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.

There is one edge case you might want to take pains to avoid. The following code will write the string {0} to the console

Console.WriteLine("{0}");

However, you use one of the implementations of WriteLine suggested by others in this answer, those implementations will throw an exception:

WriteLine("{0}"); //throws exception

The easiest fix is either to have two overloads of WriteLine or to modify the suggested code slightly to handle the edge case:

public void WriteLine(string text,params object[] args) {  var message=args.Length==0 ? text : string.Format(text, args);  this.StringList.Add(message);}

You could use params:

 public void WriteLine(string text, params object[] parameters) {     //.. }