想要将 c# 多个 Linq 转换为一个 Linq 查询?

Question

我用多个if条件编写了代码，并从每个if语句中获取ID列表。如果可能的话，我想将整个代码写入一个 linq 查询中，然后让我知道。

问题是当我们运行此代码时，可以看到我们的编译器在输出窗口上运行数百行查询。我只想优化。如果你们能帮我。 以下代码了解详细信息：

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

让我们从这个链接答案中的 MarcAndAlso方法开始，并将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);
}
}

这让我们可以写：

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);

这将生成一个查询，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,
}

我想你想把BuildYear映射到上面的一个，我们可以观察到它们以五分之一的形式出现。 所以我们可以做：

var yearNow = 2020;
var buildYear = 2019;
var ageGroupNumber = (yearNow - buildYear) / 5; // 0
var ageGroup = (AgeList) ageGroupNumber;        // _0to5 

那么我们可以做类似的事情吗？

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，您只需让 LINQ 执行筛选即可：

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，并在每个语句之间添加了 || 并修复了这个问题。

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);