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hi ...

iam using MVP Design Pattern :

I have Two Forms ( Form1 And Form2 ):

Form One Has Button which call method in Presenter Class To pass it from Form1 To Form2:

iam getting error :


System.NullReferenceException: 'Object reference not set to an instance of an object.'
adminsDepartmentAndJobTitlesDetailsPresenter was null.


but i have Declare it as


C#
public partial class FrmManageAdminsDepartmentAndJobTitles : DevExpress.XtraEditors.XtraForm, IManageAdminsDepartmentAndJobTitles
{
AdminsDepartmentAndJobTitlesDetailsPresenter adminsDepartmentAndJobTitlesDetailsPresenter;
}


What I have tried:

I have tried

C#
private void BtnAddNewDepartment_Click(object sender, EventArgs e)
        {
            Form2 frm = new Form2();
            adminsDepartmentAndJobTitlesDetailsPresenter.DepartmentAutoNumber();
            frm.ShowDialog();
        }
Posted
Updated 28-Dec-20 3:58am

1 solution

You have two different problems here: the first is the error you have noticed "Object reference not set to an instance of an object" which is known as a "null reference exception", and is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values. In this case, you know what the variable is: "adminsDepartmentAndJobTitlesDetailsPresenter" - so you need to find out what should have set it to a value and why it didn't. Which means looking the rest of your code to find out what should have set it, and using the debugger to find out why that code didn't get executed.

We can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!


The other problem is communicating between two forms - which has nothing to do with the code you show above other than one of the forms is creating an instance of the other!
That isn't difficult to do, but exactly how depends on the "relationship" between the two forms.
Have a look at these, one of them will fit your circumstances.
The form that creates an instance of another:
C#
MyForm mf = new MyForm();
mf.Show();
Is the "parent", the other form is the "child".
(This doesn't imply any formal MDI relationship)

Transferring information between two forms, Part 1: Parent to Child[^]
Transferring information between two forms, Part 2: Child to Parent[^]
Transferring information between two forms, Part 3: Child to Child[^]
   

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