How to add 2 new() constraint for to type parameters?

Question

I want something like this:

public static TTo JumpTo<TFrom, TTo>(this TFrom from_page) 
       where TTo : new() TFrom : new()
{
    ...
}

And I want to enforce that TFrom and TTo are both derived from a base type.

And I want to make this method as an extension method of TFrom type.

Is it possible ? And what's the correct syntax?

@AD.Net: And your point with that demand? Edit: and that wont compile. — leppie, Aug 27 '12 at 14:28
Btw the, the `new` constraint on the input parameter does not really make sense, unless you copying stuff internally. — leppie, Aug 27 '12 at 14:33
@AD.Net: That sounds more like VB.NET except in VB.NET it would be `Of TTo As SomeType, TFrom As SomeOtherType` so actually not at all — Ry-, Aug 27 '12 at 14:35

cadrell0 · Accepted Answer · 2012-08-27T14:39:17.947

10

Put the keyword where before each type.

public static TTo JumpTo<TFrom, TTo>(this TFrom from_page) 
    where TTo : SomeBaseType, new() 
    where TFrom : SomeOtherBaseType, new()
{
     ...
}

edited Aug 27 '12 at 14:39

answered Aug 27 '12 at 14:29

cadrell0

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Thanks, and I want to make JumpTo as an **extension method** of TFrom. Is it possible? – smwikipedia Aug 27 '12 at 14:35
@smwikipedia: Your edit just did that! You add to a static class. – leppie Aug 27 '12 at 14:36
@smwikipedia Yes you can make it an extensions method. – cadrell0 Aug 27 '12 at 14:39
Currently, I put the method into a static class. But the method does not show for the TFrom type. I am still trying on it. – smwikipedia Aug 27 '12 at 14:39
@smwikipedia please post a complete example, including the containing static class. – cadrell0 Aug 27 '12 at 14:40
I got it, I removed the new() constraint for TFrom and it shows up now. It turns out my TFrom type doesn't have a parameterless constructor. – smwikipedia Aug 27 '12 at 14:43

How to add 2 new() constraint for to type parameters?

1 Answers1