Append an AnsiString.
AnsiString& __fastcall operator +=(const AnsiString& rhs);
The operator += concatenates this AnsiString and rhs, assigning the result to this AnsiString.
The operator +=() is not supported on a property with getter/setter functions.
明确提到不能用于get/set那些“属性”上。
再看看Edit里Text的声明
__property AnsiString Text = {read=GetText, write=SetText};
所以你那里用不了很正常