c# - 有没有办法可以延迟LINQ语句中的.Where子句？

Question

我在这里有这段代码。我认为这不是很有效，我想知道是否有一种方法可以使.Where子句在开头或结尾处添加，而不是重复三遍。如果有人可以给我任何建议并展示如何做到这一点，我将不胜感激：

      List<Phrase> phrases;

        switch (options.PhraseTypeSelectId)
        {
            case 0:
                phrases = await db.Phrases
                .Where(w => ((w.CategoryId == options.CategorySelectId || options.CategorySelectId == null) ||
                             (w.ChapterId == options.ChapterSelectId || options.ChapterSelectId == null)))
                .Where(w => (w.EnglishAscii >= es1 && w.EnglishAscii <= ee1) || (w.EnglishAscii >= es2 && w.EnglishAscii <= ee2))
                .Where(w => (w.RomajiAscii >= rs1 && w.RomajiAscii <= re1) || (w.RomajiAscii >= rs2 && w.RomajiAscii <= ee2))
                .Where(w => (options.CreatedBy == 0 || w.CreatedBy == options.CreatedBy))
                .Where(w => (options.ModifiedBy == 0 || w.ModifiedBy == options.ModifiedBy))
                .Where(w => (options.JLPT == 0 || w.JLPT == options.JLPT))
                .AsNoTracking()
                .ToListAsync();
                return Ok(phrases);
            case 1:
                phrases = await db.Phrases
                .Where(w => ((w.CategoryId == options.CategorySelectId || options.CategorySelectId == null)))
                .Where(w => (w.EnglishAscii >= es1 && w.EnglishAscii <= ee1) || (w.EnglishAscii >= es2 && w.EnglishAscii <= ee2))
                .Where(w => (w.RomajiAscii >= rs1 && w.RomajiAscii <= re1) || (w.RomajiAscii >= rs2 && w.RomajiAscii <= ee2))
                .Where(w => (options.CreatedBy == 0 || w.CreatedBy == options.CreatedBy))
                .Where(w => (options.ModifiedBy == 0 || w.ModifiedBy == options.ModifiedBy))
                .Where(w => (options.JLPT == 0 || w.JLPT == options.JLPT))
                .AsNoTracking()
                .ToListAsync();
                return Ok(phrases);
            case 2:
                phrases = await db.Phrases
                .Where(w => ((w.ChapterId == options.ChapterSelectId || options.ChapterSelectId == null)))
                .Where(w => (w.EnglishAscii >= es1 && w.EnglishAscii <= ee1) || (w.EnglishAscii >= es2 && w.EnglishAscii <= ee2))
                .Where(w => (w.RomajiAscii >= rs1 && w.RomajiAscii <= re1) || (w.RomajiAscii >= rs2 && w.RomajiAscii <= ee2))
                .Where(w => (options.CreatedBy == 0 || w.CreatedBy == options.CreatedBy))
                .Where(w => (options.ModifiedBy == 0 || w.ModifiedBy == options.ModifiedBy))
                .Where(w => (options.JLPT == 0 || w.JLPT == options.JLPT))
                .AsNoTracking()
                .ToListAsync();
                return Ok(phrases);
        }
        return BadRequest();

我想在没有该语句的情况下包括switch语句，它与与PhraseTypeSelectId相关的.Where子句变得非常混乱

Answer 1

由于我无法编译该示例，因此我不确定100％是否可以编译该示例，但是您应该了解它的要旨。

您可以为不同的PhraseTypeSelecteId创建特定的表达式，并稍后在.Where()方法中使用它。

Expression<Func<Phrase, bool>> phraseTypePredicate = null;

switch (options.PhraseTypeSelectId)
{
    case 0:
        phraseTypePredicate =
            w => ((w.CategoryId == options.CategorySelectId || options.CategorySelectId == null) ||
                  (w.ChapterId == options.ChapterSelectId || options.ChapterSelectId == null));
        break;
    case 1:
        phraseTypePredicate =
            w => ((w.CategoryId == options.CategorySelectId || options.CategorySelectId == null));
        break;
    case 2:
        phraseTypePredicate =
            w => ((w.ChapterId == options.ChapterSelectId || options.ChapterSelectId == null));
        break;
}

if (phraseTypePredicate != null)
{
    List<Phrase> phrases = phrases = await db.Phrases
        .Where(phraseTypePredicate)
        .Where(w => (w.EnglishAscii >= es1 && w.EnglishAscii <= ee1) || (w.EnglishAscii >= es2 && w.EnglishAscii <= ee2))
        .Where(w => (w.RomajiAscii >= rs1 && w.RomajiAscii <= re1) || (w.RomajiAscii >= rs2 && w.RomajiAscii <= ee2))
        .Where(w => (options.CreatedBy == 0 || w.CreatedBy == options.CreatedBy))
        .Where(w => (options.ModifiedBy == 0 || w.ModifiedBy == options.ModifiedBy))
        .Where(w => (options.JLPT == 0 || w.JLPT == options.JLPT))
        .AsNoTracking()
        .ToListAsync();
    return Ok(phrases);
}

return BadRequest();