问题描述
我正在尝试使用 LINQ 链接多个值或在循环中。
情况
平台：.net 5 C# 9
我们正在为列表构建过滤器逻辑。在当前情况下，它涉及要过滤的字符串值。
用户可以搜索一个或多个值。他可以决定单个搜索词是否为 AND/OR 链接，以及某个值是否为否定。
我看到了这个条目。但由于我的值在循环中，我不能使用 ||。
https://stackoverflow.com/a/37195788/1847143
示例：
- 名称中带有“A”的所有动物
SELECT * FROM "Animal" WHERE "Name" = 'A'; - 名称中带有“A”或“B”的所有动物
SELECT * FROM "Animal" WHERE "Name" = 'A' OR "Name" = 'B'; - 名称中包含“A”或“B”或不包含“C”的所有动物（这将是无意义的搜索）
SELECT * FROM "Animal" WHERE "Name" = 'A' OR "Name" = 'B' OR "Name" != 'C' ; - 名称中带有“A”和“B”的所有动物
SELECT * FROM "Animal" WHERE "Name" = 'A' AND "Name" = 'B'; - 名称中带有“A”和“B”而不是“C”的所有动物
SELECT * FROM "Animal" WHERE "Name" = 'A' AND "Name" = 'B' AND "Name" != 'C';
问题
与 LINQ 的 AND 链接没有问题。但是如何将这些值与 OR 联系起来？
代码示例
using System.Collections.Generic;
using System.Linq;
namespace SampleProject
{
public class Program
{
public static void Main(string[] args)
{
// Or Condtion
Condition condition = Condition.Or;
var animalsQuery = Animals.AsQueryable();
// Loop over all search values to extend the query
foreach (FilterValue filterValue in FilterValues)
{
switch (filterValue.LikeType)
{
case LikeType.Left: // LIKE '%value'
animalsQuery = filterValue.IsNegated
? animalsQuery.Where(animal => !animal.Name.EndsWith(filterValue.Value))
: animalsQuery.Where(animal => animal.Name.EndsWith(filterValue.Value));
break;
case LikeType.Right: // LIKE 'value%'
animalsQuery = filterValue.IsNegated
? animalsQuery.Where(animal => !animal.Name.StartsWith(filterValue.Value))
: animalsQuery.Where(animal => animal.Name.StartsWith(filterValue.Value));
break;
case LikeType.LeftAndRight: // LIKE '%value%'
animalsQuery = filterValue.IsNegated
? animalsQuery.Where(animal => !animal.Name.Contains(filterValue.Value))
: animalsQuery.Where(animal => animal.Name.Contains(filterValue.Value));
break;
case LikeType.Equals: // Like 'value'
animalsQuery = filterValue.IsNegated
? animalsQuery.Where(animal => animal.Name != filterValue.Value)
: animalsQuery.Where(animal => animal.Name == filterValue.Value);
break;
}
}
var result = animalsQuery.ToList();
}
/// Values to filter
public static List<Animal> Animals = new()
{
new() {Name = "Lenny"},new() {Name = "Gideon"},new() {Name = "Shania"},new() {Name = "Jada"},new() {Name = "Kamil"},new() {Name = "Fariha"},};
/// Search Values
public static List<FilterValue> FilterValues = new()
{
new() {Value = "a",LikeType = LikeType.Left},new() {Value = "n",LikeType = LikeType.Right},new() {Value = "f",LikeType = LikeType.LeftAndRight},new() {Value = "k",LikeType = LikeType.Equals},};
}
public class Animal
{
public string Name { get; set; }
}
public class FilterValue
{
public string Value { get; set; }
public bool IsNegated { get; set; }
public LikeType LikeType { get; set; }
}
public enum LikeType
{
Left = 1,Right = 2,LeftAndRight = 3,Equals = 4,}
public enum Condition
{
And = 1,Or = 2,}
}
解决方法
这进入了表达式树重写的领域。好消息是：它并不复杂而且您可以同时执行您的否定步骤：
using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
using System.Linq.Expressions;
namespace SampleProject
{
public class Program
{
static Expression<Func<T,bool>> Combine<T>(Condition condition,Expression<Func<T,bool>> left,bool>> right,bool negateRight)
{
if (right is null) return left;
if (left is null)
{
return negateRight ?
Expression.Lambda<Func<T,bool>>(
Expression.Not(right.Body),right.Parameters)
: right;
}
var leftP = left.Parameters.Single();
var rightP = right.Parameters.Single();
var rightBody = right.Body;
if (!ReferenceEquals(leftP,rightP))
{
// swap all uses of rightP on rightBody to leftP
// i.e. normalize on the parameter
rightBody = new SwapVisitor(rightP,leftP).Visit(rightBody);
}
if (negateRight)
{
rightBody = Expression.Not(rightBody);
}
return Expression.Lambda<Func<T,bool>>(condition switch
{
Condition.And => Expression.AndAlso(left.Body,rightBody),Condition.Or => Expression.OrElse(left.Body,_ => throw new ArgumentOutOfRangeException(nameof(condition)),},left.Parameters);
}
class SwapVisitor : ExpressionVisitor
{
private readonly Expression _from,_to;
public SwapVisitor(Expression from,Expression to)
{
_from = from;
_to = to;
}
public override Expression Visit(Expression node)
=> ReferenceEquals(node,_from) ? _to : base.Visit(node);
}
public static void Main(string[] args)
{
// Or Condtion
Condition condition = Condition.Or;
var animalsQuery = Animals.AsQueryable();
// Loop over all search values to extend the query
Expression<Func<Animal,bool>> predicate = null;
foreach (FilterValue filterValue in FilterValues)
{
switch (filterValue.LikeType)
{
case LikeType.Left: // LIKE '%value'
predicate = Combine(condition,predicate,animal => animal.Name.EndsWith(filterValue.Value),filterValue.IsNegated);
break;
case LikeType.Right: // LIKE 'value%'
predicate = Combine(condition,animal => animal.Name.StartsWith(filterValue.Value),filterValue.IsNegated);
break;
case LikeType.LeftAndRight: // LIKE '%value%'
predicate = Combine(condition,animal => animal.Name.Contains(filterValue.Value),filterValue.IsNegated);
break;
case LikeType.Equals: // Like 'value'
predicate = Combine(condition,animal => animal.Name == filterValue.Value,filterValue.IsNegated);
break;
}
}
if (predicate is not null)
{
animalsQuery = animalsQuery.Where(predicate);
}
var result = animalsQuery.ToList();
}
/// Values to filter
public static List<Animal> Animals = new()
{
new() { Name = "Lenny" },new() { Name = "Gideon" },new() { Name = "Shania" },new() { Name = "Jada" },new() { Name = "Kamil" },new() { Name = "Fariha" },};
/// Search Values
public static List<FilterValue> FilterValues = new()
{
new() { Value = "a",LikeType = LikeType.Left },new() { Value = "n",LikeType = LikeType.Right },new() { Value = "f",LikeType = LikeType.LeftAndRight },new() { Value = "k",LikeType = LikeType.Equals },};
}
public class Animal
{
public string Name { get; set; }
}
public class FilterValue
{
public string Value { get; set; }
public bool IsNegated { get; set; }
public LikeType LikeType { get; set; }
}
public enum LikeType
{
Left = 1,Right = 2,LeftAndRight = 3,Equals = 4,}
public enum Condition
{
And = 1,Or = 2,}
}