如何将值传递给方法中的字符串 [英] How to pass a value to a string in a method
问题描述
我有一个方法，它从数据库中获取值并将值传递给方法参数中的字符串。但它总是导致0.在我的数据库中有一个名为account的表，其中包含一个名为[New Balance]的列。我有一个查询从这个列中检索一个产生500的输出值并将值传递给方法参数中的字符串，所以当我调用方法时，我可以将字符串中的值传递给文本框。
我尝试了什么：
public void AVinfo（ string str）
{
decimal paValue;
尝试
{
con = new SqlConnection（DAL.DBConn ）;
con.Open（）;
string ct = SELECT [New Balance] ]来自帐户Where WithdrawalID =（SELECT Max（WithdrawalID）From Account）;
cmd = new SqlCommand（ct）;
cmd.CommandType = CommandType.Text;
cmd.CommandText = ct;
cmd.Connection = con;
DataTable dt = new DataTable（）;
SqlDataAdapter da = new SqlDataAdapter（）;
da.SelectCommand = cmd;
da.Fill（dt）;
con.Close（）;
string newval = dt.Rows [ 0 ] [ New Balance]。ToString（）;
str = newval.ToString（）;
}
catch （Exception ex）
{
MessageBox.Show（ex.Message， 错误，MessageBoxButtons.OK，MessageBoxIcon.Error）;
}
当我调用这个方法时我想把str的值放在textbox.text中
就像
AVinfo（textbox.text）
这是因为参数总是按值传递，不是参考。如果你考虑一下，这很有意义。
如果你写这样的方法：
private void Double （ int x）
{
x = x + x;
}您可以这样称呼：
int i = 666 ;
Double （i）;之后你可能会认为我是1332。但是如果这样做了，如果你这样做会发生什么：
Double （ 666 ）;显然，你不想尝试改变常量的值！
所以当你传递一个值时 - 从任何来源 - 作为参数的方法，它被复制，而传递副本：这被称为通过值传递而不是通过引用传递实际项目被传递。
不是你可以用常量或变量调用Double，因为它没关系 - 方法之外的值不会受到其中任何变化的影响 - 因为它是传递的副本，而不是真实的东西 。
你的方法有同样的事情：你的字符串通过值传递给方法 - 发送一个副本 - 并且任何更改都不会传递回外部世界，因此当方法退出时，您给str的值将被丢弃。
有t我可以修复：通过引用传递它，或者将其作为值返回。
要通过引用传递，请执行以下操作：
public void AVinfo（ref string str）并将其称为：
string str = ;
AVinfo（ ref str）;现在改变你方法中的str会影响外部变量。
由于您没有使用现有值，您可以使用out参数执行此操作：
public void AVinfo（ out 字符串 str）并按如下方式调用：
string str;
AVinfo（ out str）;
但更好的方法是返回值而不是改变它。
public string AVinfo（）
{
...
return str;
}
并按如下方式调用：
string str = AVinfo（）;
你可能想要这个：
textbox.text = AVinfo（）;
公共字符串AVinfo（）
{
// ...你的代码
return str;
}
其他方式可行，通过引用传递参数： https ：//www.dotnetperls.com/parameter [ ^ ]
i have a method which takes a value from a database and pass the value to a string in a method argument. But it always result to 0. There is a table called account in my database with a column called [New Balance]. I have a query to retrieve a value from this column which produce an output of 500 and pass the value to string in a method argument so when i call the method i can pass the value from the string to a textbox.
What I have tried:
public void AVinfo(string str)
{
decimal paValue;
try
{
con = new SqlConnection(DAL.DBConn);
con.Open();
string ct = "SELECT [New Balance] From Account Where WithdrawalID = (SELECT Max(WithdrawalID) From Account)";
cmd = new SqlCommand(ct);
cmd.CommandType = CommandType.Text;
cmd.CommandText = ct;
cmd.Connection = con;
DataTable dt = new DataTable();
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = cmd;
da.Fill(dt);
con.Close();
string newval = dt.Rows[0]["New Balance"].ToString();
str = newval.ToString();
}
catch (Exception ex)
{
MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
When i call this method i want to put the value of str in textbox.text
like
AVinfo(textbox.text)That's because parameters are always passed by value, not reference. And if you think about it, that makes a lot of sense.
If you write a method like this:
private void Double (int x) { x = x + x; }You could call it like this:
int i = 666; Double(i);And you might expect i to be 1332 after that. But if that worked, what would happen if you did this:
Double(666);Clearly, you don't want to try and change the value of constants!
So when you pass a value - from whatever source - to a method as a parameter, it is copied, and the copy is passed instead: this is called "passing by value" instead of "passing by reference" where the actual item is passed over.
Not you can call Double with a constant or a variable because it doesn't matter - the value outside the method will not be affected by any changes inside it - because it's a copy that is passed, not the "real thing".
You method has the same thing: your string is passed by value into the method - a copy is sent - and any changes are not passed back to the outside world, so the value you give tostris discarded when the method exits.
There are two ways to "fix" that: pass it by reference, or return it as a value.
To pass by reference, you do this:
public void AVinfo(ref string str)And call it like this:
string str = ""; AVinfo(ref str);And now changes to str inside your method will affect the external variable.
Since you don't use the existing value, you could do that with an out parameter:
public void AVinfo(out string str)And call it like this:
string str; AVinfo(out str);
But the better way is to return the value instead of changing it.
public string AVinfo() { ... return str; }
And call it like this:
string str = AVinfo();
You probably want this:
textbox.text = AVinfo(); public string AVinfo() { // ... your code return str; }
Other ways are possible, pass a parameter by reference: https://www.dotnetperls.com/parameter[^]
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