What causes "Your expression '{expressionString}' cant be used. Nested accessors like 'o => o.NestedObject.Foo' at a parent level are not allowed. You should create a dedicated faker for NestedObject like new Faker ().RuleFor(o => o.Foo, ...) with its own rules that define how 'Foo' is generated. See this GitHub issue for more info: https://github.com/bchavez/Bogus/issues/115" in bchavez/Bogus?

Create a separate Faker with its own rules and use it in the parent's RuleFor to populate the nested property.

bchavez/Bogus · error · ArgumentException

Your expression '{expressionString}' cant be used. Nested ac

Error message

Your expression '{expressionString}' cant be used. Nested accessors like 'o => o.NestedObject.Foo' at a parent level are not allowed. You should create a dedicated faker for NestedObject like new Faker<NestedObject>().RuleFor(o => o.Foo, ...) with its own rules that define how 'Foo' is generated. See this GitHub issue for more info: https://github.com/bchavez/Bogus/issues/115

What it means

Thrown by PropertyName.GetMemberName when the expression string contains more than one '.' (IndexOf != LastIndexOf), indicating a nested accessor like o => o.NestedObject.Foo. Bogus only supports flat member access at the Faker<T> level; nested objects must have their own dedicated Faker.

Source

Thrown at Source/Bogus/PropertyName.cs:33

   public static string For<T>(Expression<Func<T, object>> expression)
   {
      Expression body = expression.Body;
      return GetMemberName(body);
   }

   public static string For(Expression<Func<object>> expression)
   {
      Expression body = expression.Body;
      return GetMemberName(body);
   }

   public static string GetMemberName(Expression expression)
   {
      var expressionString = expression.ToString();
      if( expressionString.IndexOf('.') != expressionString.LastIndexOf('.') )
      {
         throw new ArgumentException(
            $"Your expression '{expressionString}' cant be used. Nested accessors like 'o => o.NestedObject.Foo' at " +
            $"a parent level are not allowed. You should create a dedicated faker for " +
            $"NestedObject like new Faker<NestedObject>().RuleFor(o => o.Foo, ...) with its own rules " +
            $"that define how 'Foo' is generated. " +
            "See this GitHub issue for more info: https://github.com/bchavez/Bogus/issues/115");
      }

      MemberExpression memberExpression;

      if( expression is UnaryExpression unary )
         //In this case the return type of the property was not object,
         //so .Net wrapped the expression inside of a unary Convert()
         //expression that casts it to type object. In this case, the
         //Operand of the Convert expression has the original expression.
         memberExpression = unary.Operand as MemberExpression;
      else
         //when the property is of type object the body itself is the
         //correct expression

View on GitHub (pinned to 6ece18c5c2)

Solutions

Create a separate Faker<NestedObject> with its own rules and use it in the parent's RuleFor to populate the nested property.
Flatten the model so the property is directly on T.
Use a plain setter lambda (f => new Nested { Foo = ... }) instead of an expression accessing into the nested object.

Example fix

// before
new Faker<Order>().RuleFor(o => o.Customer.Name, f => f.Name.FullName());
// after
var customerFaker = new Faker<Customer>().RuleFor(c => c.Name, f => f.Name.FullName());
new Faker<Order>().RuleFor(o => o.Customer, f => customerFaker.Generate());

Defensive patterns

Strategy: type-guard

Validate before calling

static string SafeMember<T,P>(Expression<Func<T,P>> e) {
    var s = e.Body.ToString();
    if (s.IndexOf('.') != s.LastIndexOf('.')) throw new InvalidOperationException("nested accessor not allowed");
    return PropertyName.For(e as Expression<Func<object>>);
}

Type guard

bool IsFlatAccessor(Expression e) {
    var s = e.ToString();
    return s.IndexOf('.') == s.LastIndexOf('.');
}

Try / catch

try { f.RuleFor(expr, setter); }
catch (ArgumentException ex) when (ex.Message.Contains("Nested accessors")) {
    // extract nested faker and rewrite RuleFor at parent level
}

Prevention

Always model sub-objects with their own Faker<T>.
Lint expressions in a test: assert single-dot member access.
Review RuleFor lambdas during code review for any dot beyond x.

When it happens

Trigger: Calling RuleFor(x => x.Address.Street, ...) or FinishWith/PropertyName.For on a lambda that traverses into a sub-object.

Common situations: Modeling aggregates where a property is itself a complex type; assuming Bogus can auto-bind nested property paths like EF Core expressions.

Related errors

AI-assisted analysis of bchavez/Bogus@6ece18c5c2 (2026-08-13). Data as JSON: /api/errors/64f3fd0ddfaede5e. Report an issue: GitHub.