使用lambda表达式动态设置比较运算符 [英] Set comparison operator dynamically using lambda expression
本文介绍了使用lambda表达式动态设置比较运算符的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
要在linq查询中动态设置一个比较运算符,请执行以下操作:
To set a comparison operator in linq query dynamically, i do the following:
parameter = Expression.Parameter(typeof(SomeType));
var predicate = Expression.Lambda<Func<SomeType, bool>>(
Combine(
"=",
Expression.Property(parameter, "ID"),
Expression.Constant(150497)
), parameter);
BinaryExpression Combine(string op, Expression left, Expression right)
{
switch (op)
{
case "=":
return Expression.Equal(left, right);
case "<":
return Expression.LessThan(left, right);
case ">":
return Expression.GreaterThan(left, right);
}
return null;
}
但是我宁愿将lambda表达式作为参数left替代。那可能吗?例如:
That works. But I'd rather pass a lambda expression as parameter "left" instead. Is that possible? Something like:
var predicate = Expression.Lambda<Func<SomeType, bool>>(Combine(
"=",
c => c.ID,
Expression.Constant(150497)
), parameter);
推荐答案
不幸的是,我现在不能测试,所以如果不行,请给我知道。
What about this? Unfortunately, I cannot test it now, so give me know if it doesn't work
private class TestCalss
{
public int Id { get; set; }
}
private class SwapVisitor : ExpressionVisitor
{
public readonly Expression _from;
public readonly Expression _to;
public SwapVisitor(Expression from, Expression to)
{
_from = from;
_to = to;
}
public override Expression Visit(Expression node) => node == _from ? _to : base.Visit(node);
}
private BinaryExpression Combine<T, TResult>(string op, Expression<Func<T, TResult>> left, Expression right, ParameterExpression parameter)
{
// Need to use parameter from outer lambda expression for equality two expressions
var swap = new SwapVisitor(left.Parameters[0], parameter);
var newLeft = swap.Visit(left) as Expression<Func<T, TResult>>;
switch (op)
{
case "=":
return Expression.Equal(newLeft.Body, right);
case "<":
return Expression.LessThan(newLeft.Body, right);
case ">":
return Expression.GreaterThan(newLeft.Body, right);
}
return null;
}
...
var parameter = Expression.Parameter(typeof(TestCalss));
var predicate = Expression.Lambda<Func<TestCalss, bool>>(
Combine<TestCalss, int>("=", c => c.Id, Expression.Constant(156), parameter),
parameter);
var test = new TestCalss { Id = 156 };
var result = predicate.Compile()(test); // <- true
这篇关于使用lambda表达式动态设置比较运算符的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文