## Why is this an issue?

Using the `readonly` keyword on a field means it can’t be changed after initialization. However, that’s only partly true when applied to collections or arrays. The `readonly` keyword enforces that another instance can’t be assigned to the field, but it cannot keep the contents from being updated. In practice, the field value can be changed, and the use of `readonly` on such a field is misleading, and you’re likely not getting the behavior you expect.

This rule raises an issue when a non-private, `readonly` field is an array or collection.

## How to fix it

To fix this, you should either use an immutable or frozen collection or remove the `readonly` modifier to clarify the behavior.

### Code examples

#### Noncompliant code example

    public class MyClass
    {
      public readonly string[] strings1;  // Noncompliant
      public readonly string[] strings2;  // Noncompliant
      public readonly string[] strings3;  // Noncompliant
      // ...
    }

#### Compliant solution

    public class MyClass
    {
      public string[] strings1;                         // Compliant: remove readonly modifier
      public readonly ImmutableArray<string> strings;   // Compliant: use an Immutable collection
      private readonly string[] strings;                // Compliant: reduced accessibility to private

      // ...
    }

## Resources

### Documentation

- Microsoft Learn - readonly (C# Reference)