来源: Lambda 表达式查询条件拼接 – 鞍山老菜鸟 – 博客园
public static class PredicateBuilder
{
/// <summary> /// 可以理解为创建一个初始化为True的Predicate。 /// 注意:如果你是要创建一个OR组成的Predicate就不能把它初始化为True因为这样这个表达试永远为True了。 /// </summary> /// <typeparam name="T"></typeparam> /// <returns></returns>
public static Expression<Func<T, bool>> True<T>() { return f => true; }
/// <summary> /// 可以理解为创建一个初始化为False的Predicate。 ///注意:如果你是要创建一个AND组成的Predicate就不能把它初始化为False因为这样这个表达试永远为False了。 /// </summary> /// <typeparam name="T"></typeparam> /// <returns></returns>
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);
}
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);
}
}
}
调用方法
/*
Lambda 表达式查询条件拼接
调用实例
var predicate = PredicateBuilder.True<Orders>();
if (!string.IsNullOrWhiteSpace(select.so_text))
{
predicate = predicate.And(d => d.price > 10);
predicate = predicate.And(d => d.price < 12);
}
var list = db.Orders.Where(predicate).ToList();
全And: var where = PredicateBuilder.True<int>(); where = where.And(x => x >= 50); where = where.And(x => x <= 70); var res = list.Where(where.Compile());
全Or: var list = Enumerable.Range(1, 100); var where = PredicateBuilder.False<int>(); where = where.Or(x => x == 50); where = where.Or(x => x == 70); var res = list.Where(where.Compile());
var list = Enumerable.Range(1, 100); var where = PredicateBuilder.True<int>(); where = where.And(x => x >= 50); where = where.And(x => x <= 70); var subwhere = PredicateBuilder.False<int>(); subwhere = subwhere.Or(x => x == 60); subwhere = subwhere.Or(x => x == 61); where = where.And(subwhere); var res = list.Where(where.Compile());
*/