Do not declare read only mutable reference types
A mutable type is a type whose instance data can be modified. The System.Text.StringBuilder class is an example of a mutable reference type. It contains members that can change the value of an instance of the class. An example of an immutable reference type is the System.String class. After it has been instantiated, its value can never change.
The read-only modifier (readonly (C# Reference) in C#, ReadOnly (Visual Basic) in Visual Basic, and const (C++) in C++) on a reference type field (pointer in C++) prevents the field from being replaced by a different instance of the reference type but does not prevent the field's instance data from being modified through the reference type.
Read-only array fields are exempt from this rule but instead cause a violation of the Array fields should not be read only rule.
The following example shows a field declaration that causes a violation of this rule.