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copied!<p>You can roll your own (see the end of the answer for a more robust implementation that is thread safe and supports default values).</p> <pre><code>public class SetOnce<T> { private bool set; private T value; public T Value { get { return value; } set { if (set) throw new AlreadySetException(value); set = true; this.value = value; } } public static implicit operator T(SetOnce<T> toConvert) { return toConvert.value; } } </code></pre> <p>You can use it like so:</p> <pre><code>public class Foo { private readonly SetOnce<int> toBeSetOnce = new SetOnce<int>(); public int ToBeSetOnce { get { return toBeSetOnce; } set { toBeSetOnce.Value = value; } } } </code></pre> <p><strong>More robust implementation below</strong></p> <pre><code>public class SetOnce<T> { private readonly object syncLock = new object(); private readonly bool throwIfNotSet; private readonly string valueName; private bool set; private T value; public SetOnce(string valueName) { this.valueName = valueName; throwIfGet = true; } public SetOnce(string valueName, T defaultValue) { this.valueName = valueName; value = defaultValue; } public T Value { get { lock (syncLock) { if (!set && throwIfNotSet) throw new ValueNotSetException(valueName); return value; } } set { lock (syncLock) { if (set) throw new AlreadySetException(valueName, value); set = true; this.value = value; } } } public static implicit operator T(SetOnce<T> toConvert) { return toConvert.value; } } public class NamedValueException : InvalidOperationException { private readonly string valueName; public NamedValueException(string valueName, string messageFormat) : base(string.Format(messageFormat, valueName)) { this.valueName = valueName; } public string ValueName { get { return valueName; } } } public class AlreadySetException : NamedValueException { private const string MESSAGE = "The value \"{0}\" has already been set."; public AlreadySetException(string valueName) : base(valueName, MESSAGE) { } } public class ValueNotSetException : NamedValueException { private const string MESSAGE = "The value \"{0}\" has not yet been set."; public ValueNotSetException(string valueName) : base(valueName, MESSAGE) { } } </code></pre>

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