How to call c# server side method through javascript

I need to call a c# server method through the javascript. I have a gridview in which i have a column开发者_开发知识库 with dropdown list. When i change the dropdown's value i need to call a server side method through javascript and change the value of another text box in the gridview.

I am able to do this on the selected index change. but i am slightly worried about the performance.

i am using asp.net c#.

Please let me know how to do this.

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Can also use PageMethods with ajax like here

http://aspalliance.com/1922_PageMethods_In_ASPNET_AJAX

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You're going to need to use AJAX in a manner similar to this: http://www.codeproject.com/KB/aspnet/Ajax_Samplecode_in_C_Net.aspx