Linq-创建表达式< T1>来自表达式< T2> [英] Linq - Creating Expression<T1> from Expression<T2>

问题描述

我有一个谓词Expression<Func<T1, bool>>

我需要使用T2的T1属性将其用作谓词Expression<Func<T2, bool>>. >

供参考:

class T2 {
  public T1 T1;
}

还有

Expression<Func<T1, bool>> ConvertPredicates(Expression<Func<T2, bool>> predicate) {
  //what to do here...
}

非常感谢.

解决方案

在考虑表达式树之前，请尝试查找具有正常lambda的解决方案.

您有一个谓词

Func<T1, bool> p1

并想要一个谓词

Func<T2, bool> p2 = (x => p1(x.T1));

您可以将其构建为表达式树，如下所示:

Expression<Func<T2, bool>> Convert(Expression<Func<T1, bool>> predicate)
{
    var x = Expression.Parameter(typeof(T2), "x");
    return Expression.Lambda<Func<T2, bool>>(
        Expression.Invoke(predicate, Expression.PropertyOrField(x, "T1")), x);
}

I have a predicate Expression<Func<T1, bool>>

I need to use it as a predicate Expression<Func<T2, bool>> using the T1 property of T2 I was trying to think about several approches, probably using Expression.Invoke but couln;t get my head around it.

For reference:

class T2 {
  public T1 T1;
}

And

Expression<Func<T1, bool>> ConvertPredicates(Expression<Func<T2, bool>> predicate) {
  //what to do here...
}

Thanks a lot in advance.

解决方案

Try to find the solution with normal lambdas before you think about expression trees.

You have a predicate

Func<T1, bool> p1

and want a predicate

Func<T2, bool> p2 = (x => p1(x.T1));

You can build this as an expression tree as follows:

Expression<Func<T2, bool>> Convert(Expression<Func<T1, bool>> predicate)
{
    var x = Expression.Parameter(typeof(T2), "x");
    return Expression.Lambda<Func<T2, bool>>(
        Expression.Invoke(predicate, Expression.PropertyOrField(x, "T1")), x);
}

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