S3962 — "static readonly" constants should be "const" instead

Why is this an issue?

The value of a static readonly field is computed at runtime while the value of a const field is calculated at compile time, which improves performance.

This rule raises an issue when a static readonly field is initialized with a value that is computable at compile time.

As specified by Microsoft, the list of types that can have a constant value are:

Noncompliant code example

namespace myLib
{
  public class Foo
  {
    static readonly int x = 1;  // Noncompliant
    static readonly int y = x + 4; // Noncompliant
    static readonly string s = "Bar";  // Noncompliant
  }
}

Compliant solution

namespace myLib
{
  public class Foo
  {
    const int x = 1;
    const int y = x + 4;
    const string s = "Bar";
  }
}