Allow SetAccountCurrency after SetCash without throwing

Previously, calling SetAccountCurrency after SetCash threw an
InvalidOperationException. The portfolio manager now switches the
base account currency in place: the previous Cash entry (and its
balance) is preserved in the CashBook, and a notice is logged. When
the new account currency matches the existing one, an optional
startingCash overrides the previously set amount and the override
is logged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: AlexCatarino

Common/Messages/Messages.Securities.cs

@@ -896,6 +896,25 @@ public static class SecurityPortfolioManager
             public static string CannotChangeAccountCurrencyAfterSettingCash =
                 "Cannot change AccountCurrency after setting cash. Please move SetAccountCurrency() before SetCash().";
 
+            /// <summary>
+            /// Returns a string message saying the AccountCurrency has been changed after setting cash, reporting the
+            /// remaining amount held in the previous account currency
+            /// </summary>
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public static string AccountCurrencyChangedAfterSettingCash(Securities.Cash previousCash)
+            {
+                return Invariant($"Account currency was changed after SetCash() was called. Algorithm still holds {previousCash.Amount} {previousCash.Symbol} in the previous account currency.");
+            }
+
+            /// <summary>
+            /// Returns a string message saying the account currency starting cash has been updated from a previous amount to a new one
+            /// </summary>
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public static string AccountCurrencyCashUpdated(string accountCurrency, decimal previousAmount, decimal newAmount)
+            {
+                return Invariant($"Account currency cash updated to {newAmount} {accountCurrency} from {previousAmount} {accountCurrency}.");
+            }
+
             /// <summary>
             /// Returns a string message saying the AccountCurrency has already been set and that the new value for this property
             /// will be ignored

Common/Securities/SecurityPortfolioManager.cs

@@ -616,10 +616,17 @@ public override SecurityHolding this[Symbol symbol]
 
         /// <summary>
         /// Sets the account currency cash symbol this algorithm is to manage, as well
-        /// as the starting cash in this currency if given
+        /// as the starting cash in this currency if given.
         /// </summary>
-        /// <remarks>Has to be called before calling <see cref="SetCash(decimal)"/>
-        /// or adding any <see cref="Security"/></remarks>
+        /// <remarks>
+        /// Should be called before adding any <see cref="Security"/>.
+        /// If <see cref="SetCash(decimal)"/> was called beforehand, the previous cash entry
+        /// is preserved in the <see cref="CashBook"/>: the algorithm continues to hold that
+        /// amount in the previous currency, while <see cref="_baseCurrencyCash"/> is
+        /// repointed to the new account currency. When the new account currency matches the
+        /// existing one, the optional <paramref name="startingCash"/> simply overrides the
+        /// previously set amount.
+        /// </remarks>
         /// <param name="accountCurrency">The account currency cash symbol to set</param>
         /// <param name="startingCash">The account currency starting cash to set</param>
         public void SetAccountCurrency(string accountCurrency, decimal? startingCash = null)
@@ -645,24 +652,40 @@ public void SetAccountCurrency(string accountCurrency, decimal? startingCash = n
                     Messages.SecurityPortfolioManager.CannotChangeAccountCurrencyAfterAddingSecurity);
             }
 
-            if (_setCashWasCalled)
-            {
-                throw new InvalidOperationException("SecurityPortfolioManager.SetAccountCurrency(): " +
-                    Messages.SecurityPortfolioManager.CannotChangeAccountCurrencyAfterSettingCash);
-            }
-
-            Log.Trace("SecurityPortfolioManager.SetAccountCurrency(): " +
-                Messages.SecurityPortfolioManager.SettingAccountCurrency(accountCurrency));
+            // Capture the previous base cash and amount if SetCash() was called earlier so we can
+            // either report the leftover balance in the old currency or detect a same-currency override.
+            var previousCash = _setCashWasCalled ? _baseCurrencyCash : null;
+            var previousAmount = previousCash?.Amount;
+            var message = Messages.SecurityPortfolioManager.SettingAccountCurrency(accountCurrency);
 
             UnsettledCashBook.AccountCurrency = accountCurrency;
             CashBook.AccountCurrency = accountCurrency;
 
+            // Repoint the base cash to the new account currency entry.
             _baseCurrencyCash = CashBook[accountCurrency];
 
+            if (previousCash != null && previousCash.Symbol != accountCurrency)
+            {
+                // The CashBook.AccountCurrency setter migrates the previous amount onto the new
+                // currency entry and removes the old one. Undo that migration so the previous
+                // balance is kept in its own currency, and the new account currency starts at zero
+                // (a subsequent SetCash below will apply startingCash if provided).
+                _baseCurrencyCash.SetAmount(0);
+                CashBook.Add(previousCash.Symbol, previousAmount.Value, previousCash.ConversionRate);
+                message += ". " + Messages.SecurityPortfolioManager.AccountCurrencyChangedAfterSettingCash(previousCash);
+            }
+
             if (startingCash != null)
             {
-                SetCash((decimal)startingCash);
+                SetCash(startingCash.Value);
+                // When the account currency is unchanged, report the override of the prior amount.
+                if (previousCash?.Symbol == accountCurrency && previousAmount != startingCash)
+                {
+                    message = Messages.SecurityPortfolioManager.AccountCurrencyCashUpdated(accountCurrency, previousAmount.Value, startingCash.Value);
+                }
             }
+
+            Log.Trace("SecurityPortfolioManager.SetAccountCurrency(): " + message);
         }
 
         /// <summary>

Tests/Common/Securities/SecurityPortfolioManagerTests.cs

@@ -2578,7 +2578,7 @@ public void CanNotChangeAccountCurrencyAfterAddingASecurity()
 
         [TestCase("SetCash(decimal cash)")]
         [TestCase("SetCash(string symbol, ...)")]
-        public void CanNotChangeAccountCurrencyAfterSettingCash(string overload)
+        public void ChangeAccountCurrencyAfterSettingCashKeepsPreviousCash(string overload)
         {
             var algorithm = new QCAlgorithm();
             if (overload == "SetCash(decimal cash)")
@@ -2587,9 +2587,57 @@ public void CanNotChangeAccountCurrencyAfterSettingCash(string overload)
             }
             else
             {
-                algorithm.Portfolio.SetCash(Currencies.USD, 1, 1);
+                algorithm.Portfolio.SetCash(Currencies.USD, 10, 1);
             }
-            Assert.Throws<InvalidOperationException>(() => algorithm.Portfolio.SetAccountCurrency(Currencies.USD));
+
+            // Switch the account currency to EUR; previous USD balance must be preserved
+            // in the CashBook and the base currency cash must be repointed to EUR.
+            Assert.DoesNotThrow(() => algorithm.Portfolio.SetAccountCurrency(Currencies.EUR));
+
+            Assert.AreEqual(Currencies.EUR, algorithm.Portfolio.CashBook.AccountCurrency);
+            Assert.AreEqual(10m, algorithm.Portfolio.CashBook[Currencies.USD].Amount);
+            Assert.AreEqual(0m, algorithm.Portfolio.CashBook[Currencies.EUR].Amount);
+        }
+
+        [Test]
+        public void ChangeAccountCurrencyAfterSettingCashAppliesStartingCashToNewCurrency()
+        {
+            var algorithm = new QCAlgorithm();
+            algorithm.Portfolio.SetCash(100000);
+
+            algorithm.Portfolio.SetAccountCurrency(Currencies.EUR, 50000);
+
+            Assert.AreEqual(Currencies.EUR, algorithm.Portfolio.CashBook.AccountCurrency);
+            // Previous USD cash is retained as a residual balance in the cash book.
+            Assert.AreEqual(100000m, algorithm.Portfolio.CashBook[Currencies.USD].Amount);
+            // Starting cash applies to the new account currency.
+            Assert.AreEqual(50000m, algorithm.Portfolio.CashBook[Currencies.EUR].Amount);
+        }
+
+        [Test]
+        public void SetAccountCurrencyWithSameCurrencyOverridesStartingCash()
+        {
+            var algorithm = new QCAlgorithm();
+            algorithm.Portfolio.SetCash(100000);
+
+            // Calling SetAccountCurrency with the same currency must overwrite the previously
+            // set cash amount instead of keeping the older value.
+            algorithm.Portfolio.SetAccountCurrency(Currencies.USD, 200000);
+
+            Assert.AreEqual(Currencies.USD, algorithm.Portfolio.CashBook.AccountCurrency);
+            Assert.AreEqual(200000m, algorithm.Portfolio.CashBook[Currencies.USD].Amount);
+        }
+
+        [Test]
+        public void SetAccountCurrencyWithSameCurrencyAndNoStartingCashKeepsExistingAmount()
+        {
+            var algorithm = new QCAlgorithm();
+            algorithm.Portfolio.SetCash(100000);
+
+            algorithm.Portfolio.SetAccountCurrency(Currencies.USD);
+
+            Assert.AreEqual(Currencies.USD, algorithm.Portfolio.CashBook.AccountCurrency);
+            Assert.AreEqual(100000m, algorithm.Portfolio.CashBook[Currencies.USD].Amount);
         }
 
         [Test]