Is there a way of setting a property once only in C#

Question

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.

Answer 1

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; } }