Question

I have a dictionary containg ID which are alphanumeric (e.g. a10a10 & d10a9) from which I want the biggest ID, meaning 9 < 10 < a …

When I use the following code, d10a9 is MAX since 9 is sorted before 10

var lsd = new Dictionary<string, string>();
lsd.Add("a", "d10a10");
lsd.Add("b", "d10a9");
string max = lsd.Max(kvp => kvp.Value);

How can I get the Max value of the IDs with the Longest string combined?

Answer 1

One way would be to do this

string max =lsd.Where(kvp=>kvp.Value.Length==lsd.Max(k=>k.Value.Length)).Max(kvp => kvp.Value);

however I think that this method would evalute the max length for each item so you may be better to extract it to a variable first

int maxLength=lsd.Max(kvp=>kvp.Value.Length);
string max = lsd.Where(kvp=>kvp.Value.Length == maxLength).Max(kvp => kvp.Value);

If you are going to have null strings in there you may need to perform null checks too

int maxLength=lsd.Max(kvp=>(kvp.Value??String.Empty).Length);
string max = lsd.Where(kvp=>(kvp.Value??String.Empty).Length == maxLength).Max(kvp => kvp.Value);

Alternatively treat your string as Base36 number and convert to long for the max function and then convert back again to get the max string.

string max =lsd.Max(tvp=>tvp.Value.FromBase36()).ToBase36();

public static class Base36 {

  public static long FromBase36(this string src) {
    return src.ToLower().Select(x=>(int)x<58 ? x-48 : x-87).Aggregate(0L,(s,x)=>s*36+x);
  }

  public static string ToBase36(this long src) {
    StringBuilder result=new StringBuilder();
    while(src>0) {
      var digit=(int)(src % 36);
      digit=(digit<10) ? digit+48 :digit+87;
      result.Insert(0,(char)digit);
      src=src / 36;
      }
    return result.ToString();
   }
}

Finally just just the Agregate extension method instead of Max as this lets you do all the comparison logic….

lsd.Agregate(string.Empty,(a,b)=> a.Length == b.Length ? (a>b ? a:b) : (a.Length>b.Length ? a:b));

This could doesn’t have null checks but you easily add them in.