如何将Lambda表达式用作字符串参数C＃

Question

出于好奇，我想知道是否可以使用Lambda表达式作为String参数？

这是我的摘录：

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();
}

但是，不幸的是，我在这里收到此错误“错误7无法将lambda表达式转换为类型'string'，因为它不是委托类型” ：（字符串消息）

感谢您的帮助，我们非常感谢！

Answer 1

建立您的信息之前

string ids = selectedUsers.Select(n=>n.ToString()).Aggregate((current, next) => current + ", " + next);
// also works string.Join
// string ids = string.Join(", ",selectedUsers);
string message = "Are you sure you want to delete user(s) ID: "+ids+"?";
if (MessageBox.Show(this, message, "Confirmation Delete User", MessageBoxButtons.YesNo, MessageBoxIcon.Warning) == DialogResult.Yes)
{ 
    //DoSomething();
}

Answer 2

如果您真的想使用某种委托来返回字符串（老实说，我不确定您为什么会在这里），则需要将表达式转换为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

我认为这是你应该做的

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。 作为一种优化，您可以摆脱message参数，因为您并没有真正使用初始值。 这将变成：

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();
            }

我希望这有帮助