首先，将文本框值转换为数字。这很容易，但由于这是一个文本框，你应该这样做：

  int  myValue; 
 if （！int.TryParse（myTextBox.Text， out  myValue））
 {
 //  告诉用户他没有输入数字！ 
 ... 
 return ; 
} 
 //  一切正常 - 继续......  

然后，为您的下拉列表执行相同的操作：因为您控制了值，所以更容易：

  int  myDDLValue =  int  .Parse（myDropDownList.Text）; 

然后就这样做吧！

 myOutputTextBox.Text =（myDDLValue  -  myValue）.ToString（）; 

这是在开始编写基于表单的应用程序之前应该真正理解的基本编程。请参阅 http://www.codeproject.com/Messages/4799340/Re-Csharp.aspx [ ^ ]虽然不准确，但原则是一样的。

for example i have a dropdownlist having values 1,2,3 and a textbox having value 10,how do i subtract these values ie if choosen 1 then output should be 9.output could be shown in another textbox

First, convert the textbox value to a number. That's easy, but since this is a textbox, you shoudl do it like this:

int myValue;
if (!int.TryParse(myTextBox.Text, out myValue))
   {
   // Tell the user he didn't enter a number!
   ...
   return;
   }
// All ok - move on...

Then, do the same for your drop down list: that's even easier because you control the values:

int myDDLValue = int.Parse(myDropDownList.Text);

Then just do it!

myOutputTextBox.Text = (myDDLValue - myValue).ToString();

This is basic programming that you should really understand before you start writing form based applications. See http://www.codeproject.com/Messages/4799340/Re-Csharp.aspx[^], while not exact, the principle is the same.