{"id":25312,"date":"2022-12-09T08:46:55","date_gmt":"2022-12-09T03:16:55","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-access-violation-while-calling-forms-method\/"},"modified":"2022-12-09T08:46:55","modified_gmt":"2022-12-09T03:16:55","slug":"solved-access-violation-while-calling-forms-method","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-access-violation-while-calling-forms-method\/","title":{"rendered":"[Solved] Access Violation While Calling Form’s Method"},"content":{"rendered":"

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In your question you left out a very important piece of the puzzle. You’ve mentioned it in comments, but I repeat it here because it’s the direct trigger of your problem. In comments you said the form is created as follows:<\/p>\n

with TForm1.Create(Self) do\nbegin\n  try\n    ShowModal;\n  finally\n    Free;\n  end;\nend;\n<\/code><\/pre>\n
Your problem is that within the call-chain of ShowModal<\/code> you expect Form1<\/code> to be assigned specifically to the instance you’ve just created<\/em>. But clearly you haven’t yet done anything to set Form1<\/code>.<\/p>\n
Your “solution” was to assign Form1<\/code> within the ShowModal<\/code> call-chain. While this solves your immediate problem, it is far from correct, and I’ll explain why later. First I’ll show the simple solution that would have avoided the problem altogether (note it’s not a complete solution because there are far more problems with your code than you realise).<\/p>\n
Solution, get rid of with<\/strong>:<\/p>\n
\/\/This first line is the most important.\n\/\/It explicitly sets which variable must be assigned to the new form.\nForm1 := TForm1.Create(Self);\ntry\n  Form1.ShowModal;\nfinally\n  Form1.Free;\nend;\n<\/code><\/pre>\n
Vishal, try it out. If you do, you’ll see it works. And hopefully I have your full attention now. You didn’t believe at first, but perhaps you now realise I really do<\/strong> understand correctly<\/em>.<\/p>\n
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So why do I say your “solution” wrong? After all it seems to solve the problem….<\/p>\n
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Well, you said it yourself: you expected Form1<\/code> to be assigned at the start of FormShow<\/code>. You’re right, it should be. By assigning Form1 := Self;<\/code> inside FormShow<\/code> you’re just patching up the earlier mistake. Surely you agree it’s better to fix the original mistake than to just patch over it later?<\/p>\n<\/blockquote>\n
But there’s a deeper problem here… I’m not sure you understand the difference between “object instances” and “classes”. (If you do, then please still read the next few paragraphs for revision.<\/em>)<\/p>\n
It seems you expected that when TForm1<\/code> was created it should be automatically assigned to the Form1<\/code> variable. It’s as if you expected there would only ever be one TForm1<\/code> in memory at any point in time. But TForm1<\/code> is a class type; meaning it generally defines the behaviour for any number of<\/strong> object instances of the same type. Each time you create a TForm1<\/code>, it’s a new separate instance of the form. And each instance can have its own variable that is assigned to it. E.g.<\/p>\n
JohnsForm1 := TForm1.Create(Self);\nPaulsForm1 := TForm1.Create(Self);\n<\/code><\/pre>\n
Consider for a moment what happens with your incorrect “solution” if you needed 2 form variable? What would you write inside the FormShow<\/code> method? JohnsForm1 := Self;<\/code> or<\/em><\/strong> PaulsForm1 := Self;<\/code>?<\/p>\n
Of course you can still choose<\/em> to keep only 1 instance of TForm1<\/code> in memory at a time. But Delphi has no way to automatically know that is what you intend. So you should still do any desired assignments explicitly as above.<\/p>\n
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I mentioned there are still more serious problems with your code. It’s related to the above discussion about object instance and class types.<\/p>\n
Your TTest<\/code> class makes a bunch of unnecessary assumptions:<\/p>\n