ReadOnlyStatModifiers.cs

public readonly ReadOnlyCollection<StatModifier> StatModifiers; // Add this variable

public CharacterStat(float baseValue)
{
    BaseValue = baseValue;
    statModifiers = new List<StatModifier>();
    StatModifiers = statModifiers.AsReadOnly(); // Add this line to the constructor
}

Using_ObjectModel.cs

using System.Collections.ObjectModel; // Add this using statement