This post I wanted to post for quite a while but keep forgetting. In order to use Variable in SSIS Script Task there is no Intellisense. You have to remember the exact syntax for it. Very first you need to choose the Read/Write variable from Script Task Editor.
To read the value
string strSource = Dts.Variables[“User::varOraSource”].Value.ToString();
To write the value.
Dts.Variables[“User::varOraSource”].Value = strSourceFinal;