转换前pression&LT; Func键&LT; FromType&GT;&GT;到防爆pression&LT; Func键&LT; ToType&GT;&GT; [英] Convert Expression<Func<FromType>> to Expression<Func<ToType>>
问题描述
我怎样才能让一个普通的辅助方法,转换使用的函数功能从一种类型到另一种内的防爆pression类型
How can I make a generic helper method to convert the type used by a Func from one type to another within an Expression
我有一个防爆pression&LT; Func键&LT; IEmployee,布尔&GT;&GT;
,我想将其转换为一个
I have a Expression<Func<IEmployee, bool>>
and I want to convert it to a
Expression<Func<Employee, bool>>.
第二类始终贯彻的第一个类型。一个通用的解决方案是什么,我想要的目的。
The second Type always implements the first Type. A generic solution is what I am trying to achieve.
修改
我已经编辑了问题再清楚不过了。
I have edited the question to be clearer.
推荐答案
好了,你可以创建一个前pression是蒙上,然后转发其参数原来的EX pression:
Well, you could create an expression that casts and then forwards its argument to the original expression:
Expression<Func<IEmployee, bool>> source = ...
var param = Expression.Parameter(typeof(Employee));
// Types the argument as the type expected by the source expression
// and then forwards it...
var invocationExpr = Expression.Invoke
(source, Expression.TypeAs(param, typeof(IEmployee)));
var result = Expression.Lambda<Func<Employee, bool>>(invocationExpr, param);
如果供应商不支持调用EX pressions,你可能需要一个更 成熟的解决方案,替代参数源EX pression。
If the provider doesn't support invocation expressions, you will probably need a much more sophisticated solution that replaces Parameters in the source expression.
编辑:好吧,既然你说你的供应商不喜欢所得到的前pression,这里的替代的一个例子。这是一个的真正的一个什么样的参数替代应该像粗剪(我只是写了这个,现在为样本),但它应该能正常运行你的目的。
Ok, since you say your provider doesn't like the resulting expression, here's an example of the alternative. It's a really rough cut of what a parameter-replacer should look like (I just wrote this now as a sample), but it should work fine for your purposes.
public static class ParameterReplacer
{
// Produces an expression identical to 'expression'
// except with 'source' parameter replaced with 'target' parameter.
public static Expression<TOutput> Replace<TInput, TOutput>
(Expression<TInput> expression,
ParameterExpression source,
ParameterExpression target)
{
return new ParameterReplacerVisitor<TOutput>(source, target)
.VisitAndConvert(expression);
}
private class ParameterReplacerVisitor<TOutput> : ExpressionVisitor
{
private ParameterExpression _source;
private ParameterExpression _target;
public ParameterReplacerVisitor
(ParameterExpression source, ParameterExpression target)
{
_source = source;
_target = target;
}
internal Expression<TOutput> VisitAndConvert<T>(Expression<T> root)
{
return (Expression<TOutput>)VisitLambda(root);
}
protected override Expression VisitLambda<T>(Expression<T> node)
{
// Leave all parameters alone except the one we want to replace.
var parameters = node.Parameters.Select
(p => p == _source ? _target : p);
return Expression.Lambda<TOutput>(Visit(node.Body), parameters);
}
protected override Expression VisitParameter(ParameterExpression node)
{
// Replace the source with the target, visit other params as usual.
return node == _source ? _target : base.VisitParameter(node);
}
}
}
,然后用它为:
And then use it as:
Expression<Func<IEmployee, bool>> expression = ...
var result = ParameterReplacer.Replace
<Func<IEmployee, bool>, Func<Employee, bool>>
(expression,
expression.Parameters.Single(),
Expression.Parameter(typeof(Employee));
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