C# Academy · Lesson

Guard & validation patterns

Validate inputs early with guard clauses, throw the right exceptions, and use TryParse-style checks for user data.

Why guards

Aim: Keep methods safe and readable.

Use guard clauses at the top
Throw specific exceptions (ArgumentNullException, ArgumentOutOfRangeException)
Use TryParse for user input
Fail fast; avoid deep nesting

Early guards demo

Add small checks at the top to validate arguments; throw specific exceptions so callers know what went wrong.

using System;

// Demo: validate inputs at the top; keep main logic flat.
public class Program
{
  // Compute price with simple rules; guard early.
  public static decimal ComputePrice(string sku, int qty, decimal unitPrice)
  {
    if (sku == null) throw new ArgumentNullException("sku");
    if (sku.Length == 0) throw new ArgumentException("sku must not be empty", "sku");
    if (qty <= 0) throw new ArgumentOutOfRangeException("qty", "qty must be > 0");
    if (unitPrice < 0m) throw new ArgumentOutOfRangeException("unitPrice", "unitPrice must be >= 0");

    // Main logic stays simple
    decimal subtotal = qty * unitPrice;
    if (qty >= 10) subtotal = subtotal * 0.9m; // small bulk discount
    return subtotal;
  }

  public static void Main(string[] args)
  {
    Console.WriteLine(ComputePrice("ABC", 2, 5m));   // ok
    try { Console.WriteLine(ComputePrice("", 2, 5m)); } catch (Exception ex) { Console.WriteLine(ex.GetType().Name); }
  }
}

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