Error: Unhandled exception has occurred in your application

Question

Error: Unhandled exception has occurred in your application

VAer-4038 776

Let us say, I have ChildForm1 (selecting comboBox item, then display query result in DataGridView), and I have no problem running query on this child form.

Then I click parent form's menustrip button and switch to open ChildForm2 (at this moment, ChildForm1 is closed). If I click a button and run query from ChildForm2, sometimes it pops up this error message (Unhandled exception).

This does not only happen to ChildForm1. For most child forms, there are some query (button_click/Combo Box Selected Item Change, etc). If I open application, and randomly go to a child form, it seems it runs fine. But after running some query on this form, if I switch to another child form and running new query there, the error message could pop up.

Why?

When I open a new child form, previous child form has already closed. Quite new to Visual Studio, have no ideas what causes the issue.

Is there a way to "kill" unhandled exception before opening a new child form?

Thanks.

Daniel Zhang-MSFT 9,661 Reputation points

2021-01-07T07:42:58.197+00:00

Hi VAer-4038,
>> running query on this child form.
What is your "query"?
And I guess you did not close the database connection.
So when you switch to another child form and running new query there, the error message could pop up.
In order to find the problem more accurately, please provide the relevant code.
Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-07T19:28:34.27+00:00

Here is the code link: 1G_VWvKmu1s_Quh03rIM6GcLZkoJzjLq6

For every event (Load/Click/TextChange, etc), I have included Cn.Open and Cn.Close

There are just three child forms so far( a few more child forms to be built), the issue is not limited to one child form.

Thanks.

Edit: For above code, for some events, such as ChildForm_Load, it comes with cmd.ExecuteReader, do I need Cn.Close() for this method? Not sure if it causes the problem. Or is there anything to do with Access database?

In general, in what cases do I need Cn.Close? I tried to use Cn.Close for each Cn.Open

For this attached screenshot, it seems I don't need Cn.Close(), why? As soon as I put it Cn.Close, it becomes unreachable.
Daniel Zhang-MSFT 9,661 Reputation points

2021-01-08T10:03:33.77+00:00
Hi @VAer-4038 ,
First, the Close method is used to close the connection to the data source.
But when you use using()statement , the connection automatically closes. So, you do not need to close the connection explicitly.
Based on your code, you need to put the OdbcConnection into using statement to close the connection.
Code likes below:

Using(OdbcConnection Cn=new OdbcConnection(GlobalVariables.DatabaseConnectionString)) { Cn.Open(); String select=""; OdbcCommand cmd=new OdbcCommend(select,Cn) .... }

Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-09T01:12:16.063+00:00

Based on your code

Are you referring to Google Drive link code or attached screenshot? What causes unhandled exception?

Thanks.
Daniel Zhang-MSFT 9,661 Reputation points

2021-01-11T01:52:22.06+00:00

Hi @VAer-4038 ,
>>Based on your code
The code is referring to the screenshot.
When the code executes to an if or else statement, it returns and will not execute any further. So your Cn.Close() will not execute. This means that Odbconnectiong is not closed which causes the unhandled exception.
So I suggest you put the OdbcConnection into using statement to close the connection.
Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-11T01:57:39.047+00:00
So far, a lot of exception has been solved by below code. My code is messy, when switching from query to query, I guess a lot of exception caused by Cn.Open.

if(Cn.State != ConnectionState.Open) { Cn.Open(); }
Daniel Zhang-MSFT 9,661 Reputation points

2021-01-11T02:05:47.2+00:00
Hi @VAer-4038 ，
As I said before, when you use using()statement , the connection automatically closes. So, you do not need to close the connection explicitly.
You can put the OdbcConnection into using statement to close the connection.

Using(OdbcConnection Cn=new OdbcConnection(GlobalVariables.DatabaseConnectionString)) { ... }

Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-11T02:06:47.817+00:00

Thanks much!

Your answer

Daniel Zhang-MSFT 9,661 Reputation points

2021-01-07T07:42:58.197+00:00

Hi VAer-4038,
>> running query on this child form.
What is your "query"?
And I guess you did not close the database connection.
So when you switch to another child form and running new query there, the error message could pop up.
In order to find the problem more accurately, please provide the relevant code.
Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-07T19:28:34.27+00:00

Here is the code link: 1G_VWvKmu1s_Quh03rIM6GcLZkoJzjLq6

For every event (Load/Click/TextChange, etc), I have included Cn.Open and Cn.Close

There are just three child forms so far( a few more child forms to be built), the issue is not limited to one child form.

Thanks.

Edit: For above code, for some events, such as ChildForm_Load, it comes with cmd.ExecuteReader, do I need Cn.Close() for this method? Not sure if it causes the problem. Or is there anything to do with Access database?

In general, in what cases do I need Cn.Close? I tried to use Cn.Close for each Cn.Open

For this attached screenshot, it seems I don't need Cn.Close(), why? As soon as I put it Cn.Close, it becomes unreachable.
Daniel Zhang-MSFT 9,661 Reputation points

2021-01-08T10:03:33.77+00:00

Hi @VAer-4038 ,
First, the Close method is used to close the connection to the data source.
But when you use using()statement , the connection automatically closes. So, you do not need to close the connection explicitly.
Based on your code, you need to put the OdbcConnection into using statement to close the connection.
Code likes below:

Using(OdbcConnection Cn=new OdbcConnection(GlobalVariables.DatabaseConnectionString)) { Cn.Open(); String select=""; OdbcCommand cmd=new OdbcCommend(select,Cn) .... }

Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-09T01:12:16.063+00:00

Based on your code

Are you referring to Google Drive link code or attached screenshot? What causes unhandled exception?

Thanks.
Daniel Zhang-MSFT 9,661 Reputation points

2021-01-11T01:52:22.06+00:00

Hi @VAer-4038 ,
>>Based on your code
The code is referring to the screenshot.
When the code executes to an if or else statement, it returns and will not execute any further. So your Cn.Close() will not execute. This means that Odbconnectiong is not closed which causes the unhandled exception.
So I suggest you put the OdbcConnection into using statement to close the connection.
Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-11T01:57:39.047+00:00

So far, a lot of exception has been solved by below code. My code is messy, when switching from query to query, I guess a lot of exception caused by Cn.Open.

if(Cn.State != ConnectionState.Open) { Cn.Open(); }
Daniel Zhang-MSFT 9,661 Reputation points

2021-01-11T02:05:47.2+00:00

Hi @VAer-4038 ，
As I said before, when you use using()statement , the connection automatically closes. So, you do not need to close the connection explicitly.
You can put the OdbcConnection into using statement to close the connection.

Using(OdbcConnection Cn=new OdbcConnection(GlobalVariables.DatabaseConnectionString)) { ... }

Best Regards,
Daniel Zhang
VAer-4038 776 Reputation points

2021-01-11T02:06:47.817+00:00

Thanks much!

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Error: Unhandled exception has occurred in your application

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