C# Enable or disable menu in a form frm another form
-
I have an application developed in Visual Studio 2022 that works with forms. In this application, there is a main form, from which other forms can be opened. In this main form, there is an item in the top menu (“selection”), whose name is M_Selecao, which must be activated or deactivated depending on which form was opened. For example, when the “Expenses” form is opened, the menu must be activated (enabled). When this secondary form is closed, the “selection” menu must be disabled. I can enable the menu, but I can’t disable it. I tried to make the main form’s menu “public” in the “TelaPrincipal.Designer.cs” file:
public System.Windows.Forms.ToolStripMenuItem M_Selecao;
This way I could disable the menu in the main form when closing the secondary form, but I get the message that "an object reference is required for the method or property 'TelaPrincipal.M_Selecao'. I also don't know how to do it from the main form. Can anyone help me? Thanks.
-
I have an application developed in Visual Studio 2022 that works with forms. In this application, there is a main form, from which other forms can be opened. In this main form, there is an item in the top menu (“selection”), whose name is M_Selecao, which must be activated or deactivated depending on which form was opened. For example, when the “Expenses” form is opened, the menu must be activated (enabled). When this secondary form is closed, the “selection” menu must be disabled. I can enable the menu, but I can’t disable it. I tried to make the main form’s menu “public” in the “TelaPrincipal.Designer.cs” file:
public System.Windows.Forms.ToolStripMenuItem M_Selecao;
This way I could disable the menu in the main form when closing the secondary form, but I get the message that "an object reference is required for the method or property 'TelaPrincipal.M_Selecao'. I also don't know how to do it from the main form. Can anyone help me? Thanks.
You could derive a class from the ToolStripMenuItem and override the method ToolStripDropDownItem.OnDropDownOpened(EventArgs) Method (System.Windows.Forms) | Microsoft Learn[^] Then in your OnDropDownOpened handler, you can scan the Application.OpenForms collection to see if the form exists. If it exists, then you disable the menu item.
The difficult we do right away... ...the impossible takes slightly longer.
-
I have an application developed in Visual Studio 2022 that works with forms. In this application, there is a main form, from which other forms can be opened. In this main form, there is an item in the top menu (“selection”), whose name is M_Selecao, which must be activated or deactivated depending on which form was opened. For example, when the “Expenses” form is opened, the menu must be activated (enabled). When this secondary form is closed, the “selection” menu must be disabled. I can enable the menu, but I can’t disable it. I tried to make the main form’s menu “public” in the “TelaPrincipal.Designer.cs” file:
public System.Windows.Forms.ToolStripMenuItem M_Selecao;
This way I could disable the menu in the main form when closing the secondary form, but I get the message that "an object reference is required for the method or property 'TelaPrincipal.M_Selecao'. I also don't know how to do it from the main form. Can anyone help me? Thanks.
OK, let's take this in two stages, starting with your error message:
An object reference is required for the non-static field, method, or property 'TelaPrincipal.M_Selecao'
This 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, it 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, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values. But 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 second part is what you are trying to do, and the whole way you are going about it - so let me summarise what you are trying to achieve: You have a main form which opens a "child" form. When some of these forms are opened, a main form menu option must be enabled, and disabled again when it closes. First off, don't make the menu item public so that the c
-
You could derive a class from the ToolStripMenuItem and override the method ToolStripDropDownItem.OnDropDownOpened(EventArgs) Method (System.Windows.Forms) | Microsoft Learn[^] Then in your OnDropDownOpened handler, you can scan the Application.OpenForms collection to see if the form exists. If it exists, then you disable the menu item.
The difficult we do right away... ...the impossible takes slightly longer.
Ok, Richard. I'll try it. Thanks
-
OK, let's take this in two stages, starting with your error message:
An object reference is required for the non-static field, method, or property 'TelaPrincipal.M_Selecao'
This 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, it 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, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values. But 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 second part is what you are trying to do, and the whole way you are going about it - so let me summarise what you are trying to achieve: You have a main form which opens a "child" form. When some of these forms are opened, a main form menu option must be enabled, and disabled again when it closes. First off, don't make the menu item public so that the c
Once again you help me, OriginalGriff. I'll Think about. Thanks.
