of a switch, or to create the view data objects only once and cache them, or (as you suggested) replacing it with dependency injection …