What are switch expressions in D?

 switch (c) {
        case 1:
           //This block will execute if the passed value is equal to 1
            break;
        case 2:
            //This block will execute if the passed value is equal to 2
            break;
        case 3:
           //This block will execute if the passed value is equal to 3
            break;
        default:
            //This block will execute if the passed value is not equal to any case
            break;
        }

import std.stdio;
void main() {
    // Passing values to c
    foreach (c ; [ 1, 2,3,20 ]) {
        // Passing the values of c to the switch statment
        switch (c) {
            // Declaring case 1 code block
            case 1:
                writeln("I am from case 1");
                break;
            // Declaring case 2 code block
            case 2:
                writeln("I am from case 2");
                break;
            // Declaring case 3 code block
            case 3:
                writeln("I am from case 3");
                break;
            // Declaring default code block
            default:
                writeln("I am from default case");
                break;
        }
    }
}