Less wrong TryGetValue then null check syntax

Question

I'm a little new to C# still... I'm finding myself reusing a specific procedure over and over. Before I go writing a helper method for personal laziness, is there a shorter or less wrong way to write this sort of statement?

Dictionary<string, string> data = someBigDictionary;
string createdBy;
data.TryGetValue("CreatedBy", out createdBy);
//do that for 15 other values
...
MyEntity me = new MyEntity{
    CreatedBy = createdBy ?? "Unknown",
    //set 15 other values
    ...
}

Essentially, set a property on an object by trying to get a value, then if it's null use a default value. I have a LOT of properties, it would be better if I could just

MyEntity me = new MyEntity{
    CreatedBy = TryToGetValueOrReturnNull(data, "CreatedBy") ?? "Unknown",
    ...
}

Again, I am perfectly capable of writing my own helper function. I'm looking for an existing native functionality or shorthand before I do so.

Why not provide the default value as another parameter? You can pass in null if you want - but you can pass in "Unknown" instead. — Jon Skeet, Jun 06 '17 at 12:55
@JonSkeet may well do that if I write my own... just showing an example — Randy Hall, Jun 06 '17 at 12:56
If you can use C# 7 - you can do this: `CreatedBy = data.TryGetValue("CreatedBy", out var value) ? value : "Unknown"` — Evk, Jun 06 '17 at 13:00
@Evk I cannot for this application, but that sounds like a valid answer for anyone who comes by later using C# 7 so please post — Randy Hall, Jun 06 '17 at 13:01
IMHO it's better to leave the `CreatedBy` field blank _(dbnull)_ instead of writing a default valid _"Unknown"_ value. The UI should present "Unknown" when the `CreatedBy` field is empty. This way you can always distingues a name and _nothing_ — Jeroen van Langen, Jun 06 '17 at 13:07
@JeroenvanLangen valid point, but this was simply an example. The default values are varied — Randy Hall, Jun 06 '17 at 13:09
Helper function gives you advantage of abstraction so it eases future changes. Why are you trying to avoid it? — Antonín Lejsek, Jun 06 '17 at 13:17
@AntonínLejsek I'm only trying to avoid it if there is some sort of shorthand functionality that already exists natively. I'm perfectly fine with a helper otherwise — Randy Hall, Jun 06 '17 at 13:26

score 6 · Accepted Answer · answered Jun 06 '17 at 13:04

6

There are many similar questions (like this and this) which propose different solutions from extension methods to inheriting from dictionary and overriding indexer. However they are written before C# 7, and with C# 7 you can do this in one line:

CreatedBy = data.TryGetValue("CreatedBy", out var value) ? value : "Unknown"

answered Jun 06 '17 at 13:04

Evk

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It appears that this is not possible without a helper method pre-C# 7, so I'm accepting this as the best answer even though I cannot use it in my specific implementation. – Randy Hall Jun 06 '17 at 13:32
It is possible, just clunky. string value; MyEntity me = new MyEntity{ CreatedBy = data.TryGetValue("CreatedBy", out value) ? value : "Unknown", UpdatedBy = data.TryGetValue("UpdatedBy", out value) ? value : "Unknown" //set 15 other values ... } – mjwills Jun 06 '17 at 13:51

score 3 · Answer 2 · answered Jun 06 '17 at 12:58

public static class DictionaryExtensions
{
    public static U TryGetValueOrDefault<T, U>(this IDictionary<T, U> dict, T key, U defaultValue)
    {
        U temp;

        if (dict.TryGetValue(key, out temp))
            return temp;

        return defaultValue;
    }
}

then do something like:

Dictionary<string, string> data = someBigDictionary;
//do that for 15 other values
...
MyEntity me = new MyEntity{
    CreatedBy = data.TryGetValueOrDefault("CreatedBy", "Unknown"),
    //set 15 other values
    ...
}

score 2 · Answer 3 · answered Jun 06 '17 at 13:01

2

TryGetValue returns a bool indicating if the key was found in the dictionary. So you should use that and set the variable to the default if it was not found:

string createdBy;
if (!data.TryGetValue("CreatedBy", out createdBy)) createdBy="Unknown";

answered Jun 06 '17 at 13:01

Ofir Winegarten

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Less wrong TryGetValue then null check syntax

3 Answers3