error of method not found in C# class in VS 2013 on win7

I am changing a C# code in a solution (it has many files and projets) from VS 2013 on win 7.

I added some properties in class1. It was built well.

But, it has run-time error:

Method not found : class1.set_p1(system.string)

The code logic is as follows:

publicclass class1{publicstring p1 {get;set;}// **have updated**}

class1 is defined in a C# project that has been added in the solution.

publicabstractclass class2 : class1
  class2 ins2;
  ins2.p1 = myData.order.data_file;// myData is from an xml file after deserialize and serialize, data_file is a file path.

I have cleaned up the solution before building. I have also search the error online but none of them can solve my problems.

Any help would be appreciated.