I think it's better to create an interface for that class. And create a unit test using interface.
If it you don't have access to that class, you can create an adapter for that class.
For example:
public class RealClass
{
int DoSomething(string input)
{
// real implementation here
}
}
public interface IRealClassAdapter
{
int DoSomething(string input);
}
public class RealClassAdapter : IRealClassAdapter
{
readonly RealClass _realClass;
public RealClassAdapter() => _realClass = new RealClass();
int DoSomething(string input) => _realClass.DoSomething(input);
}
This way, you can easily create mock for your class using IRealClassAdapter.
Hope it works.