For those purposes you should use uigetfile() to get the file name and path, fullfile() to put those together into a complete name, and load() on the complete name.
If you assign the output of load() of a .mat to a variable then load() will create a struct with one field per stored variable, the way you describe. This is the recommended approach.
If you do not assign the output of load() of a .mat to a variable, then load() will create one workspace variable for each stored variable from the .mat file, overwriting any existing variables with the same name. It is not possible to "protect" any workspace variable when you do this. The Just In Time optimizer does not know about variables that are "poofed" into existence, so you might get unexpected results. This is not recommended. If you need the contents outside of a structure then you should load() returning a structure like described above, and then specifically pull out the parts you need.
filecontents = load(....)
B = filecontents.B;
C = filecontents.C;
