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feat: add defensive-programming core modules for csharp go and python
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‎LANGUAGE_PARITY_MATRIX.md‎

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This matrix tracks concept parity across C++, C#, Go, and Python.
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- Canonical order is defined by the C++ track.
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- Current priority is to complete `02-core` in C#, Go, and Python.
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- Current priority is to begin `03-advanced` parity planning in C#, Go, and Python.
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- Status labels:
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- `Done`: module implemented with example, exercises, and README.
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- `Planned`: module not implemented yet, already queued in order.
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Current parity progress in non-C++ tracks:
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- C#: `5/6` modules complete
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- Go: `5/6` modules complete
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- Python: `5/6` modules complete
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- C#: `6/6` modules complete
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- Go: `6/6` modules complete
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- Python: `6/6` modules complete
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| Order | Module | C++ | C# | Go | Python |
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| --- | --- | --- | --- | --- | --- |
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| 3 | file-io-basics | Done | Done | Done | Done |
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| 4 | sorting-and-searching | Done | Done | Done | Done |
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| 5 | maps-and-frequency-counting | Done | Done | Done | Done |
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| 6 | error-handling-and-defensive-programming | Done | Planned | Planned | Planned |
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| 6 | error-handling-and-defensive-programming | Done | Done | Done | Done |
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## Advanced and Expert
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`03-advanced` and `04-expert` currently exist only in the C++ track and are out of scope until `02-core` parity is complete in C#, Go, and Python.
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`03-advanced` and `04-expert` currently exist only in the C++ track. `02-core` parity is complete, so the next step is defining the first `03-advanced` parity batch.

‎README.md‎

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| Language | Current Levels | Module Coverage | Track Status |
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| --- | --- | --- | --- |
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| C++ | 00-setup, 01-foundations, 02-core, 03-advanced, 04-expert | Foundations, Core, Advanced, Expert, projects, assessments | Most complete and primary track |
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| C# | 01-foundations, 02-core (partial) | 8/8 foundations modules, 5/6 core modules | Foundations complete, core expansion in progress |
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| Go | 01-foundations, 02-core (partial) | 8/8 foundations modules, 5/6 core modules | Foundations complete, core expansion in progress |
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| Python | 01-foundations, 02-core (partial) | 8/8 foundations modules, 5/6 core modules | Foundations complete, core expansion in progress |
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| C# | 01-foundations, 02-core | 8/8 foundations modules, 6/6 core modules | Foundations and core complete |
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| Go | 01-foundations, 02-core | 8/8 foundations modules, 6/6 core modules | Foundations and core complete |
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| Python | 01-foundations, 02-core | 8/8 foundations modules, 6/6 core modules | Foundations and core complete |
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Parity planning reference: [LANGUAGE_PARITY_MATRIX.md](LANGUAGE_PARITY_MATRIX.md)
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‎languages/csharp/02-core/README.md‎

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3. [file-io-basics](./file-io-basics/README.md)
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4. [sorting-and-searching](./sorting-and-searching/README.md)
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5. [maps-and-frequency-counting](./maps-and-frequency-counting/README.md)
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6. [error-handling-and-defensive-programming](./error-handling-and-defensive-programming/README.md)
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Track progress in [../CHECKLIST.md](../CHECKLIST.md).
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## Study Tip
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Use `maps-and-frequency-counting` after `sorting-and-searching` to practice aggregation and grouped reporting patterns.
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Use `error-handling-and-defensive-programming` last to consolidate validation and guard-style thinking across all core modules.

‎languages/csharp/02-core/error-handling-and-defensive-programming/README.md‎

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# Error Handling and Defensive Programming (C#)
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This module teaches robust validation checks and safe failure behavior.
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## Quick Run
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~~~bash
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dotnet run --project example/error-handling-and-defensive-programming-example.csproj
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~~~
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## Topics Covered
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- Defensive input validation with retry loops.
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- Early returns for invalid states.
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- Guard conditions before risky operations.
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- Producing clear and actionable error messages.
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## Common Pitfalls
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- Continuing execution after detecting an invalid state.
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- Performing division without zero checks.
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- Printing vague errors that do not help recovery.
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## Exercise Focus
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- exercises/01.cs: validate CSV-like row format.
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- exercises/02.cs: safe division utility with retries.
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### Exercise Specs
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1. exercises/01.cs
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- Input: one row with format `name,age,city`.
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- Output: parsed fields or invalid format message.
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- Edge cases: missing commas; empty fields.
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2. exercises/02.cs
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- Input: pairs of numbers for division until valid.
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- Output: quotient or error/retry message.
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- Edge cases: divisor zero; non-numeric input.
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## Checkpoint
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- [ ] I can guard risky operations with clear checks.
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- [ ] I can stop invalid program paths early.
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- [ ] I can produce useful error feedback for users.
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- [ ] I completed exercises/01.cs.
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- [ ] I completed exercises/02.cs.

‎languages/csharp/02-core/error-handling-and-defensive-programming/example/error-handling-and-defensive-programming-example.csproj‎

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<OutputType>Exe</OutputType>
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<TargetFramework>net8.0</TargetFramework>
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<ImplicitUsings>disable</ImplicitUsings>
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<Nullable>enable</Nullable>
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</PropertyGroup>
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</Project>

‎languages/csharp/02-core/error-handling-and-defensive-programming/example/main.cs‎

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// Example purpose: show the module flow with clear, beginner-friendly steps.
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using System;
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class Program
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{
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static bool TrySafeDivide(double left, double right, out double result)
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{
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// Intent: block invalid operations early before the main logic continues.
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if (right == 0.0)
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{
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result = 0.0;
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return false;
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}
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result = left / right;
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return true;
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}
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static void Main()
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{
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// Program flow: run a fixed set of cases to show both safe and unsafe paths.
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(double left, double right)[] scenarios = new[]
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{
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(42.0, 6.0),
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(10.0, 0.0)
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};
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foreach ((double left, double right) in scenarios)
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{
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// Intent: print scenario input so behavior is easy to verify.
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Console.WriteLine($"Input: {left} {right}");
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if (!TrySafeDivide(left, right, out double quotient))
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{
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Console.WriteLine("Cannot divide by zero.");
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continue;
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}
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// Intent: print deterministic final output for quick verification.
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Console.WriteLine($"Result: {quotient}");
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}
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}
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}

‎languages/csharp/02-core/error-handling-and-defensive-programming/exercises/01.cs‎

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using System;
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class Program
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{
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static void Main()
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{
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Console.Write("Enter row (name,age,city): ");
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string row = Console.ReadLine() ?? string.Empty;
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int firstComma = row.IndexOf(',');
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if (firstComma < 0)
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{
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Console.WriteLine("Invalid format. Missing commas.");
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return;
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}
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int secondComma = row.IndexOf(',', firstComma + 1);
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if (secondComma < 0)
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{
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Console.WriteLine("Invalid format. Missing second comma.");
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return;
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}
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string name = row.Substring(0, firstComma);
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string age = row.Substring(firstComma + 1, secondComma - firstComma - 1);
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string city = row.Substring(secondComma + 1);
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if (name.Length == 0 || age.Length == 0 || city.Length == 0)
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{
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Console.WriteLine("Invalid format. Empty field detected.");
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return;
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}
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Console.WriteLine($"Name: {name}");
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Console.WriteLine($"Age: {age}");
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Console.WriteLine($"City: {city}");
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}
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}

‎languages/csharp/02-core/error-handling-and-defensive-programming/exercises/02.cs‎

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using System;
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class Program
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{
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static void Main()
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{
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while (true)
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{
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Console.Write("Enter two numbers to divide (a b): ");
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string raw = Console.ReadLine() ?? string.Empty;
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string[] parts = raw.Split(' ', StringSplitOptions.RemoveEmptyEntries);
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if (parts.Length != 2 ||
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!double.TryParse(parts[0], out double a) ||
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!double.TryParse(parts[1], out double b))
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{
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Console.WriteLine("Invalid input type. Try again.");
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continue;
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}
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if (b == 0.0)
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{
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Console.WriteLine("Divisor cannot be zero. Try again.");
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continue;
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}
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Console.WriteLine($"Result: {a / b}");
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break;
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}
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}
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}

‎languages/csharp/CHECKLIST.md‎

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- [ ] Complete `02-core/file-io-basics`.
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- [ ] Complete `02-core/sorting-and-searching`.
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- [ ] Complete `02-core/maps-and-frequency-counting`.
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- [ ] Complete `02-core/error-handling-and-defensive-programming`.
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## Parity Goals
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‎languages/csharp/README.md‎

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## Scope and Status
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This track currently covers `01-foundations` and has started `02-core`.
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This track currently covers `01-foundations` and `02-core`.
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- 8/8 foundations modules implemented.
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- 5/6 core modules implemented (`input-validation`, `algorithms-basics`, `file-io-basics`, `sorting-and-searching`, `maps-and-frequency-counting`).
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- 6/6 core modules implemented (`input-validation`, `algorithms-basics`, `file-io-basics`, `sorting-and-searching`, `maps-and-frequency-counting`, `error-handling-and-defensive-programming`).
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- Same module naming as C++, Python, and Go for parity.
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## Prerequisites
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- [file-io-basics](./02-core/file-io-basics/README.md)
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- [sorting-and-searching](./02-core/sorting-and-searching/README.md)
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- [maps-and-frequency-counting](./02-core/maps-and-frequency-counting/README.md)
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- [error-handling-and-defensive-programming](./02-core/error-handling-and-defensive-programming/README.md)
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## Progress Tracking
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