About Generic types with like T

Question

I wonder what is this? This kind of a generic method I think. It has a part with 'where'. What about that? There is also generic classes I've heard. How can I learn these can you recommend an article?

    protected T Item<T>() where T : class
    {
        return GetDataItem() as T ?? default(T);
    }

http://stackoverflow.com/questions/4737970/what-does-where-t-class-new-mean — Anton Sizikov, Sep 04 '13 at 08:30
[Read the fine manual](http://msdn.microsoft.com/en-us/library/d5x73970.aspx). — Jon, Sep 04 '13 at 08:30
MSDN reference to generics http://msdn.microsoft.com/en-us/library/512aeb7t%28VS.80%29.aspx — Gonzix, Sep 04 '13 at 08:31
yes this a generic method and where is used to constraint the type of parameters you can return from this method — Prabhu Murthy, Sep 04 '13 at 08:32
Hey this is not duplicate! It isn't the same question as mine. The question you showed is similar with only the 'where' part. — Serhat Koroglu, Sep 04 '13 at 08:46

dcastro · Accepted Answer · 2013-09-04T08:37:41.157

5

The where clause is called a "generic constraint". In that case, where T: class dictates that T must be a reference type (i.e., not a struct).

More info on generic constraints: http://msdn.microsoft.com/en-us/library/d5x73970.aspx And generic classes: http://msdn.microsoft.com/en-us/library/sz6zd40f.aspx

Edit

In the snippet you provided, the constraint is needed because otherwise the null-coalescing operator (??) wouldn't make sense, since value types (structs) can't be null.

edited Sep 04 '13 at 08:37

answered Sep 04 '13 at 08:30

dcastro

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minor footnote re your last line: `Nullable` (for any `T : struct`) is itself a `struct` and can notionally be `null` – Marc Gravell Sep 04 '13 at 08:38
Thanks for the correction. Isn't it weird that neither `T:struct` nor `T:class` allow `T` to be a Nullable? – dcastro Sep 04 '13 at 08:48

About Generic types with like T

1 Answers1