Where is the Source Code for Html.TextBoxFor
Get reflector (version 6. should be somewhere available for free), open
System.Web.Mvc.dll and search for
Get mvc source code, open in visual studio, and search for
edit: misread the question..
I guess you are looking for
InputExtensions.cs line: 371
You don't need to download the source code or use reflector extensions.
If you "go to definition" (F12), you should see the file metadata, which is the summarized method declarations. From there, hover over the tab, and you should see the (local) file path where the method came from, which also corresponds to the namespace. With that, you'll be able to look it up from the MVC source code on Codeplex:
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