method overloading vs optional parameter in C# 4.0

One good use case for 'Optional parameters' in conjunction with 'Named Parameters' in C# 4.0 is that it presents us with an elegant alternative to method overloading where you overload method based on the number of parameters.

For example say you want a method foo to be be called/used like so, foo(), foo(1), foo(1,2), foo(1,2, "hello"). With method overloading you would implement the solution like this,

///Base foo method
public void DoFoo(int a, long b, string c)
{
   //Do something
}  

/// Foo with 2 params only
public void DoFoo(int a, long b)
{
    /// ....
    DoFoo(a, b, "Hello");
}

public void DoFoo(int a)
{
    ///....
    DoFoo(a, 23, "Hello");
}

.....

With optional parameters in C# 4.0 you would implement the use case like the following,

public void DoFoo(int a = 10, long b = 23, string c = "Hello")

Then you could use the method like so - Note the use of named parameter -

DoFoo(c:"Hello There, John Doe")

This call takes parameter a value as 10 and parameter b as 23. Another variant of this call - notice you don't need to set the parameter values in the order as they appear in the method signature, the named parameter makes the value explicit.

DoFoo(c:"hello again", a:100) 

Another benefit of using named parameter is that it greatly enhances readability and thus code maintenance of optional parameter methods.

Note how one method pretty much makes redundant having to define 3 or more methods in method overloading. This I have found is a good use case for using optional parameter in conjunction with named parameters.

Optional Parameters provide issues when you expose them publicly as API. A rename of a parameter can lead to issues. Changing the default value leads to issues (See e.g. here for some info: Caveats of C# 4.0 optional parameters)

Also, optional params can only be used for compile-time constants. Compare this:

public static void Foo(IEnumerable<string> items = new List<string>()) {}
// Default parameter value for 'items' must be a compile-time constant

to this

public static void Foo() { Foo(new List<string>());}
public static void Foo(IEnumerable<string> items) {}
//all good

Update

Here's some additional reading material when a constructor with default parameters does not play nicely with Reflection.

I believe they serve different purposes. Optional parameters are for when you can use a default value for a parameter, and the underlying code will be the same:

public CreditScore CheckCredit( 
  bool useHistoricalData = false,  
  bool useStrongHeuristics = true) { 
  // ... 
}

Method overloads are for when you have mutually-exclusive (subsets of) parameters. That normally means that you need to preprocess some parameters, or that you have different code altogether for the different "versions" of your method (note that even in this case, some parameters can be shared, that's why I mentioned "subsets" above):

public void SendSurvey(IList<Customer> customers, int surveyKey) {  
  // will loop and call the other one 
} 
public void SendSurvey(Customer customer, int surveyKey) {  
  ...  
}

(I wrote about this some time ago here)