Provide value to property on construction time, without doing it inline

I would like to do a refactoring that will let me define the collection of fields somewhere else than inline at the initialization of Fields.

I'm confused by the error message as I'm still providing the value when it's constructed, I'm not modifying A after it's constructed (as far as I know).

Can someone please help me understand what I'm doing wrong and tell me how to provide a list of Fields that are not defined inline?

Definition of A:

public record A
{
    public A(String text)
    {
        this.Fields = new List<Field>();
    }

    public List<Field> Fields { get; }
}

Example before:

public static class MyClass
{
    public static A Build()
    {
        return new A("some date")
        {
            Fields =
            {
                Field1,
                Field2
            }
        }
    }
}

Example after:

public static class MyClassyClass
{
    public static List<Field> GetMeTheFields()
    {
        return new List<Field>
        {
            Field1,
            Field2
        };
    }

    public static A Build()
    {
        return new A("some date")
        {
            Fields = GetMeTheFields()
        }
    }
}

But I get the following error message:
> A property without setter or inaccessible setter cannot be assigned to

Add an init clause to the Fields property definition and you are good to go.

public List<Field> Fields { get; init; }

This will allow you to modify the Fields property inside the constructors or via an object initializer.

Your code has failed because the property without set or init can't be initialized inside the object initializer.

In case you cannot modify class A to add an init setter to Fields property, here's how you can provide a list of Fields to Fields a different way, based on your "example after":

public static class MyClassyClass
{
    public static List<Field> GetMeTheFields()
    {
        return new List<Field>
        {
            Field1,
            Field2
        };
    }

    public static A Build()
    {
        var a = new A("some date");

        a.Fields.AddRange(GetMeTheFields());

        return a;
    }
}