C# variable that can't be changed but need to be initialed in the constructor

Question

I need an attribute that can't be changed after initialisation in the constructor

somthing like this:

private const string banknr;

public ClassName(string banknr)
{
    this.banknr = banknr;
    //from now on "banknr" can't be changed something like a final or const
}

but it just doesn't work, I realy don't understand

Answer 1

That's precisely what the readonly keyword does.

private readonly string banknr;

public ClassName(string banknr)
{
    this.banknr = banknr;
    //from now on "banknr" can't be changed something like a final or const
}

readonly variables can be set in a constructor, but can not be changed.