Namespaces disambiguate class names inside of class libraries or applications.
You must reference class names by their full name.
System.Text.StringBuilder sb = new System.Text.StringBuilder();
…. or, you can employ a using directive at the top of the code file to instruct the compiler to look in those namespaces to find the class that is referenced.
using System.Text; ................ StringBuilder sb = new StringBuilder();
You must always do this if the code you are writing is outside of the namespace of the class you want to use, even if it is in the same project.
Default namespace defined in Project Properties(right clicking on Project name in Solution Explorer, select Properties……)