Non Static/Constant values in case statements within a Switch

Question

I would like to evaluate a dynamic string value within a switch statement in C#.

For example:

int i = 0;
foreach (Item item in _Items)
{
    foreach (Field theField in doc.Form.Fields)
    {
        switch (theField.Name)
        {
            case "Num" + i++.ToString(): // Dynamic Evaluation?
                theField.Value = string.Empty;
                break;
        }
    }
}

I have 20 or so fields named Num1, Num2, etc. If I can do this all in one statement/block, I'd prefer to do so.

But the compiler complains that the case statements need to be constant values. Is there a way to use dynamic variables in the case statement so I can avoid repeating code?

I just thought I'd mention, the purpose of this method is to populate the fields in PDF form, with naming conventions which I can not control. There are 20 rows of fields, with names like "Num1" - "Num20". This is why string concatenation would be helpful in my scenario.

The reason why you can't was discussed (debated?) in [C# switch statement limitations - why?](http://stackoverflow.com/questions/44905/c-sharp-switch-statement-limitations-why) I think [this answer](http://stackoverflow.com/a/45584/516797) sums it up best. — Sean U, Jan 20 '12 at 17:49
Are you sure you don't want the `i++` outside the inner `foreach`? — TrueWill, Jan 20 '12 at 17:52
The C# Switch Statement Limitations post answers my question perfectly. Thank so much! — Shawn J. Molloy, Jan 20 '12 at 18:10

score 12 · Accepted Answer · answered Jan 20 '12 at 17:43

12

No. This is simply part of the language. If the values aren't constants, you'll have to use if/else if or a similar solution. (If we knew more details about what you were trying to achieve, we may be able to give more details of the solution.)

Fundamentally, I'd question a design which has a naming convention for the fields like this - it sounds like really you should have a collection to start with, which is considerably easier to work with.

answered Jan 20 '12 at 17:43

Jon Skeet

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First off - I'm completely honored to receive an answer from you Jon. I've just purchased C# in Depth Second Edition, and finally feel like I'm crossing over from being a mediocre developer (one with enough knowledge to be dangerous) to a true professional. Amazing book! I'm in complete awe over your apparent love of the inter-workings of the .NET runtime, C# and I truely appreciate your passion. /end gushing comments. Thanks! – Shawn J. Molloy Jan 20 '12 at 18:09
I just want to quickly add - for anyone reading this, I can't put enough stress on how great C# In Depth is. There is simply nothing else like it out there - the only book I can compare it to in the quality of authorship is "JavaScript - The Good Parts" by douglas crockford. – Shawn J. Molloy Jan 20 '12 at 18:15
@Shawn: You're very kind :) Hope this answer was actually helpful to solving your problem as well, of course... – Jon Skeet Jan 20 '12 at 19:46

score 1 · Answer 2 · answered Jan 20 '12 at 17:43

1

Yes case value must be able to be evaluated at compile time. How about this instead

foreach (Field theField in doc.Form.Fields)
{
    if(theField.Name == ("Num" + i++))
    {
        theField.Value = string.Empty;
    }
}

answered Jan 20 '12 at 17:43

Ash Burlaczenko

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Seth Eden · Answer 3 · 2013-08-02T15:17:48.953

Also consider that every time you concatenate two or more strings together the compiler will create memory variables for each of the component strings and then another one for the final string. This is memory intensive and for very large strings like error reporting it can even be a performance problem, and can also lead to memory fragmentation within your programs running, for long-running programs. Perhaps not so much in this case, but it is good to develop those best-practices into your usual development routines.

So instead of writing like this:

"Num" + i++.ToString()

Consider writing like this:

string.Format("{0}{1}", "Num", i++.ToString());

Also you may want to consider putting strings like "Num" into a separate constants class. Having string constants in your code can lead to program rigidity, and limit program flexibility over time as your program grows.

So you might have something like this at the beginning of your program:

using SysConst = MyNamespace.Constants.SystemConstants;

Then your code would look like this:

string.Format("{0}{1}", SysConst.Num, i++.ToString());

And in your SystemConstants class, you'd have something like this:

/// <summary>System Constants</summary>
public static class SystemConstants
{
    /// <summary>The Num string.</summary>
    public static readonly string Num = @"Num";
}

That way if you need to use the "Num" string any place else in your program, then you can just use the 'SysConst.Num'

Furthermore, any time you decide to change "Num" to "Number", say perhaps per a customer request, then you only need to change it in one place, and not a big find-replace in your system.

score 0 · Answer 4 · answered Jan 20 '12 at 17:43

0

How about:

int i = 0;
foreach ( Item item in _Items )
    doc.Form.Fields.First(f=>f.Name == "Num" + i++.ToString()).Value = string.Empty;

Not sure what the purpose of item is in your code though.

answered Jan 20 '12 at 17:43

Chris Shain

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`First` will raise an exception if no matching elements are found. – TrueWill Jan 20 '12 at 18:11
@Krizz Unless I am missing something, it does what the pseudo-code in the OP was trying to do. What's wrong with it? – Chris Shain Jan 20 '12 at 18:11
@TrueWill Yes, but I dont know what he wants to do in case of a field not found. I was leaving that up to him. – Chris Shain Jan 20 '12 at 18:11
@ChrisShain It evaluates over items and don't use them later in the block. – Krizz Jan 20 '12 at 18:21
@Krizz so does the OP. And as my note says, "*I'm not sure what the purpose of item is in your code though.*" – Chris Shain Jan 20 '12 at 18:22

score 0 · Answer 5 · answered Jan 20 '12 at 17:59

0

No. There is no way.

How about replacing the switch with:

if (theField.Name.StartsWith("Num"))
  theField.Value = string.Empty;

or some similar test?

answered Jan 20 '12 at 17:59

Krizz

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TrueWill · Answer 6 · 2012-01-20T18:14:06.313

0

var matchingFields =
    from item in _Items
    join field in doc.Form.Fields
      on "Num" + item.PackageCount equals field.Name
    select field;

foreach (var field in matchingFields)
{
    field.Value = string.Empty;
}

For more efficiency include a DistinctBy on field Name after getting the matching fields (from MoreLINQ or equivalent).

edited Jan 20 '12 at 18:14

answered Jan 20 '12 at 18:08

TrueWill

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Non Static/Constant values in case statements within a Switch

6 Answers6