add simplification steps

csharp-fp1/Domain/Money.cs

@@ -0,0 +1,33 @@
+namespace CsharpFp1.Domain;
+
+/// Value object used to represent money and enforce simple invariants.
+/// Note that this implementation uses immutable patterns for the data
+/// by returning a new instance for each modification. This is an early
+/// recommendation for DDD with OO, but not necessarily the common practice
+/// in many real-world implementations.
+public sealed class Money
+{
+    // Potentially with a setter - see note above
+    public decimal Amount { get; }
+
+    public Money(decimal amount)
+    {
+        Amount = amount;
+    }
+
+    // In many existing DDD/OO codebases you may actually see the use
+    // of mutable value types.
+    //
+    // public void Add(Money other)
+    // {
+    //     this.Amount += other.Amount;
+    // }
+
+    // On the other hand, sometimes these helpers may be left out
+    // and operations encoded directly "from the outside":
+    // newBalance = new Money(oldBalance.Amount - charge.Amount)
+    //
+    public Money Add(Money other) => new(Amount + other.Amount);
+
+    public Money Subtract(Money other) => new(Amount - other.Amount);
+}