Enforced immutability: Difference between revisions
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};</lang>
=={{header|C sharp}}==
Fields can be made read-only (a runtime constant) with the '''readonly''' keyword.
<lang csharp>readonly DateTime now = DateTime.Now;</lang>
When used on reference types, it just means the reference cannot be reassigned. It does not make the object itself immutable.<br/>
Primitive types can be declared as a compile-time constant with the '''const''' keyword.
<lang csharp>const int Max = 100;</lang>
Parameters can be made readonly by preceding them with the '''in''' keyword. Again, when used on reference types, it just means the reference cannot be reassigned.
<lang csharp>public void Method(in int x) {
x = 5; //Compile error
}</lang>
Local variables of primitive types can be declared as a compile-time constant with the '''const''' keyword.
<lang csharp>public void Method() {
const double sqrt5 = 2.236;
...
}</lang>
To make a type immutable, the programmer must write it in such a way that mutation is not possible. One important way to this is to use readonly properties. By not providing a setter, the property can only be assigned within the constructor.
<lang csharp>public string Key { get; }</lang>
On value types (which usually should be immutable from a design perspective), immutability can be enforced by applying the '''readonly''' modifier on the type. It will fail to compile if it contains any members that are not read-only.
<lang csharp>
public readonly struct Point
{
public Point(int x, int y) => (X, Y) = (x, y);
public int X { get; }
public int Y { get; }
}</lang>
=={{header|Clojure}}==
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