IEnumerable集合的串联C＃

Question

I have 2 results of a Linq query, with which i want to do do some string operations and concatenate.

Result 1 which is the names of enabled checkboxes from group1, obtained by

var selectedCarPosts = grpBox1MCar.Controls.OfType<CheckBox>()
.Where(c => c.Checked).OrderBy(c => c.Name).Select(c => c.Name);

which yields Result 1:

NearMainGate
NearMP5WestGate

Result 2 which is the names of enabled checkboxes from group2, obtainedby

var selectedDtTypeCars = gbDataTypeMCars.Controls.OfType<CheckBox>()
.Where(c => c.Checked).OrderBy(c => c.Name).Select(c => c.Name);

which yields Result2:

WindDir
WindVel

From both results would like to get a concatenated list as follows: (Result3)

C.NearMainGate
C.NearMP5WestGate
C.WindDirNearMainGate
C.WindDirNearMP5WestGate
C.WindVelNearMainGate
C.WindVelNearMP5WestGate

These form columns in a dynamic sql query later.

I have the following code to accomplish this step by step:

var s1 = selectedCarPosts.Select(s => "C." + s); //the first 2 items in Result3
//Now to get the rest, by inserting Result2 string in each of the first 2 items of Result3
IEnumerable<string> selCarPostsArrWithC = new string[]{};
IEnumerable<string> s2 = new string[]{};
foreach (var type in selectedDtTypeCars)
{
   selCarPostsArrWithC = s1.Select(s => s.Insert(2, type));//C.WindDirNearMainGate C.WindDirNearMP5WestGate in FIRST iteration and so on
   s2 = s2.Concat(selCarPostsArrWithC);// as soon as the SECOND iteration starts, the previous s2 list is overwritten with the subsequent result in selCarPostsArrWithC 
 }

The problem here is that during code debugging, I noticed, that as soon as I tap F10 key just after the foreach line, before actually reaching the foreach block, the the previous values in s2 is overwritten with the subsequent result in selCarPostsArrWithC already. Explained below

For the first iteration s2 has result.

[0] "C.WindDirNearMainGate"
[1] "C.WindDirNearMP5WestGate"

At the beginning of second iteration before entering inside the foreach block

s2 already resets to new values with WindVel some how:

[0] "C.WindVelNearMainGate"
[1] "C.WindVelNearMP5WestGate"

Please could any one assist what am I doing wrong? How can i accomplish Result3 in bold, for the IEnumerable list?

Answer 1

Enumerable.Select doesn't do anything important when you call it, it merely sets things up so that the requested work will be performed later as desired.

So when you write

selCarPostsArrWithC = s1.Select(s => s.Insert(2, type));

this doesn't call string.Insert yet. string.Insert is only called when you (or your debugger) later starts iterating over selCarPostsArrWithC .

Normally, that doesn't matter, except for performance if you iterate over the enumerable multiple times. However, here, because string.Insert is called later than you expect, the arguments that you pass to it are also evaluated later than you expect. You only have a single type variable, and that variable already holds the next value by the time it gets read.

In general, you can either solve this by creating a new variable per iteration, that captures the value of type as seen during that iteration:

foreach (var type in selectedDtTypeCars)
{
  var type_ = type;
  selCarPostsArrWithC = s1.Select(s => s.Insert(2, type_));
  s2 = s2.Concat(selCarPostsArrWithC);
}

(Nowadays, C# already does this behind the scenes for foreach , but you may need to write it out like this if using an older compiler.)

Or, alternatively, perform all of the evaluations directly inside the loop body:

foreach (var type in selectedDtTypeCars)
{
  selCarPostsArrWithC = s1.Select(s => s.Insert(2, type)).ToList();
  s2 = s2.Concat(selCarPostsArrWithC);
}

Although in that case, it would be better to just make s2 a List<string> , and call its AddRange method.

Answer 2

Basically you have a list of prefixes, and a list of suffixes. (Note: I skimmed your code, and I couldn't find where you got the C. that was appended to each of the values.)

For every prefix, you need a suffix.

This is the use of SelectMany()

I simplified your code, because you gave quite a bit of code. But here's what I came up with:

    var location = new[]
    {
      "NearMainGate", 
      "NearMP5WestGate"
    };

    var modifier = new[]
    {
      "WindDir", 
      "WindVel"
    };

    var lambdaResult = modifier.SelectMany(s => location.Select(l => string.Format("{0}{1}{2}", "C.", s, l)));

    var queryResult =
        from m in modifier
        from l in location
        select string.Format("{0}{1}{2}", "C.", m, l);

Note that there's two solutions: a linq query syntax, and linq lambda syntax.

In this situation, I think the query syntax is cleaner and easier to read, but to each their own.

And here's a fiddle with proof of functionality: https://dotnetfiddle.net/Z61RsI