如何將Lambda表達式用作字符串參數C＃

Question

出於好奇，我想知道是否可以使用Lambda表達式作為String參數？

這是我的摘錄：

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，我在這里收到此錯誤“錯誤7無法將lambda表達式轉換為類型'string'，因為它不是委托類型” ：（字符串消息）

感謝您的幫助，我們非常感謝！

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種委托來返回字符串（老實說，我不確定您為什么會在這里），則需要將表達式轉換為Func<string>並立即調用它。

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

我認為這是你應該做的

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();
            }
        }

我們這里有一個IFFE。 作為一種優化，您可以擺脫message參數，因為您並沒有真正使用初始值。 這將變成：

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();
            }

我希望這有幫助