lambda 表达式拼接

static void Main(string[] args)
{
//模拟数据源
List<Preson> l = new List<Preson>();
for (int i = 1; i <= 100; i++)
{

Preson p = new Preson();
p.id = i;
p.age = i;
l.Add(p);
}
//定义表达式
Expression<Func<Preson, bool>> ex = t => true;
ex = ex.And(t => t.id > 30);
ex = ex.And(t => t.age > 25);
ex = ex.And(t => t.age < 35);

//查询 注意:AsQueryable
var rs = l.AsQueryable().Where(ex).ToList(); ;
foreach (Preson p in rs)
{
Console.WriteLine(p.id);
}
Console.Read();
}

 

 

 

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);
}
}
public static class LambdaHelper
{
public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
{
var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
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 Preson
{
public int id { get; set; }
public int age { get; set; }
}