How to imitate string.Format() in my own method?

You need to copy the method signature from string.format.

public void WriteLine(string text,params object[] args) {
  this.StringList.Add(string.Format(text,args));
}

As suggested by ChaosPandion you can also include overloads to prevent array creation

public void WriteLine(string text) {
  this.StringList.Add(text);
}

public void WriteLine(string text,object arg0) {
  this.StringList.Add(string.Format(text, arg0));
}

public void WriteLine(string text,object arg0, object arg1) {
  this.StringList.Add(string.Format(text, arg0, arg1));
}

public void WriteLine(string text,object arg0, object arg1, object arg2) {
  this.StringList.Add(string.Format(text, arg0, arg1, arg2));
}

I wouldn’t go past arg2 as string.format doesn’t so the benefit disappears.