Faced this problem when trying to run my web application. No error thrown in the Design view and no warning generated. However, hitting F5 gives me this error message which really cracked my head.

CS1061: ‘Telerik.Web.UI.RadComboBox’ does not contain a definition for ‘Web’ and no extension method ‘Web’ accepting a first argument of type ‘Telerik.Web.UI.RadComboBox’ could be found (are you missing a using directive or an assembly reference?)

Looking into the Error highlighted in the screenshot does not help at all. “Web” is not mentioned or used anywhere in my ASPX page.

Googling around and don’t seem to be able to find any solution. Thought it would be good to share the resolution i found after few hours!

Resolution

The solution is plain SIMPLE! Which is to Remove Attribute “Height” away. Holy Cow.

It seem that Telerik RadTextBox does not support “Height” attribute. Instead, you should use Rows or Cols….

[Updated]

Found the source of problem is that i added a RadComboBox with ID=”System“. This is a system reserved keyword. After renaming it to other name. The error is now gone. Height and Width is now working as it is!