add fp1 sample

csharp-fp1/Domain/Account.cs

@@ -0,0 +1,32 @@
+namespace CsharpFp1.Domain;
+
+public sealed record Account(Guid Id, Money Balance);
+
+public static class AccountDomain
+{
+    // Choosing a "clean" FP approach here of instantiating the Account
+    // Depending on needs, code to prevent the Account type from
+    //   being instantiated without this helper needs to be added
+    //   to the record type.
+    public static Account Open(Guid id, Money openingBalance)
+    {
+        if (openingBalance.Amount < 0)
+            throw new ArgumentOutOfRangeException(nameof(openingBalance));
+
+        return new Account(id, openingBalance);
+    }
+
+    public static Account Withdraw(Account account, Money amount)
+    {
+        if (amount.Amount <= 0)
+            throw new InvalidOperationException("Withdrawal amount must be positive.");
+
+        if (account.Balance.Amount < amount.Amount)
+            throw new InsufficientBalanceException(account.Balance, amount);
+
+        return account with
+        {
+            Balance = account.Balance.Subtract(amount),
+        };
+    }
+}