As part of the refactoring I was doing to the load code for
crawler projects I needed a way of verifying that new code was
loading data correctly. As it would be extremely time consuming
to manually compare the objects, I used Reflection to compare
the different objects and their properties. This article briefly
describes the process and provides a complete helper function
you can use in your own projects.
The ability to analyze assemblies and their component pieces is
directly built into the .NET Framework, and something I really
appreciate - I remember the nightmares of trying to work with
COM type libraries in Visual Basic many years ago!
The Type class represents a type declaration in your project,
such as a class or enumeration. You can either use the GetType
method of any object to get its underlying type, or use the
typeof keyword to access a type from its type name.
Once you have a type, you can call the GetProperties method to
return a list of PropertyInfo objects representing the
available properties of the type. Several methods, including
GetProperties, accept an argument of BindingFlags, these
flags allow you to define the type of information return, such
as public members or instance members.
In this case, I want all public instance members which can be
read from and which are not included in a custom ignore list.
Retrieving the value of a property
The PropertyInfo class has a GetValue method that can be
used to read the value of a property. Its most basic usage is to
pass in the instance object (or null if you want to read a
static property) and any index parameters (or null if no index
parameters are supported).
The sample function described in this article doesn't currently
support indexed properties.
Determining if a property can be directly compared
Some properties are simple types, such as an int or a string
and are very easy to compare. What happens if a property returns
some other object such as a collection of strings, or a complex
In this case, I try and see if the type supports IComparable
by calling the IsAssignableFrom method. You need to call this
from the type you would like to create, passing in the source
I also check the IsPrimitive and IsValueType properties of
the source type, although this is possibly redundant as all the
base types I've checked so far all support IComparable.
Directly comparing values
Assuming that I can directly compare a value, first I check if
one of the values is null - if one value is null and one false,
I immediately return a mismatch.
Otherwise, if IComparable is available, then I obtain an
instance of it from the first value and call its CompareTo
method, passing in the second value.
If IComparable is not supported, then I fallback to
If the values could not be directly compared, and do not
implement IEnumerable (as described in the next section) then
I assume the properties are objects and call the compare objects
function again on the properties.
This works nicely, but has one critical flaw - if you have a
child object which has a property reference to a parent item,
then the function will get stuck in a recursive loop. Currently
the only workaround is to ensure that such parent properties are
excluded via the ignore list functionality of the compare
If the direct compare check failed, but the property type
supports IEnumerable, then some Linq is used to obtain the
collection of items.
To save time, a count check is made and if the counts do not
match (or one of the collections is null and the other is not),
then an automatic mismatch is returned. If the counts do match,
then all items are compared in the same manner as the parent
I have tested this code on generic lists such as List<string>,
and on strongly typed collections which inherit from
Collection<TValue> with success.
Below is the comparison code. Please note that it won't handle
all situations - as mentioned indexed properties aren't
supported. In addition, if you throw a complex object such as a
DataReader I suspect it will throw a fit on that. I also
haven't tested it on generic properties, it'll probably crash on
those too. But it has worked nicely for the original purpose I
wrote it for.
Also, as I was running this from a Console application, you may
wish to replace the calls to Console.WriteLine with either
Debug.WriteLine or even return them as an out parameter.
I hope you find these helper methods useful, this article will
be updated if and when the methods are expanded with new
2010-11-05 - First published
2020-11-21 - Updated formatting
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