C#条件拼接Expression＜Func＜T, bool＞＞的使⽤
使⽤代码
特别注意：拼接条件时，所使⽤到的条件strID，strBir必须是独⽴的
var predicate = PredicateBuilder.GetTrue<Student>();
predicate = predicate.And(it => it.id.ToString().Contains(strID));
predicate = predicate.And(it => it.Birthday.ToString().Contains(strBir));
predicate = predicate.And(it => it.Sex.ToString().Contains(strSex));
predicate = predicate.And(it => it.Age == 20);
var lst = db.Queryable<Student>.Where(predicate).ToList();
⼯具类
using System;
using System.Linq.Expressions;
namespace WindowsFormsApp1.Tools
{
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> GetTrue<T>() { return f => true; }
public static Expression<Func<T, bool>> GetFalse<T>() { return f => false; }
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
{
return first.AndAlso<T>(second, Expression.AndAlso);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
{
return first.AndAlso<T>(second, Expression.OrElse);
}
private static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2, Func<Expression, Expression, BinaryExpression> func)        {
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>>(
func(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;typeof的用法
return base.Visit(node);
}
}
}
}
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件拼接Expression＜Func＜T, bool＞＞内容请搜索以前的⽂章或继续浏览下⾯的相关⽂章希望⼤家以后多多⽀持！

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