C#Result.cs
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/// <summary>
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/// A generic discriminated union representing success or failure.
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/// </summary>
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public class Result<T>
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{
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    public T? Value { get; }
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    public string? Error { get; }
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    public bool IsSuccess { get; }
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    public bool IsFailure => !IsSuccess;
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    private Result(T value) { Value = value; IsSuccess = true; }
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    private Result(string error) { Error = error; IsSuccess = false; }
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    public static Result<T> Ok(T value) => new(value);
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    public static Result<T> Fail(string error) => new(error);
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    // Implicit conversion from T
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    public static implicit operator Result<T>(T value) => Ok(value);
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    public TOut Match<TOut>(Func<T, TOut> onSuccess, Func<string, TOut> onFailure)
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        => IsSuccess ? onSuccess(Value!) : onFailure(Error!);
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    public Result<TOut> Map<TOut>(Func<T, TOut> mapper)
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        => IsSuccess ? Result<TOut>.Ok(mapper(Value!)) : Result<TOut>.Fail(Error!);
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    public override string ToString()
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        => IsSuccess ? $"Ok({Value})" : $"Fail({Error})";
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}
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public class PagedList<T>
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{
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    public IReadOnlyList<T> Items { get; }
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    public int Page { get; }
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    public int PageSize { get; }
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    public int TotalCount { get; }
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    public int TotalPages => (int)Math.Ceiling((double)TotalCount / PageSize);
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    public bool HasPreviousPage => Page > 1;
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    public bool HasNextPage => Page < TotalPages;
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    public PagedList(IEnumerable<T> items, int page, int pageSize, int totalCount)
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    {
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        Items = items.ToList().AsReadOnly();
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        Page = page;
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        PageSize = pageSize;
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        TotalCount = totalCount;
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    }
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}
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public static class QueryableExtensions
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{
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    public static async Task<PagedList<T>> ToPagedListAsync<T>(
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        this IQueryable<T> query,
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        int page,
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        int pageSize,
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        CancellationToken ct = default)
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    {
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        int totalCount = await System.Linq.AsyncEnumerable.CountAsync(query, ct);
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        var items = await System.Linq.AsyncEnumerable
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            .ToListAsync(query.Skip((page - 1) * pageSize).Take(pageSize), ct);
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        return new PagedList<T>(items, page, pageSize, totalCount);
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    }
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}
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