ExpressionHelper.cs
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using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;
namespace Yi.Framework.Core.Helper
{
public static class ExpressionHelper
{
/// <summary>
/// Expression表达式树lambda参数拼接组合
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="first"></param>
/// <param name="second"></param>
/// <param name="merge"></param>
/// <returns></returns>
public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
{
var parameterMap = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
var secondBody = LambdaParameteRebinder.ReplaceParameter(parameterMap, second.Body);
return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
}
/// <summary>
/// Expression表达式树lambda参数拼接--false
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static Expression<Func<T, bool>> False<T>() => f => false;
/// <summary>
/// Expression表达式树lambda参数拼接-true
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static Expression<Func<T, bool>> True<T>() => f => true;
/// <summary>
/// Expression表达式树lambda参数拼接--and
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="first"></param>
/// <param name="second"></param>
/// <returns></returns>
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) => first.Compose(second, Expression.And);
/// <summary>
/// Expression表达式树lambda参数拼接--or
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="first"></param>
/// <param name="second"></param>
/// <returns></returns>
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) => first.Compose(second, Expression.Or);
}
public class LambdaParameteRebinder : ExpressionVisitor
{
/// <summary>
/// 存放表达式树的参数的字典
/// </summary>
private readonly Dictionary<ParameterExpression, ParameterExpression> map;
/// <summary>
/// 构造函数
/// </summary>
/// <param name="map"></param>
public LambdaParameteRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
{
this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
}
/// <summary>
/// 重载参数访问的方法，访问表达式树参数，如果字典中包含，则取出
/// </summary>
/// <param name="node">表达式树参数</param>
/// <returns></returns>
protected override Expression VisitParameter(ParameterExpression node)
{
if (map.TryGetValue(node, out ParameterExpression expression))
{
node = expression;
}
return base.VisitParameter(node);
}
public static Expression ReplaceParameter(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
{
return new LambdaParameteRebinder(map).Visit(exp);
}
}
}