what's the WindowExpertName()'s fuciton

It returns the File_Name of the Expert_Advisor. Example Macd_Sample.mq4 >>> it'll return "Macd_Sample".

string WindowExpertName( )
Returns name of the executed expert, script, custom indicator, or library, depending on the MQL4 program, from which this function has been called.

Sample:
string name=WindowExpertName();
GlobalVariablesDeleteAll(name);