FooBarAssert.cs

private void CheckInput(int input, string methodName) {
   if (input != 42) {
      throw new ArgumentOutOfRangeException(nameof(input), $"{methodName} cannot be invoked with a non-42 value");
   }
}

public void DoFoo(int input) {
   CheckInput(input, "DoFoo");
   
   // ...
}

public void DoBar(int input) {
   CheckInput(input, "DoBar");
   
   // ...
}