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();活期利息怎么算
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>> GetFal<T>() { return f => fal; }
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> cond)
{
return first.AndAlso<T>(cond, Expression.AndAlso);
800资源}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> cond)
{
return first.AndAlso<T>(cond, Expression.OrEl);
}
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);面试通知
var right = rightVisitor.Visit(expr2.Body);
黄华溢return Expression.Lambda<Func<T, bool>>(
func(left, right), parameter);
}
private class ReplaceExpressionVisitor : ExpressionVisitor
{
演良private readonly Expression _oldValue;
private readonly Expression _newValue;孩子祝福语
public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
{
_oldValue = oldValue;
_newValue = newValue;
}
public override Expression Visit(Expression node)
{
if (node == _oldValue)
return _newValue;
return ba.Visit(node);
}
}
}
}
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