Comparing values in a hash table

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Hi everyone.

I have a requirement to check the key of a hashtable and compare it's value. I thought the following code would have worked:

C#

private void Startup()
       {
           GetSettings(); // Get the test program settings from the text file.
           if (hsh_Settings.ContainsKey("TestMode") == true)
           {
               if (hsh_Settings["TestMode"] == "Production")
               {
                    SetProductionMode();
               }
               if (hsh_Settings["TestMode"] == "Engineer")
               {
                    SetEngineeringMode();
               }
           }
           else
           {
               SetProductionMode();
           }
       }

Unfortunately I received the following warning: Possible unintended reference comparison; to get a value comparison, cast the left hand side to type 'string'

To make this warning go away, I did as asked:

C#

private void Startup()
       {
           GetSettings(); // Get the test program settings from the text file.
           if (hsh_Settings.ContainsKey("TestMode") == true)
           {
               if ((string)hsh_Settings["TestMode"] == "Production")
               {
                    SetProductionMode();
               }
               if ((string)hsh_Settings["TestMode"] == "Engineer")
               {
                    SetEngineeringMode();
               }
           }
           else
           {
               SetProductionMode();
           }
       }

Now there are no warnings and the code works. My problem is that I don't properly understand the reason for the cast requirement. Perhaps there is a cleaner way of comparing hashtable values that does not require messy casting to be done?

Thanks in advance,

Brian

Posted 25-Sep-13 4:20am

bh_

Updated 25-Sep-13 4:21am

v2

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Comments

Richard C Bishop 25-Sep-13 10:24am

The cast is required because you cannot compare two different datatypes. Another option would be to add .ToString() to the end of your hsh_Settings[""], but the cast is fine.

bh_ 25-Sep-13 10:33am

Hello richcb, thanks for the reply.

So the story behind this hashtable is that I have a text file with some settings in it, like this:

<pre lang="vb">
AutoHighLimit = 2E
ModIndex = 2B
AutoLowLimit = 2B
AutoUpdateRange = 10
ModIndexMode = Manual
TestMode = Engineering</pre>

And then I have a function called GetSettings() which iterates through the lines of the text file, first populating the keys with the text value on the far left, and then making the value of that key equal to the text on the other side of the '=' sign, not including the whitespace. So, as an example, after running GetSettings() there will be a key called AutoHighLimit and its value will be "2E".

And that's where the source of my confusion begins. My understanding was that the hashtable already contained string values. Why, then, do I have to cast to a string in the example I gave above? Clearly my understanding is not correct! :)

Thanks,

Brian

2 solutions

Solution 1

Please go through this link
http://stackoverflow.com/questions/12263715/possible-unintended-reference-comparison[^]
Hope this helps

Posted 25-Sep-13 4:29am

Jameel VM

Comments

bh_ 25-Sep-13 10:38am

Thanks Jameel. My confusion is still present because I expected the type of the hashtable value to already be a string, hence no need for a cast.

Jameel VM 25-Sep-13 10:56am

then i recommend is to use Dictionary<tkey, tvalue="">.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-09-25T04:44:00

Yes, there is much cleaner approach. Actually, your solution is pretty dirty from the very beginning: you are using a non-generic hash table which inherently based on type casting. In other words, compile-time element type is System.Object, so you need to do type casts if you store objects of other types (which are still objects but have different run-time types).

All such collections (except specialized ones) have been rendered obsolete as early as of .NET v.2.0, when generics were introduced. The were not formally marked with System.ObsoleteAttribute just because there is nothing wrong with using it in legacy software units, for compatibility, but using them in newly developed software makes no sense, ever.

In your case, you should use the type System.Collections.Generic.Dictionary<string, string>:
http://msdn.microsoft.com/en-us/library/xfhwa508.aspx[^].

See also: http://msdn.microsoft.com/en-us/library/System.Collections.Generic.aspx[^].

Non-generic non-specialized collections have gone, don't use them anymore.

[EDIT]

As I see you are still confused with the cast. You need to understand how strings are compared and how default comparison between reference-type objects is done. Normally, references are compared. But you can redefine comparison method System.Object.Equals(System.Object), to compare object in some different way, let's say, semantically. If this is done, you have a choice between "==" comparison (which can be redefined separately) and comparison which always checks referential identity, using System.Object.ReferenceEquals. Please see:
http://msdn.microsoft.com/en-us/library/system.object.aspx[^].

Compiler does not "know" that one operand is a string and compares references, which is not what you meant to do. For strings, it compares string contents. System.String is a reference class which closely mimic value semantic. It is much more complex then most people think. For understanding, please see:
http://msdn.microsoft.com/en-us/library/system.string.intern.aspx[^],
http://msdn.microsoft.com/en-us/library/system.string.isinterned.aspx[^],
http://en.wikipedia.org/wiki/String_intern_pool[^].

You really need to understand reference and value types and working with references. These concepts are fundamental to .NET and CLI.

—SA