Version Used:

Microsoft Visual Studio Community 2019
Version 16.4.2
Microsoft .NET Framework
4.8.03752

Steps to Reproduce:

In a class marked DataContract, create a private field, NOT readonly, and marked DataMember.

[DataContract]
public class MyClass
{
	[DataMember]
	private bool isReadOnly;
}

Expected Behavior:

The field cannot be readonly because it is a DataMember.

Actual Behavior:

The IDE warns with "IDE0044 (Make Field Readonly)".