NullFunction.cs

using Queries.Core.Attributes;
using Queries.Core.Parts.Columns;
using System;
using System.Collections.Generic;
using System.Linq;

namespace Queries.Core.Parts.Functions;

/// <summary>
/// "ISNULL" function
/// </summary>
[Function]
public class NullFunction : IAliasable<NullFunction>, IColumn, IEquatable<NullFunction>
{
    /// <summary>
    /// Column onto which the function must be applied.
    /// </summary>
    public IColumn Column { get; }

    /// <summary>
    /// Value to use as replacement when <see cref="Column"/>'s value is <see langword="null" />
    /// </summary>
    public IColumn DefaultValue { get; }

    /// <summary>
    /// Additionnal default values that can be used as fallback when <see cref="DefaultValue"/> returns <see langword="null" />.
    /// </summary>
    public IEnumerable<IColumn> AdditionalDefaultValues { get; }

    /// <summary>
    /// Builds a new <see cref="NullFunction"/> instance.
    /// </summary>
    /// <param name="column">The column to apply the function onto.</param>
    /// <param name="defaultValue">The default value value to use if <paramref name="column"/>'s value is <see langword="null" />.</param>
    /// <param name="additionalDefaultValues"></param>
    /// <exception cref="ArgumentNullException"> if either <paramref name="column"/> or <paramref name="defaultValue"/> is <see langword="null" /></exception>
    public NullFunction(IColumn column, IColumn defaultValue, params IColumn[] additionalDefaultValues)
    {
        Column = column ?? throw new ArgumentNullException(nameof(column));
        DefaultValue = defaultValue ?? throw new ArgumentNullException(nameof(defaultValue));
        AdditionalDefaultValues = additionalDefaultValues ?? Enumerable.Empty<IColumn>();
    }

    private string _alias;

    ///<inheritdoc/>
    public string Alias => _alias;

    /// <summary>
    /// Sets the alias of the result of the function
    /// </summary>
    /// <param name="alias">the new alias</param>
    /// <returns>The current instance</returns>
    public NullFunction As(string alias)
    {
        _alias = alias;

        return this;
    }

    ///<inheritdoc/>
    public override bool Equals(object obj) => Equals(obj as NullFunction);

    ///<inheritdoc/>
    public bool Equals(NullFunction other) => (Column, DefaultValue).Equals((other?.Column, other?.DefaultValue));

    ///<inheritdoc/>
    public override int GetHashCode() => (Column, DefaultValue).GetHashCode();

    /// <summary>
    /// Performs a deep copy of the current instance.
    /// </summary>
    /// <returns><see cref="NullFunction"/></returns>
    public IColumn Clone() => new NullFunction(Column.Clone(), DefaultValue, AdditionalDefaultValues?.ToArray());

    ///<inheritdoc/>
    public override string ToString()
    {
        IDictionary<string, object> props = new Dictionary<string, object>
        {
            ["Function"] = nameof(NullFunction),
            [nameof(Column)] = Column,
            [nameof(DefaultValue)] = DefaultValue,
            [nameof(AdditionalDefaultValues)] = AdditionalDefaultValues.Where(val => val != null)
        };

        return props.Jsonify();
    }
}