Why can I not use a "constant" within a switch statement within scope?

Question

With this code:

public partial class Form1 : Form
{
    private static readonly int TABCONTROL_BASICINFO = 0;
    private static readonly int TABCONTROL_CONFIDENTIALINFO = 1;
    private static readonly int TABCONTROL_ROLESANDSECURITY = 2;
    private static readonly int TABCONTROL_INACTIVEINFO = 3;
. . .
int ActiveTabPage = tabControlWorker.SelectedIndex;
switch (ActiveTabPage) {
    case TABCONTROL_BASICINFO:
        if (currentNode == "NodeBuckingham") {
        } else if (currentNode == "NodeNamath") {
        } else if (currentNode == "NodeParsons") {
        } else {
        }
    break;

...I have to replace "TABCONTROL_BASICINFO" with "0", or I get, "A constant value is expected"

Heavens to Murgatroyd! Can't it look up and see that TABCONTROL_BASICINFO is 0?

Frustrating huh! Gotta use an actual const rather than the readonly statics :( — mellodev
– mellodev, Commented Mar 22, 2012 at 23:08

Jon Skeet · Accepted Answer · 2012-03-22 23:05:58Z

If you want to make it a constant expression as far as the compiler is concerned, declare it as a const:

// Note that static readonly is implied here
private const int TABCONTROL_BASICINFO = 0;

Or follow .NET naming conventions...

private const int TabControlBasicInfo = 0;

Or use an enum, given that you've basically got a fixed set of values:

private enum TabControlType
{
    // Could ditch the explicit values here if you want
    BasicInfo = 0,
    ConfidentialInfo = 1,
    ...
}

By the way, you can also switch on strings in C#, so this:

 if (currentNode == "NodeBuckingham") {
 } else if (currentNode == "NodeNamath") {
 } else if (currentNode == "NodeParsons") {
 } else {
 }

Can become:

 switch (currentNode) {
     case "NodeBuckingham":
         ...
         break;
     case "NodeNamath":
         ...
         break;
     case "NodeParsons":
         ...
         break;
     default:
         ...
         break;
 }

Eric J. · Accepted Answer · 2012-03-22 23:25:18Z

14

A readonly variable is not a constant. The value is not known at compile time, but rather can be initialized either in the declaration (as you have done) or in the class constructor (in this case, the static constructor for your class).

For more see

http://msdn.microsoft.com/en-us/library/acdd6hb7(v=vs.71).aspx

You can change it to:

private const int TABCONTROL_BASICINFO = 0;

Unless you need to compute something to initialize the variable, declare it as const. It will be slightly more efficient.

edited Mar 22, 2012 at 23:25

answered Mar 22, 2012 at 23:06

Eric J.

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1 Comment

tedebus Over a year ago

This one is the real answer.

Eric Lippert · Accepted Answer · 2012-03-22 23:08:03Z

8

That's not a constant; it's a read-only variable. If you want it to be a constant then mark it as a constant.

private const int TabControlBasicInfo = 0;

Or even better:

private enum TabControl
{
    BasicInfo,
    ...
}
...
switch(whatever)
{
    case TabControl.BasicInfo:

answered Mar 22, 2012 at 23:08

Eric Lippert

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3 Comments

Eric J. Over a year ago

Why is an enum better in this case? Just because it groups related values?

asawyer Over a year ago

@EricJ. Looking at how the readonly names are structured, and how he he is trying to use them, enum would appear to be the correct structure.

B. Clay Shannon-B. Crow Raven Over a year ago

when I do that, I get, "Cannot implicitly convert type 'UserControlsOnTabPagePOCApp.Form1.TabControl' to 'int'. An explicit conversion exists (are you missing a cast?)"

Dustin Graham · Accepted Answer · 2012-03-22 23:06:26Z

5

Static readonly isn't a constant variable. It can be initialized. Whereas "const" is constant.

answered Mar 22, 2012 at 23:06

Dustin Graham

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