Question about the "FieldByName"

Hi all,
Why this always return a Null value?
-------------------------->
FieldByName('EventMessage').Value := FieldByName('EventMessage').Value + 'test';
--------------------------<

Change to the following can get the correct result.
-------------------------->
FieldByName('EventMessage').Value := FieldByName('EventMessage').AsString + 'test';
--------------------------<

I also use "FieldByName('XXXXX').Value" in other place, Do I must change them to AsFloat/AsInteger/As..... ?

(Using D3)

Thanks, supwang
LVL 2
SupWangAsked:
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DraregConnect With a Mentor Commented:

FieldByName('XXXXX').Value return a TVariant, so it maybe not possible to "work" with the result.
You can do FieldByName('X').Value := FieldByName('Y').Value but if you want to do some manipulation you have to use AsString, AsInteger , etc.
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SupWangAuthor Commented:
The DataSet is a TQuery.
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CrazyOneCommented:
SupWang I would suggest not even using Value becuase as Drareg mentioned Value is a TVariant and using variants can slow down the proccess because it takes proccessor time to convert the data to the type that it is. Try getting in the habit of using AsString, AsInteger, AsDateTime etc.


The Crazy One
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SupWangAuthor Commented:
Hi CrazyOne,
If it is a persistent field, Do I also need to change ".Value" to AsString, AsInteger, AsDateTime etc?

Regards, supwang
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CrazyOneCommented:
Yes, If you know the data type then it is best to use .AsString, .AsInteger, .AsDateTime. Experiment with this to get the overall feel of it. Example:

FieldByName('EventMessage').AsString := FieldByName('EventMessage').AsString + 'test';

One thing I do is to use a variable to capture the field's value. Mainly because I might need to use this value over and over again. Using the variable will cut down on the number of calls I need to make to the field which has the possible benefit of speeding up the process and shorting the length of the code making it easier to read. Again this something one needs to experiment with to see how it works for them.

var
  s: string;

begin

  s := FieldByName('EventMessage').AsString;
  FieldByName('EventMessage').AsString := s + 'test';

In this case the there is not much benefit in using the variable approach. But lets say we call on this field many times in our code then using the variable should improve performance. Example:

if s = 'a' then
  DoSomething
else if s = 'b' then
  DoSomething
else if s:= 'c' then
  DoSomething
else if s = 'd' then
  DoSomething
else if s = 'e' then
  DoSomething
else if s = 'f' then
  DoSomething
else if s = 'g' then
  DoSomething
else if s = 'h' then
  DoSomething;

Now a neat thing here is lets say we have label and we want to set the caption of this label to that of a field that is an integer type. We can do this rather easily by using the .AsString.

Example:

Label1.Caption := FieldByName('AnIntegerField').AsString;

This will convert the Integer field to a string for us.


The main point I am trying to make is no matter whether your code is dealing with values from a database or your code is manipulating or tracking values you create in the code try and stay away from using variants unless you absolutely need to. Variants definitely slow down the processing of the code.
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ITugayCommented:
Hi SupWang,

yes, field property Value is variant type. Empty field Value =  'NULL'.
Concatenation of string and 'NULL' always = 'NULL'.

You can notice the same behavior after executing SQL statement which has some concatenation.

e.g.

select FirstName || LastName from TheTable

the result will be empty if one of fields is NULL.

btw assignin empty string to empty field convert result to empty string.

FieldByName('EventMessage').Value := '';

FieldByName('EventMessage').Value := FieldByName('EventMessage').Value + 'test';

should return valid result.

------
Igor.
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SupWangAuthor Commented:
Hi all,
How about the TTime field? Use "ThisTime.AsDateTime" instead of "ThisTime.Value"?

Regards, supwang
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CrazyOneCommented:
AsDateTime, you can always convert it to the format you need when you actually need to use it.
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SupWangAuthor Commented:
Hi all,
Thank you very much. I had already changed many many lines in my program.

Hi CrazyOne,
Have some points for you, Please go here: http://www.experts-exchange.com/jsp/qManageQuestion.jsp?ta=delphi&qid=20276807

Hi ITugay,
Have some points for you, Please go here:
http://www.experts-exchange.com/jsp/qManageQuestion.jsp?ta=delphi&qid=20276808

Best Regards, supwang


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