Catalog / Primitives / push-down-field

Push Down Field

Moves a field from a class into the direct subclasses that need it: those whose code uses it, or whose callers reach it through a reference of the subclass's type. When nothing uses the field, every direct subclass gets a copy, so the declaration is never lost.

Arguments

None. The target is the field, by symbol: "target": { "symbol": "F:Shop.Employee.Quota" }.

Precondition

The class has at least one subclass declared in the solution.
The class does not use the field itself.
No code reaches the field through a reference of the class's type, or of a type above it, since such a reference would no longer have the field.
The field's type and initializer use nothing private to the class.
No receiving subclass already has a member of that name.
The result compiles.

Transformation

Each receiving subclass gets the field after its last field, or first when it has none, with its documentation comment, initializer and modifiers.
A use by a class further down counts as a use by the direct subclass it derives from.
In a sealed subclass a protected field becomes private.
The class's type parameters are written as the subclass's type arguments: List<TItem> in Store<TItem> becomes List<Order> in OrderStore : Store<Order>. Each subclass's file gains the usings the field needs.
A declaration of several fields is split, and only the target moves.

Preserved

Every use of the field, which now binds to the subclass's copy.
Comments around the field that belong to its neighbours.

Limitations

The field only moves one level: a field that a class further down uses goes to the direct subclass above that class, not to the class itself.
Usings the class's file no longer needs are left in place.

Error codes

Code	Meaning
`no-subclasses`	no class in the solution derives from the field's class
`used-by-base`	the field's class uses the field itself
`used-through-base`	code reaches the field through a reference of the field's class
`uses-base-private-members`	the field's type or initializer uses something private to its class
`member-exists-in-subclass`	a receiving subclass already has a member of that name
`breaks-compilation`	the result would not compile

Cases

Diffs show each file from before to after. /*[*/ … /*]*/ marks a selection and /*^*/ a caret; the runner removes them before the refactoring runs.

#generic-base

Writes the base class's type parameter as the subclass's type argument and imports what the field needs

success

Targetsymbol F:Shop.Store`1.Cache

OrderStore.csmodified

Store.csmodified

View fixture on GitHub

#keeps-comments

Moves the field's documentation comment with it and leaves its neighbours' comments alone

success

Targetsymbol F:Shop.Employee.Territory

People.csmodified

View fixture on GitHub

#member-exists-in-subclass

Refuses when a subclass that would receive the field already has a member of that name

refusal

Targetsymbol F:Shop.Employee.Quota

Refusesmember-exists-in-subclass mentioning “Salesman”; every file is left unchanged

People.csinput

View fixture on GitHub

#no-subclasses

Refuses when no class derives from the field's class

refusal

Targetsymbol F:Shop.Employee.Quota

Refusesno-subclasses; every file is left unchanged

Employee.csinput

View fixture on GitHub

#nullable-annotations

Keeps nullable annotations on the moved field

success

Targetsymbol F:Shop.Employee.Territory

Projectnullable enable

People.csmodified

View fixture on GitHub

#one-of-several-declarators

Splits a declaration of several fields and moves only the target

success

Targetsymbol F:Shop.Employee.Quota

People.csmodified

View fixture on GitHub

#sealed-subclass-becomes-private

Narrows a protected field to private in a sealed subclass, where protected means nothing

success

Targetsymbol F:Shop.Employee.Quota

People.csmodified

View fixture on GitHub

#to-every-subclass-when-unused

Copies a field no code uses into every direct subclass, so it is not lost

success

Targetsymbol F:Shop.Employee.Notes

People.csmodified

View fixture on GitHub

#to-subclass-that-uses-it

Moves a field into the one subclass that uses it, including through a subclass-typed reference in another file

success

Targetsymbol F:Shop.Employee.Quota

Employee.csmodified

Salesman.csmodified

Engineer.csunchanged

Targets.csunchanged

View fixture on GitHub

#used-by-base

Refuses when the base class itself uses the field

refusal

Targetsymbol F:Shop.Employee.Quota

Refusesused-by-base mentioning “Employee”; every file is left unchanged

People.csinput

View fixture on GitHub

#used-through-base

Refuses when code elsewhere reaches the field through a base-typed reference

refusal

Targetsymbol F:Shop.Employee.Quota

Refusesused-through-base mentioning “Payroll.cs”; every file is left unchanged

Payroll.csinput

People.csinput

View fixture on GitHub

#uses-base-private-members

Refuses when the field's initializer uses a member private to the base class

refusal

Targetsymbol F:Shop.Employee.Quota

Refusesuses-base-private-members mentioning “DefaultQuota”; every file is left unchanged

People.csinput

View fixture on GitHub

	1	+	using System.Collections.Generic;
	2	+
1	3		namespace Shop
2	4		{
3	5		public class Order
4	6		{
5	7		}
6	8
7	9		public class OrderStore : Store<Order>
8	10		{
	11	+	protected List<Order> Cache = new List<Order>();
	12	+
9	13		public override int Count() => Cache.Count;
10	14		}
11	15		}

1	1		using System.Collections.Generic;
2	2
3	3		namespace Shop
4	4		{
5	5		public abstract class Store<TItem>
6	6		{
7		−	protected List<TItem> Cache = new List<TItem>();
8		−
9	7		public abstract int Count();
10	8		}
11	9		}

1	1		namespace Shop
2	2		{
3	3		public class Employee
4	4		{
5	5		// Everyone has a name.
6	6		public string Name;
7		−
8		−	/// <summary>Where a salesman sells.</summary>
9		−	public string Territory;
10	7
11	8		// Pay is monthly.
12	9		public decimal Salary;
13	10		}
14	11
15	12		public class Salesman : Employee
16	13		{
17	14		// Sales so far this year.
18	15		public decimal Sales;
	16	+	/// <summary>Where a salesman sells.</summary>
	17	+	public string Territory;
19	18
20	19		public string Region() => Territory.ToUpperInvariant();
21	20		}
22	21		}

1		namespace Shop
2		{
3		public class Employee
4		{
5		public int Quota;
6		}
7
8		public class Salesman : Employee
9		{
10		public new string Quota = "high";
11		}
12		}

1	1		namespace Shop
2	2		{
3	3		public class Employee
4	4		{
5		−	public string? Territory;
6	5		}
7	6
8	7		public class Salesman : Employee
9	8		{
	9	+	public string? Territory;
	10	+
10	11		public string Region() => Territory ?? "none";
11	12		}
12	13		}

1	1		namespace Shop
2	2		{
3	3		public class Employee
4	4		{
5		−	protected int Grade, Quota;
	5	+	protected int Grade;
6	6
7	7		public int Level() => Grade;
8	8		}
9	9
10	10		public class Salesman : Employee
11	11		{
	12	+	protected int Quota;
	13	+
12	14		public int Target() => Quota * Grade;
13	15		}
14	16		}

1	1		namespace Shop
2	2		{
3	3		public abstract class Employee
4	4		{
5		−	public string Notes = "";
6		−
7	5		public abstract decimal Pay();
8	6		}
9	7
10	8		public class Salesman : Employee
11	9		{
	10	+	public string Notes = "";
	11	+
12	12		public override decimal Pay() => 100;
13	13		}
14	14
15	15		public class Engineer : Employee
16	16		{
17	17		public int Level;
	18	+	public string Notes = "";
18	19
19	20		public override decimal Pay() => 200;
20	21		}
21	22		}

1	1	namespace Shop
2	2	{
3	3	public static class Targets
4	4	{
5	5	public static void Raise(Salesman salesman) => salesman.Quota += 10;
6	6	}
7	7	}

1		namespace Shop
2		{
3		public static class Payroll
4		{
5		public static int QuotaOf(Employee employee) => employee.Quota;
6		}
7		}

1		namespace Shop
2		{
3		public class Employee
4		{
5		private const int DefaultQuota = 10;
6		protected int Quota = DefaultQuota;
7		}
8
9		public class Salesman : Employee
10		{
11		public int Target() => Quota;
12		}
13		}