What UserInfo.getDefaultCurrency() actually returns

The method returns a String — never null in a properly licensed org

UserInfo.getDefaultCurrency() returns a non-null ISO 4217 code for any running user in an org with currencies enabled. If you're seeing null, the most common reasons are:

• The org has multi-currency disabled. In that case the method still returns the org's single currency — null indicates something else is wrong.
• You're running in a context where UserInfo isn't fully populated — anonymous Apex in a fresh session, certain batch contexts, or test execution with SeeAllData=false and no test user setup.
• The org has currencies disabled at the platform level — extremely rare, but possible in trial/demo orgs.

Single-currency vs multi-currency — the behaviour you actually need to know

Single-currency org

One currency, set at org creation. Every user's default currency equals the org's currency. UserInfo.getDefaultCurrency() returns that single code:

String c = UserInfo.getDefaultCurrency();
// c == 'USD' (or 'EUR', 'GBP', etc — whatever the org was created with)
// Same value for every user in the org.

Multi-currency org

Multiple currencies enabled. Each user has a personal default currency (configurable on their User record). Records have their own CurrencyIsoCode field. UserInfo.getDefaultCurrency() returns the running user's personal default — not the org's corporate currency, not the record's currency:

// In a multi-currency org:
String userDefault = UserInfo.getDefaultCurrency();
// userDefault == the running user's personal default (e.g. 'EUR' for a European sales rep)

// If you need the record's currency, read it off the record:
Account a = [SELECT Id, CurrencyIsoCode FROM Account WHERE Id = :acctId];
String recordCurrency = a.CurrencyIsoCode;

How to get the org's corporate currency safely

In a single-currency org there is only one currency, so any approach works. In a multi-currency org you usually want the corporate currency — the one with IsCorporate = true on the CurrencyType object:

String corporateCurrency;
if (UserInfo.isMultiCurrencyOrganization()) {
  CurrencyType corp = [
    SELECT IsoCode FROM CurrencyType WHERE IsCorporate = true LIMIT 1
  ];
  corporateCurrency = corp.IsoCode;
} else {
  // Single-currency org — UserInfo is authoritative
  corporateCurrency = UserInfo.getDefaultCurrency();
}

UserInfo.isMultiCurrencyOrganization() is the canonical gate. It also lets your code compile cleanly in both org types — selecting from CurrencyTypein a single-currency org throws a compile error, so the conditional matters.

What to use when

• Reporting a value in the running user's preferred unit: UserInfo.getDefaultCurrency() is correct.
• Persisting a value on a record: use the record's CurrencyIsoCode field. Don't rely on the user's default — the user changes, the record's currency shouldn't.
• Reporting at the org level: read the IsCorporate entry on CurrencyType.
• Cross-org / managed package code: always gate on UserInfo.isMultiCurrencyOrganization() first.