Why Type.GetType() return null for concatenated strings? [on hold]

I have this code:

string x = "DimensionUpdater.Tasks."+className;
string y = "DimensionUpdater.Tasks.DownloadDimensionUpdateTask";
var type = Type.GetType(x);
var type2 = Type.GetType(y);
var task = Activator.CreateInstance(type);

As below the on the image visible, the x and y variables are totally the same, but Type.GetType() return with null for x

Why Type.GetType() return null for a string value which was concatenated? And what is the workaround for this?