Lambda表达式关于or的写法

常见的lambda表达式都是这么写的

var query= ThreadCache.CourseSelectionDbContext.Set<SelectionCondition>().Where(s => s.SelectionClassId == selectionClassId);

如果想附加条件，都是在后面接着添加where

var query= query.Where(s => s.ConditionType == (int)ScoreConditionTypeEnum.SingleSubject);

where解释为sql语句的时候，会解释为多个条件并且的关系。但是，总有时候，我们需要多个条件或的关系，那么或怎么写呢？

在这里提供一个写lambda表达式的基础类，通过这个基础类可以任意使用And和Or的表达式

public static class PredicateBuilder
{
        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        public static Expression<Func<T, bool>> False<T>() { return f => false; }
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression 
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.And);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.Or);
        }
}

    public class ParameterRebinder : ExpressionVisitor
    {
        private readonly Dictionary<ParameterExpression, ParameterExpression> map;

        public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
        {
            this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
        }

        public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
        {
            return new ParameterRebinder(map).Visit(exp);
        }

        protected override Expression VisitParameter(ParameterExpression p)
        {
            ParameterExpression replacement;
            if (map.TryGetValue(p, out replacement))
            {
                p = replacement;
            }
            return base.VisitParameter(p);
        }
    }

 

使用方式如下：

　　//定义总的lambda树
　　var lambdaTree = PredicateBuilder.True<ScoreInfo>();
   lambdaTree = lambdaTree.And(s => s.SelectionInfoId == selectionInfoId && studentIdList.Contains(s.UserId));  
　　//或lambda树  
   var orLambdaTree = PredicateBuilder.False<ScoreInfo>();
   foreach (var item in subjectScoreList)
   {
       orLambdaTree = orLambdaTree.Or(s => (s.SubjectId == item.SubjectId && s.Score >= item.Score));
   }
    //将或lambda树并入总的      
   lambdaTree = lambdaTree.And(orLambdaTree);
   //执行查询      
   int count = ThreadCache.XYY_CourseSelectionEntities.ScoreInfo.Where(lambdaTree).GroupBy(s => s.UserId).Where(s => s.Count() == subjectScoreList.Count).Count();            

这样就可以实现任意条件或的使用了。