C# 6 and VB 12 are about to add pattern matching

Another concept originating from functional programming languages ​​has joined the C# and VB camps, which is a feature called Pattern Matching. At first glance, pattern matching functions like a switch/select statement block, but its function is much more powerful.

Please note: Since the VB version specification is not yet complete, these examples are mainly from the pattern matching specification in C#.

　is or Matches operator

　 Pattern matching in .NET is mainly supported through the "is/Matches" operator. This less familiar operator breaks a class into its component parts. The following example is based on the Cartesian record class http://www.infoq.com/news/2014/08/Record-Class (record class) mentioned in Tuesday's news report.

public static bool operator is(Cartesian c, out double x, out double y)
    x = c.X;
    y = c.Y;
    return true;
}

The is operator is not limited to use in its defining class. The following example defines an operator in another way, allowing it to decompose a Cartesian object and match a Polar object.

public static class Polar {
    public static bool operator is( Cartesian c, out double R, out double Theta)
   {
        R = Math.Sqrt(c.X*c.X + c.Y*c.Y);
        Theta = Math.Atan2(c.Y, c.X);
        return c.X != 0 || c.Y != 0;
   }
}
var c = Cartesian(3, 4);
if (c is Polar(var R, *))
   Console.WriteLine(R);

Type Pattern

　The simplest pattern is the type pattern, which essentially tries to perform a type conversion and assign it to a variable at the same time. Here is an example of such a pattern:

if (expr is Type v)
   { // code using v }

Recursive Patterns

Most patterns appear as recursive patterns, meaning they are composed of simpler patterns. Take a look at an example of this pattern:

var a = new Location(1, 2, 3); //x=1, y=2, z=3
if (a is Location(1, var y, *))

This recursive pattern includes a constant pattern, a var pattern and a wildcard pattern.

　Constant mode

　This mode can match an attribute with a constant value. Constant matching uses the object.Equals(left, right) method to determine whether the two match.

　Var pattern

　 Var pattern will definitely match successfully. The variable corresponding to this pattern will be assigned the value provided when calling the is operator, and the type of the variable is determined by the expression. static type.

Wildcard pattern

Wildcard pattern is essentially a var expression, but you don’t need to care about the matching result.

　Internal Implementation

Let us continue to discuss the example of this location class. The compiler will perform the following steps during the compilation phase:

Create variables $x, $y and $z

Call Location.is( a, out $x, out $y, out $z) method and confirm that the return result is true

Constant mode: check the result of the object.Equals($x, 1) call

Var mode: assign $y to y

Wildcard pattern: ignore $z variable

　Switch/Select Case statement block

　The functionality of the Switch statement block will be expanded and it will be able to use pattern matching features. What this actually means is that you can write statements as follows:

case null:
case String s
case Location(1, var y, *):

限制

　　在目前的规格草稿中，还没有对于范围检查的支持。这意味着你不能编写类似于“a is Location( > 0, 1 to 5, <= 10)”这样的代码。此外目前也不支持在一个列表或迭代器中进行元素匹配。