Question

Try the following code

public enum Color
{
    Blue=1,
    Red=2, 
    Green=3 
}

public List<Color> ConvertColorEnum()
{
    var intColor = new List<int>(){1,2,3};
    return intColor.Cast<Color>().ToList();
}

Do you think the ConvertColorEnum() will return a list of color, i.e., List<Color>(){Color.Blue, Color.Red, Color.Green}?

I tested this on 2 machines, one with .net 3.5 ( mscorlib version 2.0.50727.1433), another with .net 3.5 SP1 ( mscorlib version 2.0.50727.3082). The results were different– the .net 3.5 threw an InvalidCastException because couldn’t convert integer to enum, whereas .net 3.5 SP1 could run successfully, with correct results returned.

Anyone would like to try this on his/her machine and report the result or explain why this is so?

Answer 1

You can read about the difference between the SP1 and the original release of the .net 3.5 framework in the release notes.

Here’s what it says for this particular issue:

In LINQ query expressions over
non-generic collections such as
System.Collections.ArrayList, the from
clause of the query is rewritten by
the compiler to include a call to the
Cast operator. Cast converts
all element types to the type
specified in the from clause in the
query. In addition, in the original
release version of Visual C# 2008, the
Cast operator also performs some
value type conversions and
user-defined conversions. However,
these conversions are performed by
using the System.Convert class instead
of the standard C# semantics. These
conversions also cause significant
performance issues in certain
scenarios. In Visual C# 2008 SP1, the
Cast operator is modified to throw
an InvalidCastException for numeric
value type and user-defined
conversions. This change eliminates
both the non-standard C# cast
semantics and the performance issue.
This change is illustrated in the
following example.

You can also get more details in this blog post.