C#条件拼接Expression<Func<T, bool>>
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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; return base.Visit(node); } } } }
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