How can we change the value of a constant at run time?

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1.00/5 (1 vote)

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Please help me to const string value at runtime

here,i want to change "cWeatherID" in loop

What I have tried:

public partial class Form1 : Form
{

const string cWeatherID

private void button1_Click_1(object sender, EventArgs e)
      {

foreach (string s in listBox2.Items)
{

cWeatherID =s

const string lURL = cURL + "?" + cWeatherID + "&" + cUnitID + "&format=" + cFormat;
?
}
}

Posted 5-Feb-20 18:04pm

sgramesh75

Updated 5-Feb-20 21:00pm

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F-ES Sitecore 6-Feb-20 4:38am

Just to add to what the others have said, constants aren't variables even though they look like them. If I write this code;

const x = 5;
int y = 2 * x;

then what is actually compiled is

int y = 2 * 5;

The compiler does a literally replacement of everywhere you use "x" with "5". That's why you can't change a constant at run time; they don't exist at run time, only compile time.

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phil.o · Answer 1 · 2020-02-05T19:59:00

Solution 1

You cannot. A constant is... constant.
If you want the variable to be mutable, do not define it as constant.
const keyword - C# Reference | Microsoft Docs[^]

Posted 5-Feb-20 19:59pm

phil.o

Updated 5-Feb-20 20:01pm

v2

OriginalGriff · Answer 2 · 2020-02-05T20:56:00

The const keyword is there to explicitly say "this will always have the value I give it now: it cannot be changed by anyone, ever" and the system will enforce that. But looking at your code, why would you need it at all? Change to this:

C#

foreach (string cWeatherID in listBox2.Items)
    {
    string lURL = cURL + "?" + cWeatherID + "&" + cUnitID + "&format=" + cFormat;
    ...
    }

Or better (if you use a recent version of VS)

C#

foreach (string cWeatherID in listBox2.Items)
    {
    string lURL = $"{cURL}?{cWeatherID}&{cUnitID}&format={cFormat}";
    ...
    }

And it should start to work.

Patrice T · Answer 3 · 2020-02-05T21:01:00

Solution 3

Quote:
How can we change the value of a constant at run time?

You don't, it is the principle of constant.
The keyword constant is used explicitly to say that the value never change at runtime. And any attempt to do so is reported as an error at compile time.

Posted 5-Feb-20 21:01pm

Patrice T

Updated 5-Feb-20 21:02pm

v2