Question Remove Event Handler

[arun]

hi
I want to remove the Combobox Event Handler "SelectedIndexChanged"
combobox.SelectedIndexChanged -= new System.EventHandler(this.cboDrugTypeEdit_SelectedIndexChanged);

I am getting a green scoogly underline this.cboDrugTypeEdit_SelectedIndexChanged
Nullability of reference types in type of parameter 'sender' of void selectedIndexChanged(object sender, eventargs e) doesnt match the target delegate EventHandler

What this Warning mean ? However the program works

Thank u

 

[jmcilhinney]

What version of VS and .NET are you using?

 

[jmcilhinney]

Try changing this:

combobox.SelectedIndexChanged -= new System.EventHandler(this.cboDrugTypeEdit_SelectedIndexChanged);

to this:

combobox.SelectedIndexChanged -= cboDrugTypeEdit_SelectedIndexChanged;

and see whether that makes a difference.

By the way, do you really have a ComboBox control named combobox? Not cool. Why is it using a method to handle its event that is seemingly intended for a ComboBox named cboDrugTypeEdit?

 

[arun]

What version of VS and .NET are you using?

I am using VS2022 and .Net 6

 

[arun]

Try changing this:

combobox.SelectedIndexChanged -= new System.EventHandler(this.cboDrugTypeEdit_SelectedIndexChanged);

to this:

combobox.SelectedIndexChanged -= cboDrugTypeEdit_SelectedIndexChanged;

and see whether that makes a difference.

By the way, do you really have a ComboBox control named combobox? Not cool. Why is it using a method to handle its event that is seemingly intended for a ComboBox named cboDrugTypeEdit?

Eh sorry it should have been ComboBox typo

 

[Skydiver]

That's even worse...

 

[Skydiver]

Nullability of reference types in type of parameter 'sender' of void selectedIndexChanged(object sender, eventargs e) doesnt match the target delegate EventHandler

C# 8.0 and higher versions and compiler enables stricter null checking. .NET 6 uses C# 10 as I recall. The green squiggly lines are just warnings, but in general you should heed them.

If you notice the new EventHandler is declared as:

public delegate void EventHandler(object? sender, EventArgs e);

where as the older versions of the would have:

public delegate void EventHandler(object sender, EventArgs e);

Note the nullable mark ? after the object.

Your event handler was likely declared as:

void cboDrugTypeEdit_SelectedIndexChanged(object sender, EventArgs e)

and so the compiler was highlighting that it was expecting to see object? for the delegate being passed to it, but yours only has object.