Question

According to the MSDN:

Because the Equals and GetHashCode
methods on anonymous types are defined
in terms of the Equals and GetHashcode
of the properties, two instances of
the same anonymous type are equal only
if all their properties are equal.

However, the following code demonstrates the compiler generated implementation for Equals() isn’t behaving as expected.:

 DateTime start = new DateTime(2009,1,1);
 DateTime end = new DateTime(2010, 12,31);

 // months since year 0
 int startMonth = start.Date.Year * 12 + start.Date.Month - 1;
 int endMonth = end.Date.Year * 12 + end.Date.Month -1 ;

 // iterate through month-year pairs
 for (int i = startMonth; i <= endMonth ; i++)
 {
  var yearMonth = new { Year = (int)Math.Truncate(i/12d), Month = (i % 12) + 1};

  if (yearMonth.Year == 2009 &&  yearMonth.Month == 2)
   Console.WriteLine("BOOM");

  if (yearMonth == new{Year = 2009, Month = 2})
   Console.WriteLine("I'm never called!");

  Console.WriteLine(yearMonth);
 }

Am I missing something? I am looking at the generated MSIL but don’t see an obvious error. Is there a way for MSIL level debugging (besides WinDbg maybe)? Am I overlooking something?

I have tested .NET 3.5 (VS 2008 SP1 compiler). For reference, here’s the generated Equals method:

public override bool Equals(object value)
{
    var type = value as <>f__AnonymousType3<<Year>j__TPar, <Month>j__TPar>;
    return (((type != null) && EqualityComparer<<Year>j__TPar>.Default.Equals(this.<Year>i__Field, type.<Year>i__Field)) && EqualityComparer<<Month>j__TPar>.Default.Equals(this.<Month>i__Field, type.<Month>i__Field));
}

Answer 1

== is not Equals() – I believe that your code should work as expected when you do this:

if (yearMonth.Equals(new{Year = 2009, Month = 2}))

See also this SO question.