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Code Analyzer for C#

There is a newer version: 10.2.0.105762

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)