using System;
using System.Linq;

public class Test
{
	public static void Main()
	{
		string str = "someId=00000-000-0000-000000;someotherId=123456789;someIdentifier=3030;";
        var idLookup = str.Split(new[]{';'}, StringSplitOptions.RemoveEmptyEntries)
            .Select(token => new { 
                keyvalues=token.Split(new[]{'='}, StringSplitOptions.RemoveEmptyEntries)
            })
            .ToLookup(x => x.keyvalues.First(), x => x.keyvalues.Last());
        
        // now you can lookup a key to get it's value similar to a Dictionary but with duplicates allowed
        string someotherId = idLookup["someotherId"].First();
        Console.Write("someotherId: " + someotherId);
	}
}