通过LINQ表达式树动态构建查询条件

第一种方法：

public static class PredicateExtensions
    {
        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<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1,
           Expression<Func<T, bool>> expression2)
        {
            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters
                    .Cast<Expression>());
 
            return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression),
            expression1.Parameters);
        }
 
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1,
              Expression<Func<T, bool>> expression2)
        {
            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters
                 .Cast<Expression>());
 
            return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body,
                   invokedExpression), expression1.Parameters);
        }
    }

第二种方法：

    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);
        }
    }

注意，经我实际应用，发现第一种方法PredicateExtensions类有个缺陷，就是仅适合在DbDataContext中使用，若用在EntityContext中，则会报错：LINQ to Entities 不支持 LINQ 表达式节点类型“Invoke”。而使用第二种方法则不会有这个问题，至于问题的根源我还没有找到原因，还望高手指教，谢谢！