private void SimpleLambda()
{
dynamic showMessage = x => MessageBox.Show(x);
showMessage("Hello World!");
}
错误消息为: 无法将 lambda 表达式转换为动态类型,因为它不是委托类型。
需要帮助,
private void SimpleLambda()
{
dynamic showMessage = x => MessageBox.Show(x);
showMessage("Hello World!");
}
这与MessageBox
无关 - 就像错误消息所说的那样,您不能将Lambda表达式转换为dynamic
,因为编译器不知道要创建哪种委托类型的实例。
您想要:
Action<string> action = x => MessageBox.Show(x);
甚至可以使用方法组转换,但这样你必须匹配返回类型:
Func<string, DialogResult> func = MessageBox.Show;
如果你想的话,你可以使用 dynamic
:
dynamic showMessage = action; // Or func
showMessage("Hello World!");
或者,您可以在显式委托实例表达式中指定lambda表达式:
dynamic showMessage = new Action<string>(x => MessageBox.Show(x));
private void SimpleLambda()
{
Action<string> showMessage = x => MessageBox.Show(x);
showMessage("Hello World!");
}
你需要声明委托类型,否则它不知道这个Lambda表达式应该是什么类型——x
可以是任何东西。这应该可以解决问题:
Action<string> showMessage = x => MessageBox.Show(x);
Action<T>
以了解此委托类型的详细信息。var fn = Func.F( (string x) => MessageBox.Show(x) );
或者
var fn = Func.F( (double x, double y) => x + y );
using System;
namespace System
{
/// <summary>
/// Make type inference in C# work harder for you. Normally when
/// you want to declare an inline function you have to type
///
/// Func<double, double, double> fn = (a,b)=>a+b
///
/// which sux! With the below methods we can write
///
/// var fn = Func.F((double a, double b)=>a+b);
///
/// which is a little better. Not as good as F# type
/// inference as you still have to declare the args
/// of the function but not the return value which
/// is sometimes not obvious straight up. Ideally
/// C# would provide us with a keyword fun used like
///
/// fun fn = (double a, double b)=>a+b;
///
/// but till then this snippet will make it easier
///
/// </summary>
public static class Func
{
public static Func<A> F<A>(Func<A> f)
{
return f;
}
public static Func<A,B> F<A, B>(Func<A, B> f)
{
return f;
}
public static Func<A,B,C> F<A, B,C>(Func<A, B,C> f)
{
return f;
}
public static Func<A,B,C,D> F<A,B,C,D>(Func<A,B,C,D> f)
{
return f;
}
public static Func<A,B,C,D,E> F<A,B,C,D,E>(Func<A,B,C,D,E> f)
{
return f;
}
public static Action A(Action f)
{
return f;
}
public static Action<_A> A<_A>(Action<_A> f)
{
return f;
}
public static Action<_A,B> A<_A, B>(Action<_A, B> f)
{
return f;
}
public static Action<_A,B,C> A<_A, B,C>(Action<_A, B,C> f)
{
return f;
}
public static Action<_A,B,C,D> A<_A,B,C,D>(Action<_A,B,C,D> f)
{
return f;
}
public static Action<_A,B,C,D,E> A<_A,B,C,D,E>(Action<_A,B,C,D,E> f)
{
return f;
}
}
}
Action<string>
或者Func<string, DialogResult>
- Mario Sdynamic
视为一种高级功能,在正常代码中应避免使用。 - Brian