C# EF Core中如何使用WhereIf

只需要在项目中添加以下文件即可：

using System.Linq.Expressions;

namespace System.Linq
{

  public static class LinqHelper
  {

    public static IQueryable<T> WhereIf<T>(this IQueryable<T> source, Expression<Func<T, bool>> predicate, bool condition)
    {
      return condition ? source.Where(predicate) : source;
    }

    public static IQueryable<T> WhereIf<T>(this IQueryable<T> source, Expression<Func<T, int, bool>> predicate, bool condition)
    {
      return condition ? source.Where(predicate) : source;
    }

    public static IEnumerable<T> WhereIf<T>(this IEnumerable<T> source, Func<T, bool> predicate, bool condition)
    {
      return condition ? source.Where(predicate) : source;
    }

    public static IEnumerable<T> WhereIf<T>(this IEnumerable<T> source, Func<T, int, bool> predicate, bool condition)
    {
      return condition ? source.Where(predicate) : source;
    }

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
      HashSet<TKey> seenKeys = new HashSet<TKey>();
      foreach (TSource element in source)
      {
        if (seenKeys.Add(keySelector(element)))
        {
          yield return element;
        }
      }
    }
  }
}