feat: create request projects for basic CQRS

src/request.result/_errors/Error.cs

@@ -0,0 +1,56 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+#pragma warning disable CA1716
+
+namespace Geekeey.Request.Result;
+
+/// <summary>
+/// An error containing a simple message. Makes up the other half of a <see cref="Result{T}"/> which might be an error.
+/// </summary>
+/// <remarks>
+/// An error is conceptually very similar to an exception but without the ability to be thrown, meant to be a more
+/// lightweight type meant to be wrapped in a <see cref="Result{T}"/>.
+/// An error fundamentally only contains a single string message, however other more concrete types such as
+/// <see cref="ExceptionError"/> or <see cref="AggregateError"/> may define other properties.
+/// Errors are meant to be small, specific, and descriptive, such that they are easy to match over and provide specific
+/// handling for specific kinds of errors.
+/// </remarks>
+public abstract class Error
+{
+	/// <summary>
+	/// A statically accessible default "Result has no value." error.
+	/// </summary>
+	internal static Error DefaultValueError { get; } = new StringError("The result has no value.");
+
+	/// <summary>
+	/// The message used to display the error.
+	/// </summary>
+	public abstract string Message { get; }
+
+	/// <summary>
+	/// Gets a string representation of the error. Returns <see cref="Message"/> by default.
+	/// </summary>
+	public override string ToString()
+	{
+		return Message;
+	}
+
+	/// <summary>
+	/// Implicitly converts a string into a <see cref="StringError"/>.
+	/// </summary>
+	/// <param name="message">The message of the error.</param>
+	public static implicit operator Error(string message)
+	{
+		return new StringError(message);
+	}
+
+	/// <summary>
+	/// Implicitly converts an exception into an <see cref="ExceptionError"/>.
+	/// </summary>
+	/// <param name="exception">The exception to convert.</param>
+	public static implicit operator Error(Exception exception)
+	{
+		return new ExceptionError(exception);
+	}
+}