.NET中的只读字段是否可以为空？

public class MyClass
{
    private readonly string _myReadOnlyField;

    public MyClass(string myReadOnlyField)
    {
        _myReadOnlyField = myReadOnlyField;
    }
}

public class MyClass
{
    private readonly string _myReadOnlyField;

    public MyClass()
    {
        _myReadOnlyField = null; // 编译错误：只读字段不能为空
    }
}