WriteOnce

WriteOnce<T>

WriteOnce<T> is a thread-safe value container whose value can be set exactly once. Install the WriteOnceContainer NuGet package.

using WriteOnceContainer;

var writeOnce = new WriteOnce<int>();
var value = writeOnce.Value; // throws ValueNotSetException because value has not been set yet
writeOnce.ThrowIfNotSet();   // throws ValueNotSetException because value has not been set yet
writeOnce.Value = 1337;      // OK because value is being set for the first time
writeOnce.Value = 9001;      // throws ValueAlreadySetException because value has already been set
writeOnce.ThrowIfNotSet();   // OK because value has been set

// override the default error messages with your own
var writeOnceWithCustomErrorMessages = new WriteOnce<int>(
    () => new ValueNotSetException("Value not set!"),
    () => new ValueAlreadySetException("Value already set!"));

Fluent Assertions

Install the WriteOnceContainer.FluentAssertions NuGet package. This NuGet package extends the FluentAssertions library so that assertions can be performed against WriteOnce<T> instances.

using FluentAssertions;
using WriteOnceContainer;
using WriteOnceContainer.FluentAssertions;

// assert that a WriteOnce<T> instance holds a value
const int VALUE = 1337;
var writeOnceWithValue = new WriteOnce<int>();
writeOnceWithValue = VALUE;
writeOnceWithValue.Should().HaveValue().AndValue.Should().Be(VALUE);

// assert that a WriteOnce<T> instance does not hold a value
var writeOnceWithoutValue = new WriteOnce<string>();
writeOnceWithoutValue.Should().NotHaveValue();

Blazor Components

Install the WriteOnceContainer.Blazor NuGet package. This NuGet package defines components used for rendering WriteOnce<T> in Blazor applications.

@using WriteOnceContainer
@using WriteOnceContainer.Blazor

<WriteOnceMatch WriteOnce="_container">
    <ValueSet Context="number">
        <!-- displayed if WriteOnce<T> container holds a value -->
        <p>@number</p>
    </ValueSet>
    <ValueNotSet>
        <!-- displayed if WriteOnce<T> container does not hold a value -->
        <p>No value set!</p>
    </ValueNotSet>
</WriteOnceMatch>

@code {
    private readonly WriteOnce<int> _container;
}