3 · Reliability and project · 45 MIN

Project · A tested expense report

Pure calculations make success and failure easy to verify.

This report groups whole-unit amounts in a Dictionary and uses checked addition. An invalid record or overflow throws before the method returns its local dictionary, so callers receive either a complete result or a failure. The example’s small Check helper runs regardless of build configuration. Convert these checks to your preferred test framework when the project grows. Persisting balances would require a transaction; this in-memory function deliberately does not imply cross-request or cross-server durability.

Keep the dictionary local until every row has passed validation.

Read the example

using System;
using System.Collections.Generic;
static Dictionary<string,int> Report((string Category,int Amount)[] rows) {
  var totals = new Dictionary<string,int>();
  foreach (var row in rows) {
    if (string.IsNullOrWhiteSpace(row.Category) || row.Amount < 0) throw new ArgumentException("Invalid expense");
    totals.TryGetValue(row.Category, out var previous);
    totals[row.Category] = checked(previous + row.Amount);
  }
  return totals;
}
static void Check(bool ok) { if (!ok) throw new Exception("Check failed"); }
Check(Report(Array.Empty<(string,int)>()).Count == 0);
Check(Report(new[]{("food",5),("food",7)})["food"] == 12);
Console.WriteLine("report checks passed");

Check the expected output

report checks passed

Your challenge

Build a test project covering grouping, invalid records and overflow; add stable sorted report formatting without changing calculation order.

Solution cost: Expected O(n) grouping with O(k) category storage. time · Account for collection storage separately from the returned result. space

Common trap

Returning partial results after a caught exception silently changes an all-or-nothing contract.

Study the project implementation

using System;
using System.Collections.Generic;
static Dictionary<string,int> Report((string Category,int Amount)[] rows) {
  var totals = new Dictionary<string,int>();
  foreach (var row in rows) {
    if (string.IsNullOrWhiteSpace(row.Category) || row.Amount < 0) throw new ArgumentException("Invalid expense");
    totals.TryGetValue(row.Category, out var previous);
    totals[row.Category] = checked(previous + row.Amount);
  }
  return totals;
}
static void Check(bool ok) { if (!ok) throw new Exception("Check failed"); }
Check(Report(Array.Empty<(string,int)>()).Count == 0);
Check(Report(new[]{("food",5),("food",7)})["food"] == 12);
Console.WriteLine("report checks passed");

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