C# – way of setting a property once only in C#

cnet

I'm looking for a way to allow a property in a C# object to be set once only. It's easy to write the code to do this, but I would rather use a standard mechanism if one exists.

public OneShot<int> SetOnceProperty { get; set; }

What I want to happen is that the property can be set if it is not already set, but throw an exception if it has been set before. It should function like a Nullable value where I can check to see if it has been set or not.

Best Answer

There is direct support for this in the TPL in .NET 4.0;

(edit: the above sentence was written in anticipation of System.Threading.WriteOnce<T> which existed in the "preview" bits available at the time, but this seems to have evaporated before the TPL hit RTM/GA)

until then just do the check yourself... it isn't many lines, from what I recall...

something like:

public sealed class WriteOnce<T>
{
    private T value;
    private bool hasValue;
    public override string ToString()
    {
        return hasValue ? Convert.ToString(value) : "";
    }
    public T Value
    {
        get
        {
            if (!hasValue) throw new InvalidOperationException("Value not set");
            return value;
        }
        set
        {
            if (hasValue) throw new InvalidOperationException("Value already set");
            this.value = value;
            this.hasValue = true;
        }
    }
    public T ValueOrDefault { get { return value; } }

    public static implicit operator T(WriteOnce<T> value) { return value.Value; }
}

Then use, for example:

readonly WriteOnce<string> name = new WriteOnce<string>();
public WriteOnce<string> Name { get { return name; } }
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