string.Format()

string s = name + " " + balanceDue + " " + discountRate + " " + sinceYear;
return s;

In the above code a value is assigned to variable s which type is string then s is returned.

But in the following statement string.Format() method is directly returned. Why we can't do above similar thing when we use string.Format() method that means assigned to variable and that variable is returned. I wish to know the reason for the differences in both cases. 

return string.Format("{0} {1} {2} {3}", name, balanceDue, discountRate, sinceYear);

using System;
class Customer
{
protected string name;
protected double balanceDue;

public Customer()
{
name = "Maha";
balanceDue = 0;
}
}
class PreferredCustomer : Customer
{
private double discountRate;
private int sinceYear;

public PreferredCustomer()
{
discountRate = 0.08;
sinceYear = 1997;
}
public new string ToString()
{
//string s = name + " " + balanceDue + " " + discountRate + " " + sinceYear;
//return s;

return string.Format("{0} {1} {2} {3}", name, balanceDue, discountRate, sinceYear);
}
}
class DemoPreferredCustomerConstructor2
{
public static void Main()
{
PreferredCustomer pc = new PreferredCustomer();
Console.WriteLine("The preferred customer is {0}", pc.ToString());

Console.ReadKey();
}
}
/*
The preferred customer is Maha 0 0.08 1997
*/

What happens when you try doing it the way you think cannot be done? You are probably getting an error that is not relevant and then you assume it cannot be done.

You could in fact do it like this:

   string s = string.Format("{0} {1} {2} {3}", name, balanceDue, discountRate, sinceYear); 

   return s;

You could also do this:

   return name + " " + balanceDue + " " + discountRate + " " + sinceYear;

Which you prefer is a matter of taste.

Many developers would prefer to avoid creating an unnecessary local variable and just return the string expression directly. Others would feel that creating an intermediate variable and then returning that was clearer.

Similarly, whether you prefer to use String.Format or just concatenate the strings yourself is also a matter of a taste. Personally, I've always liked the former as I think it's neater and less likely to lead to errors, but I'm not always consistent in what I use.

Note that I would prefer to help Maha understand why Maha misunderstood, so that is why I did not write a lengthy explanation. I wanted to help Maha get a deeper understanding. We might never be able to do that. I say that because most people are going to think that I did not write more only because I wanted to save time and I did not care enough about helping to spend more time writing an explanation.

Thank you for your explanation.