Question

只是出于好奇，我想知道是否可以将Lambda Expression用作String参数？

这是我的snipet：

List<Int64> selectedUsers = GetSelectedUsers();
if (MessageBox.Show(this, ((String message) =>
{
    message = "Are you sure you want to delete user(s) ID";
    foreach (Int64 id in selectedUsers)
    {
        message += " " + id.ToString();
    }
    message += "?";
    return message;
}), "Confirmation Delete User", MessageBoxButtons.YesNo, MessageBoxIcon.Warning) == DialogResult.Yes)
{ 
    //DoSomething();
}

但遗憾的是，我收到此错误“错误7无法将lambda表达式转换为类型'字符串'，因为它不是委托类型”这里:(字符串消息）

感谢您的帮助，非常感谢！

Answer 1

在

之前构建您的消息

class Base
{ 
    public : 
        Base() { /*...*/}
        Base(const Base& other){/*...*/}
        virtual ~Base() {/*...*/}  
    private:
        /*...*/
};

class Derived : public Base
{ 
    public : 
        Derived() {/*...*/} 
        ~Derived() {/*...*/} 
        Derived(const Base& other) : Base(other){/*...*/}

};

int main()
{
    Base b;
    Derived d(b);
}

Answer 2

如果真的想要使用某种代理来返回一个字符串（我老实说你不确定为什么会这样），你需要将表达式强制转换为Func<string>并立即调用它。

if (MessageBox.Show(this, 
    ((Func<string>)(() =>
        {
            var message = "Are you sure you want to delete user(s) ID";
            foreach (Int64 id in selectedUsers)
            {
                message += " " + id.ToString();
            }
            message += "?";
            return message;
       })
    )(), 
    "Confirmation Delete User", MessageBoxButtons.YesNo, MessageBoxIcon.Warning) == DialogResult.Yes)
    { 
        //DoSomething();
    }
}

Answer 3

我认为这就是你应该做的事情

List<Int64> selectedUsers = GetSelectedUsers();

            if (MessageBox.Show(this, ((Func<string, string>)(message =>
            {
                message = "Are you sure you want to delete user(s) ID";
                foreach (Int64 id in selectedUsers)
                {
                    message += " " + id.ToString();
                }
                message += "?";
                return message;
            }))(""), "Confirmation Delete User", MessageBoxButtons.YesNo, MessageBoxIcon.Warning) == DialogResult.Yes)
            {
                //DoSomething();
            }
        }

我们这里有一个IFFE。作为优化，你可以摆脱消息参数，因为你并没有真正使用初始值。这将成为：

List<Int64> selectedUsers = GetSelectedUsers();

            if (MessageBox.Show(this, ((Func<string>)(() =>
            {
                var message = "Are you sure you want to delete user(s) ID";
                foreach (Int64 id in selectedUsers)
                {
                    message += " " + id.ToString();
                }
                message += "?";
                return message;
            }))(), "Confirmation Delete User", MessageBoxButtons.YesNo, MessageBoxIcon.Warning) == DialogResult.Yes)
            {
                //DoSomething();
            }

我希望这会有所帮助

如何将Lambda Expression用作String参数C＃

3 个答案: