Can anyone please tell me what is the main advantage and feature of objective c over c#?

For starters, Objective-C is mainly used to make iOS or OSX apps, and C# is used for Windows apps. Objective C compiles to machine code, which makes it faster but the program is processor-specific while C# is compiled into CIL bytecode for use by a CLR, which makes it more portable. There's still some more in terms of syntax but mainly your choice depends on which OS you are developing for.