Task: Sum to 100
C#: using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        
        var expressionsPlus = Enumerable.Range(0, (int)Math.Pow(3, 8)).AsParallel().Select(i => new Expression(i, 1));
        
        var expressionsMinus = Enumerable.Range(0, (int)Math.Pow(3, 8)).AsParallel().Select(i => new Expression(i, -1));
        var expressions = expressionsPlus.Concat(expressionsMinus);
        var results = new Dictionary<int, List<Expression>>();
        foreach (var e in expressions)
        {
            if (results.Keys.Contains(e.Value))
                results[e.Value].Add(e);
            else
                results[e.Value] = new List<Expression>() { e };
        }
        Console.WriteLine("Show all solutions that sum to 100");
        foreach (Expression e in results[100])
            Console.WriteLine("  " + e);
        Console.WriteLine("Show the sum that has the maximum number of solutions (from zero to infinity)");
        var summary = results.Keys.Select(k => new Tuple<int, int>(k, results[k].Count));
        var maxSols = summary.Aggregate((a, b) => a.Item2 > b.Item2 ? a : b);
        Console.WriteLine("  The sum " + maxSols.Item1 + " has " + maxSols.Item2 + " solutions.");
        Console.WriteLine("Show the lowest positive sum that can't be expressed (has no solutions), using the rules for this task");
        var lowestPositive = Enumerable.Range(1, int.MaxValue).First(x => !results.Keys.Contains(x));
        Console.WriteLine("  " + lowestPositive);
        Console.WriteLine("Show the ten highest numbers that can be expressed using the rules for this task (extra credit)");
        var highest = from k in results.Keys
                      orderby k descending
                      select k;
        foreach (var x in highest.Take(10))
            Console.WriteLine("  " + x);
    }
}
public enum Operations { Plus, Minus, Join };
public class Expression
{
    protected Operations[] Gaps;
    
    
    public int Value; 
    protected int _one;
    
    public Expression(int serial, int one)
    {
        _one = one;
        Gaps = new Operations[8];
        
        int divisor = 2187; 
        int times;
        for (int i = 0; i < 8; i++)
        {
            times = Math.DivRem(serial, divisor, out serial);
            divisor /= 3;
            if (times == 0)
                Gaps[i] = Operations.Join;
            else if (times == 1)
                Gaps[i] = Operations.Minus;
            else
                Gaps[i] = Operations.Plus;
        }
        
        
        Value = Evaluate();
    }
    public override string ToString()
    {
        string ret = _one.ToString();
        for (int i = 0; i < 8; i++)
        {
            switch (Gaps[i])
            {
                case Operations.Plus:
                    ret += "+";
                    break;
                case Operations.Minus:
                    ret += "-";
                    break;
            }
            ret += (i + 2);
        }
        return ret;
    }
    private int Evaluate()
        
    {
        var numbers = new int[9];
        int nc = 0;
        var operations = new List<Operations>();
        int a = 1;
        for (int i = 0; i < 8; i++)
        {
            if (Gaps[i] == Operations.Join)
                a = a * 10 + (i + 2);
            else
            {
                if (a > 0)
                {
                    if (nc == 0)
                        a *= _one;
                    numbers[nc++] = a;
                    a = i + 2;
                }
                operations.Add(Gaps[i]);
            }
        }
        if (nc == 0)
            a *= _one;
        numbers[nc++] = a;
        int ni = 0;
        int left = numbers[ni++];
        foreach (var operation in operations)
        {
            int right = numbers[ni++];
            if (operation == Operations.Plus)
                left = left + right;
            else
                left = left - right;
        }
        return left;
    }
}

VB: 
Function plusOne(iAry() As Integer, spot As Integer) As Integer
    Dim spotLim As Integer = If(spot = 0, 1, 2) 
    If iAry(spot) = spotLim Then 
        If spot = 0 Then Return -1 
        iAry(spot) = 0 
        Return plusOne(iAry, spot - 1) 
    Else
        iAry(spot) += 1 
    End If
    Return spot
End Function


Function generate(iAry() As Integer, terms As String) As String
    Dim operations As String() = {"", "-", "+"} 
    generate = ""
    For i As Integer = 0 To iAry.Count - 1
        generate &= operations(iAry(i)) & Mid(terms, i + 1, 1).ToString()
    Next
End Function


Function eval(sequence As String) As Integer
    eval = 0
    Dim term As Integer = 0, operation As Integer = 1
    For Each ch As Char In sequence
        Select Case ch
            Case "-", "+" 
                eval += If(operation = 0, -term, term) : term = 0 
                operation = If(ch = "-", 0, 1) 
            Case Else 
                term = term * 10 + Val(ch)
        End Select
    Next
    eval += If(operation = 0, -term, term) 
End Function


Sub reSort(ByRef first As List(Of Integer), ByRef second As List(Of Integer))
    Dim lou As New List(Of ULong) 
    For i As Integer = 0 To first.Count - 1
        lou.Add((CULng(first(i)) << 32) + second(i)) 
    Next
    lou.Sort()
    For k As Integer = 0 To first.Count - 1
        first(k) = lou(k) >> 32 
        second(k) = lou(k) And &H7FFFFFFF 
    Next
End Sub



Function firstMiss(loi As List(Of Integer))
    Dim low As Integer = 0, high As Integer = loi.Count - 1, middle = (low + high) \ 2
    Do
        If loi(middle) = middle Then low = middle + 1 Else high = middle - 1
        middle = (low + high) \ 2
    Loop Until high <= low
    Return middle + If(loi(middle) = middle, 1, 0)
End Function



Sub Solve100(Optional terms As String = "123456789",
             Optional targSum As Integer = 100,
             Optional highNums As Integer = 10)
    Dim lastDig As Integer = Len(terms) - 1 
    Dim iAry() As Integer = New Integer(lastDig) {} 
    Dim seq As String 
    Dim sVal As Integer 
    Dim sCnt As Integer = 1 
    Dim res As New List(Of Integer) 
    Dim tally As New List(Of Integer) 
    Console.WriteLine("List of solutions that evaluate to 100:")
    Do 
        seq = generate(iAry, terms) 
        sVal = eval(seq) 
        If sVal >= 0 Then 
            If res.Contains(sVal) Then tally(res.IndexOf(sVal)) += 1 _
                                  Else res.Add(sVal) : tally.Add(1)
            If sVal = targSum Then _
                Console.WriteLine(" {0,2} {1}", sCnt, seq) : sCnt += 1
        End If
    Loop Until plusOne(iAry, lastDig) < 0
    reSort(tally, res) 
    Console.WriteLine("The sum that has the the most solutions is {0}, (at {1}).",
                      res.Last, tally.Last)
    reSort(res, tally) 
    Console.WriteLine("The lowest positive sum that can
                      firstMiss(res))
    Console.WriteLine("The ten highest numbers that can be expressed are:")
    res.Reverse() 
    sCnt = 0 
    For Each item As Integer In res.Take(highNums)
        Console.Write("{0, -11}", item)
        sCnt = (sCnt + 1) Mod 5 : If sCnt = 0 Then Console.WriteLine()
    Next
End Sub

Sub Main()
    Solve100() 
End Sub