How do I pass a parameter by ref to a member variable?

Question

Quite new to C#, from a long time ago C++ background and so I seem to be having my trouble transitioning away from pointers to ref in C#.

I have a class (EColour) which I create using the constructor shown.

I assign (or at least try to) a reference to cellTemplate to the variable m_template.

Looking in debug, at the time of construction, m_template is most definitely NOT null.

However, by the time I come to handle OnMouseClick event, I get a null exception error because m_template has magically turned to null.

Could anyone please shed light on what I have done wrong and how to fix it?

public EColour(ref ICellTemplate cellTemplate)
{
    m_template = (ColourTemplate)cellTemplate;
}

protected override void OnMouseClick(DataGridViewCellMouseEventArgs e)
{
    ColorDialog dlg = new ColorDialog();
    dlg.AnyColor = m_template.AnyColour; // This throws an exception because m_template is null
    
    base.OnMouseClick(e);
}

ColourTemplate m_template;

Answer 1

In C# we have two main kinds of types:

value type - its all digit types (int, float, double, long ...)
reference type -its types that inherited from object

ICellTemplate is a reference class. So what you need - just send it in argument as a regular variable.

public class EColour
{
    private ColourTemplate m_tamplate;

    public EColour(ICellTemplate cellTemplate)
    {
        m_template = (ColourTemplate)cellTemplate;
    }
}