Group linq results by value and group null or invalid values by empty string

Question

I am trying to group by a partial zip code and if any are zip codes that are null or less than 3 characters group them as ""

I've seen some example of using a nullable comparer but not sure of how to fit something like that in the syntax in the context of below.

Also the QBModel.ResultsTable is a dynamic list and the CallerZipCode is a char(10) so something with the valid value, could be "96701-----"

  var newset = (from rst in QBModel.ResultsTable
          group rst by rst.CallerZipCode.Substring(0, 3) into newGroup
          select new DataSourceRecord()
          {
            State = ToTitleCase(newGroup.Select(i => i.CallerState).FirstOrDefault()),
            ZipCode = newGroup.Where(z => z.CallerZipCode.StartsWith(newGroup.Key)).Select(x => x.CallerZipCode.Substring(0, 3)).FirstOrDefault()
          }).ToList();

Here is a nullable comparer I found but probably needs work if I'm going to check for zip codes less than 2 characters:

public class NullableComparer<T> : IEqualityComparer<T?> where T : struct
{
    public bool Equals(T? x, T? y)
    {
        if (x == null || y == null)
            return false;
        return x.Equals(y);
    }

    public int GetHashCode(T? obj)
    {
        return obj.GetHashCode();
    }
}

How can I change this code to accomplish what I am after?

[Edit]

Just tried something like this, but it didn't seem to work very well

  var newset = (from rst in QBModel.ResultsTable
          group rst by rst.CallerZipCode == null || rst.CallerZipCode.Trim().Length() < 3 ? "<null>" : rst.CallerZipCode.Substring(0, 3) into newGroup
          select new DataSourceRecord()
          {
            State = ToTitleCase(newGroup.Select(i => i.CallerState).FirstOrDefault()),
            ZipCode = newGroup.Where(z => z.CallerZipCode.StartsWith(newGroup.Key)).Select(x => x.CallerZipCode.Substring(0, 3)).FirstOrDefault()
          }).ToList();

Thomas D. · Accepted Answer · 2019-07-28T11:55:33.613

I don't think that I have completely understood what you are after, but here is my take:

You want to group by the zip code, but if the zip code is null or empty or less than 3 characters long, you want to put them in the group "<null>".

If that is what you want, you could try something like the following:

  var newset = (from rst in QBModel.ResultsTable
          group rst by GetGroupRepresentation(rst.CallerZipCode) into newGroup
          select new DataSourceRecord()
          {
            // ...
          }).ToList();

With the following implementation for GetGroupRepresentation:

private string GetGroupRepresentation(string zipCode)
{
    if (string.IsNullOrEmpty(zipCode) || zipCode.Length < 3)
    {
        return "<null>";
    }

    return zipCode;
}

I didn't understand why you are using the Substring-method or StartsWith-method for, so I just removed it.

Here is a full example:

static void Main(string[] args)
{
    var zipcodes = new List<string> { "1234", "4321", null, "", "12" };

    // LINQ Query Syntax
    var groups = from code in zipcodes
                 group code by GetGroupRepresentation(code) into formattedCode
                 select formattedCode;

    // I think this is easier to read in LINQ Method Syntax.
    // var groups = zipcodes.GroupBy(code => GetGroupRepresentation(code));
}

private static string GetGroupRepresentation(string zipCode)
{
    if (string.IsNullOrEmpty(zipCode) || zipCode.Length < 3)
    {
        return "<null>";
    }

    return zipCode;
}

Hi Thomas, that's great thank you! Yes I forgot add that bit in, I was using the startswith because generally a zip code is 6 characters long, but I'm just using the first 3 to group and report out. — Hank, Jul 28 '19 at 12:25
Opened a followup question if you're interested: https://stackoverflow.com/questions/57247012/group-linq-results-by-value-and-group-null-or-invalid-values-and-do-counts — Hank, Jul 29 '19 at 03:53

Group linq results by value and group null or invalid values by empty string

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