想要将 c# 多个 Linq 转换为一个 Linq 查询？

Question

我编写了包含多个 if 条件的代码，并从每个 if 语句中获取了 Id 列表。如果可能的话，我想将整个代码写入一个 linq 查询，然后告诉我。

问题是当我们运行这段代码时，可以看到我们的编译器在输出窗口上运行了数百行查询。我只想优化。如果你们能帮助我。 详细代码如下：

if (queryObj.AgeCategory.Contains((int)AgeList._0to5))
{
    var list = queryRecord.Where(v => v.BuildYear <= yearNow && v.BuildYear > year5).Select(s => s.Id);
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._6to10))
{
    var list = queryRecord.Where(v => v.BuildYear <= year5 && v.BuildYear > year10).Select(s => s.Id);
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._11to15))
{
    var list = queryRecord.Where(v => v.BuildYear <= year10 && v.BuildYear > year15).Select(s => s.Id);
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._16to20))
{
    var list = Ids.Concat(queryRecord.Where(v => v.BuildYear <= year15 && v.BuildYear > year20).Select(s => s.Id));
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._21to25))
{
    var list = Ids.Concat(queryRecord.Where(v => v.BuildYear <= year20 && v.BuildYear > year25).Select(s => s.Id));
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._26to30))
{
    var list = Ids.Concat(queryRecord.Where(v => v.BuildYear <= year25 && v.BuildYear > year30).Select(s => s.Id));
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._31to35))
{
    var list = Ids.Concat(queryRecord.Where(v => v.BuildYear <= year30 && v.BuildYear > year35).Select(s => s.Id));
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._36to40))
{
    var list = Ids.Concat(queryRecord.Where(v => v.BuildYear <= year35 && v.BuildYear > year40).Select(s => s.Id));
    Ids = Ids.Concat(list);
}
if (queryObj.AgeCategory.Contains((int)AgeList._Over41Years))
{
    var list = Ids.Concat(queryRecord.Where(v => v.BuildYear <= year40).Select(s => s.Id));
    Ids = Ids.Concat(list);
}
queryRecord = queryRecord.Where(v => Ids.Contains(v.Id));

我使用以下代码进行了测试，但由于同一列它满足第一个条件。

                    Ids = queryRecord.Where(st => ((queryObj.AgeCategory.Contains((int)AgeList._11to15)) ? st.BuildYear <= year10 && st.BuildYear > year15 : true) &&
                        ((queryObj.AgeCategory.Contains((int)AgeList._6to10)) ? st.BuildYear <= year5 && st.BuildYear > year10 : true))
                        .Select(s => s.Id);

Answer 1

让我们从 Marc 的 AndAlso 方法从 this linked answer 开始，将 AndAlso 更改为 OrElse：

public static Expression<Func<T, bool>> OrElse<T>(
    this Expression<Func<T, bool>> expr1,
    Expression<Func<T, bool>> expr2)
{
    var parameter = Expression.Parameter(typeof(T));

    var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
    var left = leftVisitor.Visit(expr1.Body);

    var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
    var right = rightVisitor.Visit(expr2.Body);

    return Expression.Lambda<Func<T, bool>>(
        Expression.OrElse(left, right), parameter);
}

private class ReplaceExpressionVisitor
    : ExpressionVisitor
{
    private readonly Expression _oldValue;
    private readonly Expression _newValue;

    public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
    {
        _oldValue = oldValue;
        _newValue = newValue;
    }

    public override Expression Visit(Expression node)
    {
        if (node == _oldValue)
            return _newValue;
        return base.Visit(node);
    }
}

我们可以这样写：

Expression<Func<Whatever, bool>> query = t => false;

if (queryObj.AgeCategory.Contains((int)AgeList._0to5))
{
    query = query.OrElse(v => v.BuildYear <= yearNow && v.BuildYear > year5);
}
if (queryObj.AgeCategory.Contains((int)AgeList._6to10))
{
    query = query.OrElse(v => v.BuildYear <= year5 && v.BuildYear > year10);
}
... and so on...

queryRecord = whatever.Where(query);

这会生成一个查询，如下所示：

WHERE (BuildYear <= yearNow AND BuildYear > year5) OR (BuildYear < year5 AND BuildYear > year10) etc

Answer 2

假设您所指的AgeList 是这样的枚举：

public enum AgeList
{
    _0to5   = 0,
    _6to10  = 1,
    _11to15 = 2,
    _16to20 = 3,
    _21to25 = 4,
    _26to30 = 5,
    _31to35 = 6,
    _36to40 = 7,
}

我认为您想将BuildYears 映射到上述其中之一，我们可以观察到它们成对出现。 所以我们可以这样做：

var yearNow = 2020;
var buildYear = 2019;

var ageGroupNumber = (yearNow - buildYear) / 5; // 0
var ageGroup = (AgeList) ageGroupNumber;        // _0to5 

那么我们可以做如下的事情吗？

Ids = queryObj.AgeCategory
    .SelectMany(category => // For every category :
        queryRecord.Where(v => // Select the items in this category :
            (DateTime.Now.Year - v.BuildYear) / 5 == (int) category 
        )
        .Select(s => s.Id)
    );

我不知道这是否会对您有所帮助，或者我是否正确理解了您的问题，但也许这会 给你一些想法。

Answer 3

另一种解决方案是创建一个转换器，将BuildAge 转换为年龄类别。这样您就可以简单地让 LINQ 进行过滤：

private void FilterQueryRecord(IEnumerable<Record> queryRecord, ? queryObj)
{
  this.Ids = queryRecord
    .Where(item => queryObj.AgeCategory.Contains(ConvertBuildYearToAgeCategory(item.BuildYear)))
    .Select(item => item.Id)
    .ToList();
}

private int ConvertBuildYearToAgeCategory(int buildYear) 
{
  switch (buildYear)
  {
    case int value when value <= yearNow && value > year5: return (int) AgeList._0to5;
    case int value when value <= year5 && value > year10: return (int) AgeList._6to10;
    case int value when value <= year10 && value > year15: return (int) AgeList._11to15;
    case int value when value <= year15 && value > year20: return (int) AgeList._16to20;
    case int value when value <= year20 && value > year25: return (int) AgeList._21to25;
    case int value when value <= year25 && value > year30: return (int) AgeList._26to30;
    case int value when value <= year30 && value > year35: return (int) AgeList._31to35;
    case int value when value <= year35 && value > year40: return (int) AgeList._36to40;
    case int value when value < year40: return (int) AgeList._Over41Years;
    default: return (int) AgeList.Undefined
  }
}

Answer 4

我只是在每种情况下都添加了 false 并添加 ||在每个语句之间并修复此问题。

Ids = queryRecord.Where(st => 
                    ((queryObj.AgeCategory.Contains((int)AgeList._11to15)) ? st.BuildYear <= year10 && st.BuildYear > year15 : false) ||
                    ((queryObj.AgeCategory.Contains((int)AgeList._6to10)) ? st.BuildYear <= year5 && st.BuildYear > year10 : false)
                    ).Select(s => s.Id);