Can parameters be constant?

Question

I'm looking for the C# equivalent of Java's final. Does it exist?

Does C# have anything like the following:

public Foo(final int bar); 

In the above example, bar is a read only variable and cannot be changed by Foo(). Is there any way to do this in C#?

For instance, maybe I have a long method that will be working with x, y, and z coordinates of some object (ints). I want to be absolutely certain that the function doesn't alter these values in any way, thereby corrupting the data. Thus, I would like to declare them readonly.

public Foo(int x, int y, int z) {      // do stuff      x++; // oops. This corrupts the data. Can this be caught at compile time?      // do more stuff, assuming x is still the original value. } 

Answer 1

Unfortunately you cannot do this in C#.

The const keyword can only be used for local variables and fields.

The readonly keyword can only be used on fields.

NOTE: The Java language also supports having final parameters to a method. This functionality is non-existent in C#.

from http://www.25hoursaday.com/CsharpVsJava.html

EDIT (2019/08/13): I'm throwing this in for visibility since this is accepted and highest on the list. It's now kind of possible with in parameters. See the answer below this one for details.

Answer 2

This is now possible in C# version 7.2:

You can use the in keyword in the method signature. MSDN documentation.

The in keyword should be added before specifying a method's argument.

Example, a valid method in C# 7.2:

public long Add(in long x, in long y) {     return x + y; } 

While the following is not allowed:

public long Add(in long x, in long y) {     x = 10; // It is not allowed to modify an in-argument.     return x + y; } 

Following error will be shown when trying to modify either x or y since they are marked with in:

Cannot assign to variable 'in long' because it is a readonly variable

Marking an argument with in means:

This method does not modify the value of the argument used as this parameter.