When debugging a project line-by-line in Visual Studio, you may receive this error:

Step into: Stepping over method without symbols ‘namespace’

This error occurs when you attempt to debug a DLL or EXE that is lacking a symbols (.pdb) file. 

Check the project’s binDebug folder to ensure the DLL/EXE in question has a corresponding pdb file.  If not, be sure to build the DLL/EXE with its project configuration set to “Debug” so that it will generate a pdb file. 

If the problem occurs with a third-party library, you may be out of luck because most third-party libraries do not include a pdb file, and therefore you cannot debug into them.

A StackOverflow article says this error may also occur if you attempt to debug a yield expression in a method that returns an IEnumerable, though I have not confirmed this.