S3450 — Parameters with "[DefaultParameterValue]" attributes should also be marked "[Optional]"

Why is this an issue?

There is no point in providing a default value for a parameter if callers are required to provide a value for it anyway. Thus, [DefaultParameterValue] should always be used in conjunction with [Optional].

Noncompliant code example

public void MyMethod([DefaultParameterValue(5)] int j) //Noncompliant, useless
{
  Console.WriteLine(j);
}

Compliant solution

public void MyMethod(int j = 5)
{
  Console.WriteLine(j);
}

or

public void MyMethod([DefaultParameterValue(5)][Optional] int j)
{
  Console.WriteLine(j);
}