CA1034: Nested types should not be visible

Property	Value
Rule ID	CA1034
Title	Nested types should not be visible
Category	Design
Fix is breaking or non-breaking	Breaking
Enabled by default in .NET 9	No

Cause

An externally visible type contains an externally visible type declaration. Nested enumerations, protected types, and builder patterns are exempt from this rule.

Rule description

A nested type is a type declared within the scope of another type. Nested types are useful for encapsulating private implementation details of the containing type. Used for this purpose, nested types should not be externally visible.

Do not use externally visible nested types for logical grouping or to avoid name collisions; instead, use namespaces.

Nested types include the notion of member accessibility, which some programmers do not understand clearly.

Protected types can be used in subclasses and nested types in advance customization scenarios.

How to fix violations

If you do not intend the nested type to be externally visible, change the type's accessibility. Otherwise, remove the nested type from its parent. If the purpose of the nesting is to categorize the nested type, use a namespace to create the hierarchy instead.

When to suppress warnings

Do not suppress a warning from this rule.

Example

The following example shows a type that violates the rule.

public class ParentType
{
    public class NestedType
    {
        public NestedType()
        {
        }
    }

    public ParentType()
    {
        NestedType nt = new NestedType();
    }
}

Imports System

Namespace ca1034

    Class ParentType

        Public Class NestedType
            Sub New()
            End Sub
        End Class

        Sub New()
        End Sub

    End Class

End Namespace