Validate a value in property

Question

So I heard that validating a value in a property like this:

//dummy example, let's assume that I want my value without dots
public string MyProp
{
    set
    {
        if(value.Contains('.'))
            throw new ArgumentException("Must not contain '.'", "value");
    }
}

is wrong and I should avoid it.

But in earlier days I was told that this is the good way. We could use encapsulation, there is just one place to check, DRY, etc.

What's wrong with my little example?

Answer 1

There is nothing wrong with throwing exceptions in a property setter. But you should throw an ArgumentException, and also actually set the value of the property!

private string _myprop;
public string MyProp{
    set{
       if(value.Contains('.')) throw new ArgumentException("Must not contain .");
       this._myprop=value;
    }
    get { return this._myprop; }
}

From an article on best practices in MSDN:

Property getters should be simple operations without any preconditions. If a getter might throw an exception, consider redesigning the property to be a method. This recommendation does not apply to indexers. Indexers can throw exceptions because of invalid arguments.

It is valid and acceptable to throw exceptions from a property setter.