decomposition of data types

Decomposition.cs

// composition
var expr = Add(Mult(Const(2), X()), Const(1));

// decomposition
switch (expr)
{
    case Add(Mult(var a, var x), var b): ...;
}

Expr.cs

abstract class Expr;
class X() : Expr;
class Const(double Value) : Expr;
class Add(Expr Left, Expr Right) : Expr;
class Mult(Expr Left, Expr Right) : Expr;
class Neg(Expr Value) : Expr;